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Chapter 2 · Free lesson

Compounds: formulas and names

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YOUR CHAPTER PLAN

What you will study

  1. Recognize the two compound types
  2. Ion reference and predictable charges
  3. Ionic formulas in both directions; subscripts and parentheses
  4. Ionic names in both directions
  5. Molecular formulas and names
  6. Acids, strong bases, and mixed route practice

By the end, you can…

  1. Recognize ionic and molecular compounds and explain what holds each together.
  2. Use the ion reference and periodic table to identify common charges.
  3. Write a neutral ionic formula, identify its ions, and read subscripts and parentheses.
  4. Name an ionic compound and write its formula from a name.
  5. Read a molecular formula and use prefixes to connect formulas and names.
  6. Recognize aqueous acids and common strong bases, and choose the acid naming route.

These are the skills to check, not just pages to finish. Use the section goals while studying, then try the chapter checks without hints.

First: some elements come in pairs

Paired atoms represent H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. Each molecule has two atoms of the same element.
Remember H2, N2, O2, F2, Cl2, Br2, and I2. These are their common elemental forms under ordinary conditions. O2 is an element; CO2 is a compound because it contains different elements. In equations, write the pair; do not replace O2 with O.

2.1 Recognize the compound: ionic or molecular?

A compound is two or more elements chemically combined. Before you write or name one, decide whether it is ionic (held by attraction between ions) or molecular (held by shared electrons). That one choice sets the route for the rest of the chapter.

What you’ll use from earlier
  • Ions and their charges (1.3).
  • Groups and periods on the periodic table (1.0).

Need a refresher? Open the table of contents and revisit that topic before starting.

The idea
Periodic table colored by type. Metals (blue) are left of a zigzag staircase line that starts just left of boron. Nonmetals (green) are right of it, plus hydrogen. Metalloids (gold) touch the line: B, Si, Ge, As, Sb, Te.
Left of the staircase: metals (blue). Right: nonmetals (green), plus hydrogen. Touching it: metalloids (gold).
Three kinds of element
KindWhere on the tableTypical behavior
MetalsLeft of the staircase line (most elements)Lose electrons and form positive ions.
NonmetalsRight of the line, plus hydrogen (H) at top leftGain or share electrons.
MetalloidsTouching the line: B, Si, Ge, As, Sb, TeProperties in between.
The staircase line runs down the right side of the table, starting just left of boron (B).
Chapter route map. Ionic path: metal plus nonmetal. Molecular path: two nonmetals. Both decision clues highlighted in section 2.1. Ammonium compounds are also ionic without a metal. Green: earlier material. Gold: current decision or upcoming route. Gray: later material. Follow ionic charges/formulas/names or molecular atom counts/prefixes, then acids, strong bases, and mixed practice.
Metal + nonmetal → usually ionic; nonmetals only → usually molecular. Ammonium compounds are ionic without a metal. Green = earlier, gold = now, gray = later; colors do not measure mastery.
Ionic or molecular?
CompareIonic compoundsMolecular compounds
What holds it togetherAttraction between positive and negative ionsAtoms sharing electrons in covalent bonds
First clueA metal with a nonmetal, or recognizable ions. Ammonium compounds are ionic without a metal.Nonmetals combined into molecules, such as CO2 or H2O
What the formula countsSimplest whole-number ratio of ions: 1 Mg2+ to 2 Cl− in MgCl2Actual atoms in one molecule: 1 C and 2 O in CO2
Formula routeUse ion charges: criss-cross, simplify, check neutralUse given atom counts or naming prefixes; no criss-cross
Cations and anions
Cation = positive ion
Metals usually lose electrons.
Na → Na+ (loses 1 electron).
Removing a negative charge leaves the ion more positive.
Anion = negative ion
Nonmetals usually gain electrons to form single-atom anions.
Cl → Cl− (gains 1 electron).
Adding a negative charge drives the total down.
Cartoon sodium cation Na⁺ and chloride anion Cl⁻ attract because their charges are opposite. An alternating array represents the ionic compound NaCl.
Opposite charges attract. In solid NaCl, each smaller Na+ (blue) is surrounded by larger Cl− ions (green), and each Cl− by Na+, in a repeating cube pattern.
Sodium chloride is neutral
Na+ + Cl−: (+1) + (−1) = 0 → NaCl
NaCl gives the simplest ratio. The solid is a repeating array of many ions, not one isolated pair.
A carbon dioxide molecule has one carbon atom between two oxygen atoms, O=C=O. Covalent bonds represent shared electron pairs, not attraction between simple ions.
CO2 is molecular: one C and two O atoms share electrons through covalent bonds.
Molecular compounds
  • Many nonmetal compounds are made of molecules: separate groups of atoms held by covalent bonds.
  • Covalent means the atoms share electrons.
  • CO2 is not a carbon cation balanced by oxide anions. On the periodic table: C and O are both right of the staircase, so both are nonmetals.
1 · Recognize the clue
Look for this:a formula and the words "ionic or molecular": check whether a metal or ammonium (NH4+) is present, or only nonmetals.
2 · Follow the route
  1. Find each element on the periodic table: metal, nonmetal or metalloid.
  2. Metal + nonmetal, or a recognizable ion such as NH4+: treat it as ionic.
  3. Nonmetals only: treat it as molecular.
  4. Ionic: balance ion charges to zero. Molecular: count the actual atoms.

On the periodic table: Sodium (Na) is in Group 1; Group 1 atoms lose 1 electron, so sodium forms Na+.

On the periodic table: Chlorine (Cl) is in Group 17; Group 17 atoms gain 1 electron, so chlorine forms Cl−.

On the periodic table: Magnesium (Mg) is in Group 2, so it forms Mg2+.

On the periodic table: Carbon (C) and oxygen (O) are right of the staircase line, so both are nonmetals.

Worked example

How many Cl− ions balance one Mg2+? Write the formula.

One Mg²⁺ ion and two Cl⁻ ions: +2 minus 1 minus 1 equals zero, giving the ion ratio in MgCl₂.
One magnesium ion needs two chloride ions to balance its +2 charge.
  1. 1
    Read the charges

    On the periodic table: Mg is in Group 2, so it forms Mg2+. Cl is in Group 17, so it forms Cl−.

    IonCharge
    Mg2++2
    Cl−−1
  2. 2
    Try one chloride

    The total is still positive, so one chloride is not enough.

    (+2) + (−1) = +1
  3. 3
    Add a second chloride

    Now the charges balance.

    (+2) + (−1) + (−1) = 0
  4. 4
    Write the formula

    The unwritten 1 means one Mg2+; the small 2 means two Cl−.

    IonCount× Charge= Total
    Mg2+1+2+2
    Cl−2−1−2
    Sum0
  5. ✓
    Answer

    MgCl2

2.2 Ions and their charges: your reference

Every ionic formula starts with the ions’ charges. Many charges can be read straight from the periodic table; polyatomic ions come from a short reference list; a few metals need their charge read each time. This lesson is your charge reference.

The idea
Chapter route map. Ionic path: metal plus nonmetal. Molecular path: two nonmetals. Both decision clues highlighted in section 2.1. Ammonium compounds are also ionic without a metal. Green: earlier material. Gold: current decision or upcoming route. Gray: later material. Follow ionic charges/formulas/names or molecular atom counts/prefixes, then acids, strong bases, and mixed practice.
Metal + nonmetal → ionic; two nonmetals → molecular. Ammonium compounds are ionic without a metal. Green = earlier, gold = now, gray = later; colors do not measure mastery.
Polyatomic ions: your reference
Ion formulaNameAtomsCharge
NH4+Ammonium (the cation to know)1 N, 4 H+1
OH−Hydroxide1 O, 1 H−1
NO3−Nitrate1 N, 3 O−1
NO2−Nitrite1 N, 2 O−1
SO42−Sulfate1 S, 4 O−2
SO32−Sulfite1 S, 3 O−2
CO32−Carbonate1 C, 3 O−2
HCO3−Hydrogen carbonate (bicarbonate)1 H, 1 C, 3 O−1
PO43−Phosphate1 P, 4 O−3
C2H3O2−Acetate (also CH3COO−)2 C, 3 H, 2 O−1
CN−Cyanide1 C, 1 N−1
MnO4−Permanganate1 Mn, 4 O−1
CrO42−Chromate1 Cr, 4 O−2
O22−Peroxide2 O−2
ClO−Hypochlorite1 Cl, 1 O−1
ClO2−Chlorite1 Cl, 2 O−1
ClO3−Chlorate1 Cl, 3 O−1
ClO4−Perchlorate1 Cl, 4 O−1
Polyatomic ion: a bonded group of atoms with one overall charge. Check this list against your instructor’s requirements; not every course needs every entry.
Sulfate: SO₄²⁻. Atom counts: 1 S, 4 O. Overall charge −2. Connectivity only; bond orders and geometry are not shown.
Sulfate as an atom-counting sketch: each sphere is one atom; the brackets and charge belong to the whole ion. Not a complete Lewis structure.

Practice the list as flashcards: formula → name and charge; name → formula and charge; ion pair → neutral formula.

Use the periodic table to choose common ion charges. Blue: common fixed-charge metals. Green: selected nonmetals forming simple anions. Gold: variable-charge examples Fe, Cu, Sn, Pb. Their group alone does not give one fixed charge. Badges: Group 1 METALS +1; Group 2 +2; simple anions N/P −3, O/S −2, F/Cl/Br/I −1.
Blue: common fixed-charge metals. Green: nonmetals that form simple anions. Gold: variable-charge metals (Fe, Cu, Sn, Pb).
Predictable charges: read them from the table
Where to lookUsual ion chargeExamples
Group 1 metals+1Li+, Na+, K+
Group 2 metals+2Mg2+, Ca2+, Ba2+
Other fixed-charge metalsAl +3; Zn +2; Ag +1Al3+, Zn2+, Ag+
Group 17 single-atom anions−1F−, Cl−, Br−, I−
Group 16 single-atom anions−2O2−, S2−
Group 15 single-atom anions−3N3− (nitride), P3− (phosphide)
Choose the charge from the column before balancing. One, two or three gained electrons give −1, −2 or −3.
Limits of the column rule
  • The rule gives the charge of the ion. A neutral Mg atom has zero charge; its usual ion is Mg2+.
  • Hydrogen sits above Group 1 but is a nonmetal, so the +1 metal rule is not a general rule for H.
  • These are single-atom ion charges, not rules for every compound containing the element. Polyatomic charges come from the reference list.
Why these charges? The nearest noble gas
Atom (electrons)ChangeIonElectrons nowSame as
Na (11)loses 1Na+10neon (Ne)
Mg (12)loses 2Mg2+10neon (Ne)
O (8)gains 2O2−10neon (Ne)
Cl (17)gains 1Cl−18argon (Ar)
Noble gases (Group 18) have a full outer shell, a very stable arrangement, so they almost never react or form ions. Other main-group atoms take the shortest trip to a noble gas’s electron count.
Row 3 of the periodic table. Na, Mg and Al lose 1, 2 and 3 electrons to have 10 electrons like neon, forming Na⁺, Mg²⁺ and Al³⁺. P, S and Cl gain 3, 2 and 1 electrons to have 18 electrons like argon, forming P³⁻, S²⁻ and Cl⁻. Silicon rarely forms a simple ion. Argon is a noble gas with a full outer shell.
Metals on the left are a few electrons past a noble gas, so they lose them. Nonmetals on the right are a few short, so they gain them. Transition metals such as Fe and Cu do not follow this shortcut. Chapter 3 shows the shells behind it.
Variable-charge metals: read the charge each time
MetalCommon ionsFormula check
IronFe2+ iron(II); Fe3+ iron(III)FeCl2: 2 Cl− require Fe2+. FeCl3: 3 Cl− require Fe3+.
CopperCu+ copper(I); Cu2+ copper(II)CuCl: 1 Cl− requires Cu+. CuCl2: 2 Cl− require Cu2+.
TinSn2+ tin(II); Sn4+ tin(IV)SnCl2 uses Sn2+; SnCl4 uses Sn4+.
LeadPb2+ lead(II); Pb4+ lead(IV)PbO uses Pb2+; PbO2 uses Pb4+, since each oxide is O2−.
A Roman numeral gives the positive charge, not a count of metal atoms. Given only a formula, use the known anion charges to work out the metal’s charge.
Same balance, different reason
MgCl2
Mg is in Group 2, so you already know Mg2+.
(+2) + 2(−1) = 0
FeCl2
Iron can vary. Two Cl− tell you this formula holds Fe2+.
(+2) + 2(−1) = 0
1 · Recognize the clue
Look for this:"what charge" or "usual ion" with an element name, or a polyatomic ion formula that needs its name and charge.
2 · Follow the route
  1. Polyatomic group? Look up its charge in the reference list.
  2. Main-group element? Read the charge from its group.
  3. Variable-charge metal? Use the Roman numeral, or balance it against the known anions.
  4. Check: a main-group ion should have the same electron count as the nearest noble gas.

On the periodic table: Group 1 metals form +1 ions and Group 2 metals form +2 ions.

On the periodic table: Group 17 forms −1, Group 16 forms −2 and Group 15 forms −3 single-atom anions.

On the periodic table: Group 18 noble gases have full outer shells and almost never form ions.

On the periodic table: The table does not give polyatomic-ion charges or a single charge for Fe, Cu, Sn or Pb.

Try the route yourself

Why does sulfur form S2− rather than S6+?

  1. 1
    Compare the two trips

    On the periodic table: sulfur (S) has 16 electrons. Argon is just after it; neon is well before it.

    OptionElectron changeElectrons nowSame as
    S2−gain 218argon (Ar)
    S6+lose 610neon (Ne)
  2. ✓
    Answer

    Gaining 2 electrons to match argon is a much shorter trip than losing 6 to match neon, so sulfur forms S2−.

2.3 Build and read ionic formulas

An ionic formula lists ions in the ratio that makes the total charge zero. This lesson builds formulas from charges, reads the ions back out of a formula, and counts atoms through subscripts and parentheses.

The idea
What each number in a formula means
MarkExampleWhat it tells you
SubscriptH2OAtoms of the element before it: 2 H, 1 O. No subscript means 1.
Subscript inside an ionNO3−3 O atoms in one nitrate. The charge belongs to the whole ion.
Outside parenthesesCa(NO3)2Two whole nitrate groups: 2 N and 6 O. Ca is outside, so it stays 1.
Coefficient in front2H2OMultiplies the entire formula.
SuperscriptMg2+Electrical charge, not an atom count.
Polyatomic ion: a bonded group of atoms with one charge, such as hydroxide OH− or nitrate NO3−.
Chapter route map. Ionic path: metal plus nonmetal. Molecular path: two nonmetals. Ammonium compounds are also ionic without a metal. Green: earlier material. Gold: current decision or upcoming route. Gray: later material.
Metal + nonmetal → ionic; two nonmetals → molecular. Ammonium compounds are ionic without a metal. Green = earlier, gold = now, gray = later; colors do not measure mastery.
Neutral means zero total charge
(count × cation charge) + (count × anion charge) = 0
Use the smallest whole-number counts. Write the positive ion first.
Criss-cross: each charge size becomes the other ion’s count
IonsFormulaAl3+O2−Al2O3drop thesignsCheck: 2(+3) + 3(−2) = 0
Cross the sizes, drop the signs, then reduce to the smallest whole-number ratio.
Polyatomic groups in the formula
  • Learning layout: put every polyatomic group in parentheses, even with an outside 1, e.g. Na1(NO3)1.
  • Finished formula: drop a single group’s parentheses and outside 1, giving NaNO3.
  • Keep parentheses only when a group repeats: Ca(NO3)2.
  • Reduce counts of whole groups, never the subscripts inside the group.
  • Use criss-cross for ionic compounds only, not molecular ones such as CO2.
Can criss-cross run backward?
Ions → formula
Yes. Cross the charge sizes, then reduce.
Mg2+ + O2− → Mg2O2 → MgO
Formula → ions
Not directly. MgO is 1:1, yet its ions are Mg2+ and O2−.
Recognize a known ion, total its charge, then find the charge that balances it.

The examples identify the ions in each compound. They do not claim every compound dissolves in water.

1 · Recognize the clue
Look for this:two ions given with charges ("write the formula from") or a formula where you must "identify ions and counts".
2 · Follow the route
  1. Read each ion’s charge.
  2. Cross the charge sizes, or find the smallest matching positive and negative totals.
  3. Reduce to the smallest whole-number ratio; keep polyatomic groups intact.
  4. Write the positive ion first; leave out 1s and use parentheses only for a repeated group.
  5. Check that the total charge is zero.

On the periodic table: Main-group charges come from the group: Group 1 forms 1+, Group 2 forms 2+, aluminum (Group 13) forms 3+.

On the periodic table: Group 16 forms 2− (O2−) and Group 17 forms 1− (Cl−).

On the periodic table: The table does not give polyatomic-ion charges or which iron ion is present; use the supplied charge.

On the periodic table: The table identifies elements; subscripts in the formula, not atomic numbers, give atom counts.

Worked example 1

Write the neutral formula for Mg2+ and Cl−.

  1. 1
    Cross the charge sizes

    Sizes 2 and 1: 1 goes to Mg, 2 goes to Cl. On the periodic table: Mg is Group 2 (Mg2+); Cl is Group 17 (Cl−).

    Mg2+   Cl− → Mg1Cl2
  2. 2
    Simplify

    Omit the 1.

    Mg1Cl2 → MgCl2
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    Mg2+1+2+2
    Cl−2−1−2
    Sum0
  4. ✓
    Answer

    MgCl2

Worked example 2

Write the neutral formula for Al3+ and O2−.

  1. 1
    Cross the charge sizes

    2 goes to Al and 3 goes to O. On the periodic table: Al is Group 13 (Al3+); O is Group 16 (O2−).

    Al3+   O2− → Al2O3
  2. 2
    Simplify

    The 2:3 ratio is already simplest.

    Al2O3 → Al2O3
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    Al3+2+3+6
    O2−3−2−6
    Sum0
  4. ✓
    Answer

    Al2O3

Worked example 3

Write the neutral formula for Mg2+ and O2−.

  1. 1
    Cross the charge sizes

    Both sizes are 2. On the periodic table: Mg is Group 2 (Mg2+); O is Group 16 (O2−).

    Mg2+   O2− → Mg2O2
  2. 2
    Simplify

    Both counts divide by 2: 2:2 reduces to 1:1.

    Mg2O2 → MgO
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    Mg2+1+2+2
    O2−1−2−2
    Sum0
  4. ✓
    Answer

    MgO

Worked example 4

Write the neutral formula for Na+ and NO3−.

  1. 1
    Cross the charge sizes

    The outside 1 counts one whole nitrate group. On the periodic table: Na is Group 1 (Na+).

    Na+   NO3− → Na1(NO3)1
  2. 2
    Simplify

    Drop sodium’s 1 and the single group’s parentheses and outside 1. Keep nitrate’s inner 3.

    Na1(NO3)1 → NaNO3
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    Na+1+1+1
    NO3−1−1−1
    Sum0
  4. ✓
    Answer

    NaNO3

Worked example 5

Write the neutral formula for Ca2+ and NO3−.

  1. 1
    Cross the charge sizes

    1 goes to Ca; 2 goes to the whole nitrate group. On the periodic table: Ca is Group 2 (Ca2+).

    Ca2+   NO3− → Ca1(NO3)2
  2. 2
    Simplify

    Two nitrate groups keep their parentheses. The inner 3 stays; the outside 2 counts groups.

    Ca1(NO3)2 → Ca(NO3)2
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    Ca2+1+2+2
    NO3−2−1−2
    Sum0
  4. ✓
    Answer

    Ca(NO3)2

Worked example 6

Write the neutral formula for Al3+ and SO42−.

  1. 1
    Cross the charge sizes

    2 goes to Al; 3 goes to sulfate. On the periodic table: Al is Group 13 (Al3+).

    Al3+   SO42− → Al2(SO4)3
  2. 2
    Simplify

    Already simplest. Keep SO4 intact; three groups need parentheses.

    Al2(SO4)3 → Al2(SO4)3
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    Al3+2+3+6
    SO42−3−2−6
    Sum0
  4. ✓
    Answer

    Al2(SO4)3

Worked example 7

Write the neutral formula for NH4+ and SO42−.

  1. 1
    Cross the charge sizes

    2 goes to ammonium; 1 goes to sulfate. Both groups are shown in parentheses first.

    NH4+   SO42− → (NH4)2(SO4)1
  2. 2
    Simplify

    One sulfate group needs no parentheses or outside 1. Keep the 4 inside sulfate.

    (NH4)2(SO4)1 → (NH4)2SO4
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    NH4+2+1+2
    SO42−1−2−2
    Sum0
  4. ✓
    Answer

    (NH4)2SO4

Worked example 8

Write the neutral formula for Fe3+ and Cl−.

  1. 1
    Cross the charge sizes

    1 goes to Fe; 3 goes to Cl. On the periodic table: Cl is Group 17 (Cl−). The supplied Fe3+ tells you which iron ion.

    Fe3+   Cl− → Fe1Cl3
  2. 2
    Simplify

    Omit the 1.

    Fe1Cl3 → FeCl3
  3. 3
    Check the charge

    Positives and negatives must cancel.

    IonCount× Charge= Total
    Fe3+1+3+3
    Cl−3−1−3
    Sum0
  4. ✓
    Answer

    FeCl3

Worked example 9

Which ions make up MgCl2? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    Chloride is Cl−. On the periodic table: Cl is Group 17.

    2 Cl−: 2(−1) = −2
  2. 2
    Balance the other ion

    One Mg must supply +2. On the periodic table: Mg is Group 2, so Mg2+ matches.

    1 Mg = +2 → Mg2+
  3. 3
    List the ions

    Counts come from the subscripts.

    IonCount× Charge= Total
    Mg2+1+2+2
    Cl−2−1−2
    Sum0
  4. ✓
    Answer

    1 Mg2+ and 2 Cl−

Worked example 10

Which ions make up Al2O3? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    Oxide is O2−. On the periodic table: O is Group 16.

    3 O2−: 3(−2) = −6
  2. 2
    Balance the other ion

    Two Al ions share +6. On the periodic table: Al (Group 13) usually forms Al3+, which matches.

    2 Al = +6
    each Al = +62 = +3 → Al3+
  3. 3
    List the ions

    Two of one, three of the other.

    IonCount× Charge= Total
    Al3+2+3+6
    O2−3−2−6
    Sum0
  4. ✓
    Answer

    2 Al3+ and 3 O2−

Worked example 11

Which ions make up MgO? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    No subscripts means one ion of each. On the periodic table: Mg is Group 2 (Mg2+); O is Group 16 (O2−).

    1 O2−: 1(−2) = −2
  2. 2
    Balance the other ion

    Read Mg’s +2 from its group. The unwritten 1 counts ions; it does not set their charge.

    1 Mg = +2 → Mg2+
  3. 3
    List the ions

    The 1:1 ratio hides the charge size of 2.

    IonCount× Charge= Total
    Mg2+1+2+2
    O2−1−2−2
    Sum0
  4. ✓
    Answer

    1 Mg2+ and 1 O2−

Worked example 12

Which ions make up NaNO3? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    Recognize nitrate. With no parentheses or outside number, one group is understood.

    1 NO3−: 1(−1) = −1
  2. 2
    Balance the other ion

    One sodium supplies +1. On the periodic table: Na is Group 1, so Na+ matches.

    1 Na = +1 → Na+
  3. 3
    List the ions

    The 3 counts O atoms inside nitrate, not nitrate groups.

    IonCount× Charge= Total
    Na+1+1+1
    NO3−1−1−1
    Sum0
  4. ✓
    Answer

    1 Na+ and 1 NO3−

Worked example 13

Which ions make up Ca(NO3)2? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    Recognize nitrate. The outside 2 means two nitrate ions.

    2 NO3−: 2(−1) = −2
  2. 2
    Balance the other ion

    One calcium supplies +2. On the periodic table: Ca is Group 2, so Ca2+ matches.

    1 Ca = +2 → Ca2+
  3. 3
    List the ions

    Do not split nitrate into separate N and O ions.

    IonCount× Charge= Total
    Ca2+1+2+2
    NO3−2−1−2
    Sum0
  4. ✓
    Answer

    1 Ca2+ and 2 NO3−

Worked example 14

Which ions make up Al2(SO4)3? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    Sulfate is SO42−; the outside 3 means three of them.

    3 SO42−: 3(−2) = −6
  2. 2
    Balance the other ion

    Two Al ions share +6. On the periodic table: Al (Group 13) usually forms Al3+, which matches.

    2 Al = +6
    each Al = +62 = +3 → Al3+
  3. 3
    List the ions

    Sulfate stays one intact ion.

    IonCount× Charge= Total
    Al3+2+3+6
    SO42−3−2−6
    Sum0
  4. ✓
    Answer

    2 Al3+ and 3 SO42−

Worked example 15

Which ions make up (NH4)2SO4? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    Recognize both groups: ammonium NH4+ and sulfate SO42−. Sulfate appears once.

    1 SO42−: 1(−2) = −2
  2. 2
    Balance the other ion

    The outside 2 means two ammonium ions.

    2 NH4+: 2(+1) = +2
  3. 3
    List the ions

    +2 − 2 = 0.

    IonCount× Charge= Total
    NH4+2+1+2
    SO42−1−2−2
    Sum0
  4. ✓
    Answer

    2 NH4+ and 1 SO42−

Worked example 16

Which ions make up FeCl3? Give each ion’s formula, charge and count.

  1. 1
    Spot the known ion

    Each chloride is Cl−. On the periodic table: Cl is Group 17.

    3 Cl−: 3(−1) = −3
  2. 2
    Balance the other ion

    The one iron ion must supply +3. Iron’s charge comes from the balance, not its group.

    1 Fe = +3 → Fe3+
  3. 3
    List the ions

    This is iron(III) chloride, not iron(II) chloride.

    IonCount× Charge= Total
    Fe3+1+3+3
    Cl−3−1−3
    Sum0
  4. ✓
    Answer

    1 Fe3+ and 3 Cl−

Try the route yourself

Write neutral formulas for K+ with S2−; Ca2+ with OH−; Mg2+ with PO43−; NH4+ with Cl−.

Work all four on paper first. Check each charge total, then reveal one answer at a time.

  1. 1
    K+ and S2−

    Two K+ balance one S2−.

    K2S: 2(+1) + (−2) = 0
  2. 2
    Ca2+ and OH−

    Two hydroxide groups, so hydroxide needs parentheses.

    Ca(OH)2: (+2) + 2(−1) = 0
  3. 3
    Mg2+ and PO43−

    Charge sizes 2 and 3 cross to 3 Mg and 2 phosphate groups.

    Mg3(PO4)2: 3(+2) + 2(−3) = 0
  4. 4
    NH4+ and Cl−

    Learning layout (NH4)1Cl1; one ammonium group drops its parentheses and outside 1.

    NH4Cl: (+1) + (−1) = 0
  5. ✓
    Answer

    K2S, Ca(OH)2, Mg3(PO4)2 and NH4Cl

Worked example 17

Count every kind of atom in Ca(NO3)2.

Route for counting atoms
  • Multiply each atom inside parentheses by the outside subscript.
  • Leave atoms outside the parentheses unchanged.
  • Apply any leading coefficient last.
  1. 1
    Read calcium

    Calcium is outside the parentheses with no subscript.

    Ca → 1 Ca atom
  2. 2
    Read one nitrate group

    The small 3 belongs only to oxygen, inside one group.

    NO3 → 1 N and 3 O
  3. 3
    Repeat the whole group

    The outside 2 multiplies both N and O.

    AtomIn one group× outside 2= Total
    N1× 22
    O3× 26
  4. 4
    Check the whole formula

    The group multiplier does not change calcium. On the periodic table: the table identifies Ca, N and O; the subscripts give the counts.

    CaNO
    126
  5. ✓
    Answer

    1 Ca, 2 N and 6 O

2.4 Ionic formulas and names

An ionic name is just its two ions, positive first. This lesson runs the route both ways: from a formula to its name, and from a name back to a balanced formula.

The idea
Chapter route map. Ionic path: metal plus nonmetal. Molecular path: two nonmetals. Both decision clues highlighted in section 2.1. Ammonium compounds are also ionic without a metal. Green: earlier material. Gold: current decision or upcoming route. Gray: later material. Follow ionic charges/formulas/names or molecular atom counts/prefixes, then acids, strong bases, and mixed practice.
Metal + nonmetal → ionic; two nonmetals → molecular. Ammonium compounds are ionic without a metal. Green = earlier, gold = now, gray = later; colors do not measure mastery.
How each ion is named
IonRuleExamples
Fixed-charge cationElement nameNa+ sodium; Ca2+ calcium
Variable-charge cationElement name + charge as a Roman numeralFe2+ iron(II); Fe3+ iron(III)
Polyatomic ionKeep its own nameNH4+ ammonium; NO3− nitrate; SO42− sulfate
Single-atom anionElement root + -ideCl− chloride; O2− oxide
Positive ion first, negative ion second.
Two directions
Formula → name
First choose the rule: ionic, molecular or acid.
Identify the ions, then name them in order.
Variable metal: work out its charge first.
Name → formula
Ionic names: use ion charges. Molecular names: use prefixes.
Turn each name into an ion, then balance the charges to zero.
1 · Recognize the clue
Look for this:"Name" plus a metal–nonmetal formula, or a name like "copper(II) oxide" that asks for the formula.
2 · Follow the route
  1. Identify the positive ion and the negative ion.
  2. Formula → name: name the cation, add a Roman numeral if its charge varies, then name the anion (-ide or polyatomic name).
  3. Name → formula: write each ion with its charge.
  4. Balance the charges to zero; keep polyatomic groups whole and use parentheses only for a repeated group.

On the periodic table: Sodium (Na) is in Group 1, so it forms Na+; calcium (Ca) is in Group 2, so it forms Ca2+.

On the periodic table: Chlorine (Cl) is in Group 17, so it forms Cl−; oxygen (O) is in Group 16, so oxide is O2−.

On the periodic table: The table gives iron’s symbol, Fe, but not its charge; read that from the formula or the Roman numeral.

Worked example 1

Formula → name: NaCl

  1. 1
    Identify the ions

    On the periodic table: Na (sodium) is in Group 1, so Na+. Cl (chlorine) is in Group 17, so Cl−.

    IonChargeName
    Na++1sodium
    Cl−−1chloride
  2. 2
    Name in order

    Positive ion first. A one-element chlorine anion is chloride. No prefixes or Roman numeral needed.

    sodium + chloride → sodium chloride
  3. ✓
    Answer

    sodium chloride

Worked example 2

Formula → name: CaCl2

  1. 1
    Identify the ions

    On the periodic table: Ca (calcium) is in Group 2, so Ca2+. Cl (chlorine) is in Group 17, so Cl−.

    IonChargeName
    Ca2++2calcium
    Cl−−1chloride
  2. 2
    Name in order

    The subscript 2 does not make this “dichloride”.

    calcium + chloride → calcium chloride
  3. ✓
    Answer

    calcium chloride

Worked example 3

Formula → name: Ca(NO3)2

  1. 1
    Identify the ions

    On the periodic table: the symbol Ca belongs to calcium. Recognize nitrate inside the parentheses.

    IonChargeName
    Ca2++2calcium
    NO3−−1nitrate
  2. 2
    Name in order

    Keep nitrate’s name. The parentheses do not change the naming rule.

    calcium + nitrate → calcium nitrate
  3. ✓
    Answer

    calcium nitrate

Worked example 4

Formula → name: (NH4)2SO4

  1. 1
    Identify the ions

    Both ions are polyatomic.

    IonChargeName
    NH4++1ammonium
    SO42−−2sulfate
  2. 2
    Name in order

    Positive group first, then sulfate.

    ammonium + sulfate → ammonium sulfate
  3. ✓
    Answer

    ammonium sulfate

Worked example 5

Formula → name: FeO

  1. 1
    Identify the anion

    On the periodic table: Fe is iron. Oxygen (O) is in Group 16, so oxide is O2−.

    1 O2−: 1(−2) = −2
  2. 2
    Find iron’s charge

    One O2− requires one Fe2+.

    1 Fe = +2 → Fe2+
  3. 3
    Name with a Roman numeral

    Roman numeral II shows iron’s +2 charge.

    iron(II) + oxide → iron(II) oxide
  4. ✓
    Answer

    iron(II) oxide

Worked example 6

Formula → name: Fe(NO3)3

  1. 1
    Total the anion charge

    On the periodic table: Fe is iron. There are three nitrate ions, each −1.

    3 NO3−: 3(−1) = −3
  2. 2
    Find iron’s charge

    The total −3 requires one Fe3+.

    1 Fe = +3 → Fe3+
  3. 3
    Name with a Roman numeral

    Roman numeral III shows iron’s +3 charge.

    iron(III) + nitrate → iron(III) nitrate
  4. ✓
    Answer

    iron(III) nitrate

Worked example 7

Name → formula: sodium chloride

  1. 1
    Write the ions

    On the periodic table: sodium (Na) is in Group 1, so Na+. Chlorine (Cl) is in Group 17, so Cl−.

    sodium = Na+   chloride = Cl−
  2. 2
    Balance the charges

    One of each ion balances.

    IonCount× Charge= Total
    Na+1+1+1
    Cl−1−1−1
    Sum0
  3. ✓
    Answer

    NaCl

Worked example 8

Name → formula: calcium chloride

  1. 1
    Write the ions

    On the periodic table: calcium (Ca) is in Group 2, so Ca2+. Chlorine (Cl) is in Group 17, so Cl−.

    calcium = Ca2+   chloride = Cl−
  2. 2
    Balance the charges

    One calcium needs two chlorides.

    (+2) + 2(−1) = 0 → CaCl2
  3. ✓
    Answer

    CaCl2

Worked example 9

Name → formula: calcium nitrate

  1. 1
    Write the ions

    On the periodic table: calcium (Ca) is in Group 2, so Ca2+. Nitrate comes from the reference list.

    calcium = Ca2+   nitrate = NO3−
  2. 2
    Balance the charges

    Use two complete nitrate groups; two groups need parentheses.

    IonCount× Charge= Total
    Ca2+1+2+2
    NO3−2−1−2
    Sum0
  3. ✓
    Answer

    Ca(NO3)2

Worked example 10

Name → formula: ammonium sulfate

  1. 1
    Write the ions

    Both come from the reference list.

    ammonium = NH4+   sulfate = SO42−
  2. 2
    Balance the groups

    Two ammoniums balance one sulfate.

    2(+1) + 1(−2) = 0 → (NH4)2(SO4)1
  3. 3
    Tidy the formula

    Drop the single sulfate group’s parentheses and outside 1; keep its inner 4.

    (NH4)2(SO4)1 → (NH4)2SO4
  4. ✓
    Answer

    (NH4)2SO4

Worked example 11

Name → formula: iron(II) oxide

  1. 1
    Write the ions

    Iron(II) means Fe2+. On the periodic table: iron’s symbol is Fe; oxygen (O) is in Group 16, so oxide is O2−.

    iron(II) = Fe2+   oxide = O2−
  2. 2
    Balance and reduce

    A crossed 2:2 ratio reduces to 1:1: one of each.

    Fe2O2 → FeO: (+2) + (−2) = 0
  3. ✓
    Answer

    FeO

Worked example 12

Name → formula: iron(III) nitrate

  1. 1
    Write the ions

    Iron(III) means Fe3+. On the periodic table: iron’s symbol is Fe.

    iron(III) = Fe3+   nitrate = NO3−
  2. 2
    Balance the charges

    Three nitrate groups balance one iron ion.

    IonCount× Charge= Total
    Fe3+1+3+3
    NO3−3−1−3
    Sum0
  3. ✓
    Answer

    Fe(NO3)3

2.5 Molecular formulas and names

Molecular compounds are nonmetals sharing electrons (from 2.1). Their formulas count the actual atoms in one molecule, and their names use number prefixes instead of charges.

The idea
Chapter route map. Ionic path: metal plus nonmetal. Molecular path: two nonmetals. Both decision clues highlighted in section 2.1. Ammonium compounds are also ionic without a metal. Green: earlier material. Gold: current decision or upcoming route. Gray: later material. Follow ionic charges/formulas/names or molecular atom counts/prefixes, then acids, strong bases, and mixed practice.
Metal + nonmetal → ionic; two nonmetals → molecular. Ammonium compounds are ionic without a metal. Green = earlier, gold = now, gray = later; colors do not measure mastery.
Two different formula routes
Ionic
Identify the ions and balance charges.
The formula is the simplest ion ratio, so reduce it.
Molecular
Read actual atom counts from supplied information or prefixes.
The formula is one whole molecule, so never reduce it.
Prefixes count atoms
monoditritetrapentahexaheptaoctanonadeca
12345678910
Used for binary (two-element) molecular compounds.
Naming rules
  • Name the first element; usually omit mono on it.
  • Name the second element with a prefix and an -ide ending.
  • Familiar spellings drop a vowel before oxide: monoxide, tetroxide.
  • Keep the actual counts. Do not criss-cross ion charges.

You cannot work out every molecular formula from the elements alone; you need given counts or a name. Chapter 8 explains covalent bonding in more detail.

1 · Recognize the clue
Look for this:a formula or name made of only nonmetals, often with prefixes like di-, tri-, or tetra-.
2 · Follow the route
  1. Confirm it is molecular: nonmetals only.
  2. Formula → counts: each subscript counts atoms; no subscript means 1.
  3. Formula → name: prefix + first element (omit mono), prefix + second element root + -ide.
  4. Name → formula: turn each prefix into a subscript and do not reduce.

On the periodic table: C is carbon, O is oxygen, N is nitrogen, H is hydrogen and S is sulfur.

On the periodic table: Carbon, nitrogen and oxygen are right of the staircase line, so all are nonmetals.

Worked example 1

Formula → atom counts: CO2

  1. 1
    Read each symbol

    No subscript on C means one carbon. The small 2 means two oxygens. On the periodic table: C is carbon and O is oxygen.

    AtomCOTotal
    Count123
  2. 2
    Keep it molecular

    Do not criss-cross or turn CO2 into C4+ and O2−. These atoms share electrons.

    CO2 = 1 C + 2 O (shared electrons, no ions)
  3. ✓
    Answer

    1 carbon atom and 2 oxygen atoms per molecule

Worked example 2

Atom counts → molecular formula: two N and four O

  1. 1
    Write the actual counts

    Each count becomes a subscript.

    2 N + 4 O → N2O4
  2. 2
    Do not reduce

    Ionic formulas use the simplest ratio; molecular formulas keep the atoms in one molecule.

    Correct
    N2O4
    A different molecule
    NO2
  3. ✓
    Answer

    N2O4

Worked example 3

Formula → atom counts: how many of each kind of atom are in one H2SO4 molecule?

  1. 1
    Read each subscript

    H2 means 2 H. S has an understood 1. O4 means 4 O. On the periodic table: H is hydrogen, S is sulfur and O is oxygen.

    AtomHSO
    Count214
  2. 2
    Add the total

    Sum the three counts.

    2 + 1 + 4 = 7 atoms
  3. ✓
    Answer

    2 H, 1 S and 4 O: 7 atoms total

Worked example 4

Formula → name: CO2

  1. 1
    Choose the rule

    Two nonmetals: binary molecular. On the periodic table: carbon (C) and oxygen (O) are both right of the staircase line.

    C + O: nonmetal + nonmetal → prefixes
  2. 2
    Turn counts into prefixes

    One carbon: omit mono on the first element. Two oxygens: di + oxide.

    ElementCountName part
    C1carbon
    O2dioxide
  3. ✓
    Answer

    carbon dioxide

Worked example 5

Formula → name: N2O4

  1. 1
    Choose the rule

    Binary molecular. On the periodic table: N is nitrogen and O is oxygen, both nonmetals on the right side.

    N + O: nonmetal + nonmetal → prefixes
  2. 2
    Turn counts into prefixes

    Two nitrogens give dinitrogen; four oxygens give tetroxide.

    ElementCountName part
    N2dinitrogen
    O4tetroxide
  3. ✓
    Answer

    dinitrogen tetroxide

Worked example 6

Name → formula: carbon dioxide

  1. 1
    Read the prefixes

    No prefix on the first element means one C. Dioxide means two O. On the periodic table: carbon’s symbol is C and oxygen’s is O.

    Name partCountSymbol
    carbon1C
    dioxide2O
  2. 2
    Write the formula

    Do not balance ion charges here.

    C1O2 → CO2
  3. ✓
    Answer

    CO2

Worked example 7

Name → formula: dinitrogen tetroxide

  1. 1
    Read the prefixes

    Di means 2 N; tetra in tetroxide means 4 O. On the periodic table: nitrogen’s symbol is N and oxygen’s is O.

    Name partCountSymbol
    dinitrogen2N
    tetroxide4O
  2. 2
    Write the formula

    Prefixes give the real molecule’s atom counts. Do not reduce 2:4.

    N2O4, not NO2
  3. ✓
    Answer

    N2O4

2.6 Acids, strong bases, and mixed route practice

Acids and strong bases have their own recognition clues, and acids have their own naming route, separate from ionic and molecular names. This lesson shows how to spot them and move between acid formulas and names.

What you’ll use from earlier
  • Positive and negative ions, ionic formulas, and polyatomic ions (sections 2.2–2.4).

Need a refresher? Open the table of contents and revisit that topic before starting.

The idea
Chapter route map. Ionic path: metal plus nonmetal. Molecular path: two nonmetals. Both decision clues highlighted in section 2.1. Ammonium compounds are also ionic without a metal. Green: earlier material. Gold: current decision or upcoming route. Gray: later material. Follow ionic charges/formulas/names or molecular atom counts/prefixes, then acids, strong bases, and mixed practice.
Metal + nonmetal → ionic; two nonmetals → molecular. Ammonium compounds are ionic without a metal. Green = earlier, gold = now, gray = later; colors do not measure mastery.
Acid or hydroxide base?
Acid (in water)
(aq) means aqueous: dissolved in water.
An acid donates H+ to water, forming H3O+ (hydronium).
Hydroxide base
NaOH is ionic: Na+ and the polyatomic ion OH−.
Dissolving separates the ions; OH− stays together.

See the particles in action: Chapter 6.4: acid in water → base in water → make water.

Strong-base hydroxides: find the lowercase b. Green highlights resemble a lowercase b: Li, Na, K, Rb, Cs; then Ca, Sr, Ba. A +1 metal needs one OH⁻: NaOH. A +2 metal needs two OH⁻: Ca(OH)₂. Do not include H, Be, or Mg in this introductory memory pattern; radioactive Fr/Ra are omitted. Strong means the dissolved base dissociates completely. It does not mean concentrated or very soluble.
Strong bases: look for the green lowercase b. Li, Na, K, Rb, Cs; then Ca, Sr, Ba.
Strong-base hydroxides
MetalsIon chargeHydroxidesIn water
Group 1: Li, Na, K, Rb, Cs+1: one OH−LiOH, NaOH, KOH, RbOH, CsOHNaOH → Na+ + OH−
Group 2: Ca, Sr, Ba+2: two whole OH− groupsCa(OH)2, Sr(OH)2, Ba(OH)2Ca(OH)2 → Ca2+ + 2OH−
Strong means the dissolved base separates completely into ions.
Limits of the memory aid
  • It covers common course examples, not every hydroxide.
  • H is not a metal. Be and Mg are not on this introductory strong-base list. Radioactive Fr and Ra are left out.
  • Strength is not concentration: a strong base can be dilute.
  • Calcium hydroxide has limited solubility; the part that dissolves separates completely.
One H₂SO₄ molecule: one central sulfur, four oxygens, and two hydrogens attached to oxygens.
H2SO4 is a molecular compound. In water it acts as an acid, so use acid naming, not binary molecular prefixes.
The acid naming route
Acid typeRuleExample
Binary acid in water (no oxygen)hydro- + root + -ic acidHCl(aq): hydrochloric acid
Oxygen acid from an -ate ion-ate → -ic acidnitrate → nitric acid (HNO3)
Oxygen acid from an -ite ion-ite → -ous acidnitrite → nitrous acid (HNO2)
Ion with per- or hypo-Keep the prefixperchlorate → perchloric acid; hypochlorite → hypochlorous acid
First choose the rule: ionic, molecular or acid.
1 · Recognize the clue
Look for this:H written first with (aq), the word "acid", or "strong-base hydroxide" of a Group 1 metal ion or Ca2+, Sr2+, Ba2+.
2 · Follow the route
  1. Formula starts with H and is in water: use the acid route. Metal + OH−: a hydroxide base.
  2. Find the anion: single-atom (-ide) or polyatomic (-ate or -ite).
  3. Name it: -ide → hydro-…-ic acid; -ate → -ic acid; -ite → -ous acid.
  4. Name → formula: add enough H+ to balance the anion’s charge.

On the periodic table: Sodium (Na) is in Group 1, so it forms Na+; Group 1 hydroxides use one OH−.

On the periodic table: Barium (Ba) is in Group 2, so it forms Ba2+; Ca, Sr and Ba hydroxides use two OH−.

On the periodic table: Chlorine (Cl) is in Group 17, so it forms Cl− and gives the root chlor-.

Worked example 1

Metal ion → strong-base formula: write the neutral hydroxide of Ba2+ and OH−.

  1. 1
    Read the charges

    On the periodic table: barium (Ba) is in Group 2, so it forms Ba2+.

    Ba2+   OH−
  2. 2
    Balance the charges

    One Ba2+ needs two OH− ions.

    IonCount× Charge= Total
    Ba2+1+2+2
    OH−2−1−2
    Sum0
  3. 3
    Write the formula

    Put parentheses around the whole hydroxide group. Ba is on the strong-base list.

    Ba(OH)2 = barium hydroxide
  4. ✓
    Answer

    Ba(OH)2, barium hydroxide, a strong base

Worked example 2

Formula → name: HNO3

  1. 1
    Recognize the anion

    HNO3 is an acid containing nitrate.

    HNO3 = H+ + NO3− (nitrate)
  2. 2
    Change the ending

    Nitrate’s -ate becomes -ic acid. Do not add hydro-.

    nitrate → nitric acid
  3. ✓
    Answer

    nitric acid

Worked example 3

Formula → name: HCl(aq)

  1. 1
    Choose the rule

    HCl(aq) is a binary acid in water.

    H + Cl, in water, no oxygen → hydro-…-ic acid
  2. 2
    Build the name

    On the periodic table: Cl is chlorine, which gives the root chlor-.

    hydro + chlor + ic acid → hydrochloric acid
  3. ✓
    Answer

    hydrochloric acid (HCl gas is hydrogen chloride)

Worked example 4

Name → formula: nitric acid

  1. 1
    Trace back to the ion

    -ic acid points back to an -ate ion: nitrate.

    nitric acid → nitrate, NO3−
  2. 2
    Balance with H+

    One H+ balances one nitrate ion.

    (+1) + (−1) = 0 → H1(NO3)1
  3. 3
    Tidy the formula

    Drop the 1s and the parentheses around the single nitrate group.

    H1(NO3)1 → HNO3
  4. ✓
    Answer

    HNO3

Worked example 5

Name → formula: hydrochloric acid

  1. 1
    Identify the ions

    Hydrochloric acid is H+ with Cl− in water. On the periodic table: chlorine (Cl) is in Group 17, so it forms Cl−.

    hydrochloric acid → H+ + Cl−
  2. 2
    Balance and add the state

    One of each balances. The (aq) marks it as the acid.

    (+1) + (−1) = 0 → HCl(aq)
  3. ✓
    Answer

    HCl(aq)

Mixed chapter review: choose the route, then answer

Use the ion reference when needed. Write an answer before opening the dropdowns.

Mixed chapter review · 22 questions · tap to open

What this set is for: problems from every section of the chapter, mixed together and not labeled, the way a test asks them. Your job is to pick the right method yourself. Do this before the skill check.

Practice 1

Is MgCl2 ionic or molecular?

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Ionic

On the periodic table: Magnesium (Mg) is a Group 2 metal and chlorine (Cl) is a Group 17 nonmetal.

Explain the worked problem to me

What you know: The formula MgCl2. What you want: Whether it is ionic or molecular.

  1. Find each element on the periodic table. Mg is left of the staircase line, so it is a metal. Cl is right of it, so it is a nonmetal.
  2. Apply the first clue: metal + nonmetal → ionic.
  3. Confirm with charges. Mg forms Mg2+ and Cl forms Cl−. One Mg2+ and two Cl− give (+2) + 2(−1) = 0. The ions attract, so the compound is Ionic.

Check: The subscript 2 matches the two chloride ions needed to balance +2.

On the periodic table: Magnesium (Mg) is in Group 2 on the left side, so it is a metal. Chlorine (Cl) is in Group 17 on the right side, so it is a nonmetal.

That’s right: Ionic

YOUR PERIODIC-TABLE REFERENCE

12MgMagnesium24.31Group 2 · Period 3
17ClChlorine35.45Group 17 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 2

Is CO2 ionic or molecular?

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Molecular

On the periodic table: Carbon (C) and oxygen (O) are both nonmetals on the right side of the table.

Explain the worked problem to me

What you know: The formula CO2. What you want: Whether it is ionic or molecular.

  1. Find each element. Carbon and oxygen are both right of the staircase line, so both are nonmetals.
  2. Apply the first clue: two nonmetals → molecular.
  3. Think about what holds it together. The atoms share electrons in covalent bonds. One molecule has 1 C and 2 O. So CO2 is Molecular.

Why it matters: The type decides the route. Molecular compounds use prefixes and actual atom counts, not ion charges.

Watch out: Do not crisscross CO2 into C4+ and O2−. These atoms are not separate simple ions.

On the periodic table: Carbon (C) and oxygen (O) are both to the right of the staircase line, so both are nonmetals.

That’s right: Molecular

YOUR PERIODIC-TABLE REFERENCE

6CCarbon12.01Group 14 · Period 2
8OOxygen16.00Group 16 · Period 2

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 3

Is NH4Cl ionic even though it has no metal?

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Yes

Explain the worked problem to me

What you know: NH4Cl contains only nonmetals. What you want: Whether it is still ionic.

  1. Look for a recognizable ion. NH4 is ammonium, NH4+, a common polyatomic cation.
  2. The rest is Cl, which forms the anion Cl−.
  3. A cation and an anion attract each other: (+1) + (−1) = 0. That attraction makes the compound ionic. So the answer is Yes.

Why it matters: "Metal + nonmetal" is a starting clue, not the full rule. Ammonium compounds are the common ionic exception without a metal.

Watch out: Seeing only nonmetals and calling it molecular. Check for NH4+ first.

On the periodic table: Nitrogen (N), hydrogen (H), and chlorine (Cl) are all nonmetals, so NH₄Cl has no metal. Chlorine is in Group 17, so it forms Cl⁻.

That’s right: Yes

YOUR PERIODIC-TABLE REFERENCE

7NNitrogen14.01Group 15 · Period 2
17ClChlorine35.45Group 17 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 4

What charge does the usual magnesium ion have?

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: 2+

On the periodic table: Magnesium (Mg) is in Group 2, so its usual ion is Mg²⁺.

Explain the worked problem to me

What you know: The element magnesium. What you want: The charge of its usual ion.

  1. Find Mg on the periodic table. It is a metal in Group 2.
  2. Use the column rule: Group 2 metals lose 2 electrons. Losing two negative charges leaves the atom 2 units more positive.
  3. So the usual ion is Mg2+, with charge 2+.

Check: This matches MgCl2: one Mg2+ needs two Cl−.

Watch out: A neutral magnesium atom has zero charge. The 2+ belongs to its ion.

On the periodic table: Magnesium (Mg) is in Group 2. Group 2 atoms lose 2 electrons, so magnesium forms Mg²⁺.

That’s right: 2+

YOUR PERIODIC-TABLE REFERENCE

12MgMagnesium24.31Group 2 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 5

What charge does the usual potassium ion have?

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: 1+

On the periodic table: Potassium (K) is in Group 1, so its usual ion is K⁺.

Explain the worked problem to me

What you know: The element potassium. What you want: The charge of its usual ion.

  1. Find K on the periodic table. It is a metal in Group 1.
  2. Use the column rule: Group 1 metals lose 1 electron. Losing one negative charge leaves the atom one unit more positive.
  3. So the usual ion is K+, with charge 1+.

Check: KCl balances 1:1, since (+1) + (−1) = 0.

Watch out: Metals lose electrons and become positive. Do not give potassium a negative charge.

On the periodic table: Potassium (K) is in Group 1. Group 1 atoms lose 1 electron, so potassium forms K⁺.

That’s right: 1+

YOUR PERIODIC-TABLE REFERENCE

19KPotassium39.10Group 1 · Period 4

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 6

Give the name and charge of SO42−.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Sulfate; 2−

Explain the worked problem to me

What you know: The polyatomic ion SO42−. What you want: Its name and charge.

  1. Recognize the group: one S with four O is a common polyatomic ion.
  2. Recall its name: SO42− is sulfate. Its name ends in -ate, which is normal for many oxygen-containing anions.
  3. Read the charge from the superscript. It belongs to the whole group. Answer: Sulfate; 2−.

Check: Two Na+ balance one sulfate in Na2SO4: 2(+1) + (−2) = 0.

Watch out: Do not give each oxygen its own O2− charge, and do not call it "sulfide" (S2−).

That’s right: Sulfate; 2−

Practice 7

Write the formula from Ca2+ and Cl−.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: CaCl2

On the periodic table: Calcium (Ca) is in Group 2 (Ca²⁺) and chlorine (Cl) is in Group 17 (Cl⁻).

Explain the worked problem to me

What you know: Ions Ca2+ and Cl−. What you want: The neutral formula.

  1. Start with one Ca2+: +2. One Cl− gives (+2) + (−1) = +1, still positive.
  2. Add a second Cl−: (+2) + 2(−1) = 0. Now it balances.
  3. Write the cation first, then the anion, with counts as subscripts: CaCl2.

Check: Crisscross gives the same: Ca's 2 goes to Cl, Cl's 1 goes to Ca. 1:2 is already simplest.

Watch out: Writing CaCl leaves a net +1 charge.

On the periodic table: Calcium (Ca) is in Group 2, so it forms Ca²⁺. Chlorine (Cl) is in Group 17, so it forms Cl⁻.

That’s right: CaCl2

YOUR PERIODIC-TABLE REFERENCE

20CaCalcium40.08Group 2 · Period 4

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 8

Write the formula from Al3+ and O2−.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Al2O3

On the periodic table: Aluminum (Al) is in Group 13 (Al³⁺) and oxygen (O) is in Group 16 (O²⁻).

Explain the worked problem to me

What you know: Ions Al3+ and O2−. What you want: The neutral formula.

  1. Crisscross the charge sizes. Oxygen's 2 becomes aluminum's count; aluminum's 3 becomes oxygen's count.
  2. That gives 2 Al3+ and 3 O2−. The ratio 2:3 cannot be reduced.
  3. Check neutrality: 2(+3) + 3(−2) = +6 − 6 = 0. Formula: Al2O3.

Check: +6 and −6 is the smallest total both charges can reach together.

Watch out: AlO leaves +3 + (−2) = +1. The charges must cancel exactly.

On the periodic table: Aluminum (Al) is in Group 13 and forms Al³⁺. Oxygen (O) is in Group 16 and forms O²⁻.

That’s right: Al2O3

YOUR PERIODIC-TABLE REFERENCE

8OOxygen16.00Group 16 · Period 2
13AlAluminum26.98Group 13 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 9

Write the formula from Mg2+ and NO3−.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Mg(NO3)2

On the periodic table: Magnesium (Mg) is in Group 2, so it forms Mg²⁺.

Explain the worked problem to me

What you know: Ions Mg2+ and NO3− (nitrate). What you want: The neutral formula.

  1. One Mg2+ is +2. Each nitrate is −1, so you need two: (+2) + 2(−1) = 0.
  2. Put the whole nitrate group in parentheses and write 2 outside: (NO3)2. The 3 inside stays unchanged.
  3. Formula: Mg(NO3)2.

Check: Atoms: 1 Mg, 2 × 1 = 2 N, 2 × 3 = 6 O.

Watch out: MgNO32 or MgN2O6 loses the nitrate group. Parentheses show two whole NO3− units.

On the periodic table: Magnesium (Mg) is in Group 2, so it forms Mg²⁺.

That’s right: Mg(NO3)2

YOUR PERIODIC-TABLE REFERENCE

7NNitrogen14.01Group 15 · Period 2
12MgMagnesium24.31Group 2 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 10

Write the formula from NH4+ and SO42−.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: (NH4)2SO4

Explain the worked problem to me

What you know: Ions NH4+ (ammonium) and SO42− (sulfate). What you want: The neutral formula.

  1. Sulfate is −2. Each ammonium is +1, so you need two: 2(+1) + (−2) = 0.
  2. Put ammonium in parentheses with a 2 outside: (NH4)2. Keep its internal 4 unchanged.
  3. One sulfate needs no parentheses or written 1. Cation first: (NH4)2SO4.

Check: Atoms: 2 N, 8 H, 1 S, 4 O.

Watch out: Writing NH42SO4 or changing the 4 inside ammonium. Count whole groups, not internal subscripts.

That’s right: (NH4)2SO4

YOUR PERIODIC-TABLE REFERENCE

7NNitrogen14.01Group 15 · Period 2
16SSulfur32.06Group 16 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 11

Identify ions and counts in Ca(OH)2.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: 1 Ca2+ and 2 OH−

On the periodic table: Calcium (Ca) is in Group 2, so it forms Ca²⁺.

Explain the worked problem to me

What you know: The formula Ca(OH)2. What you want: Which ions, and how many of each.

  1. Recognize the ions. Ca is a Group 2 metal, so Ca2+. The group in parentheses, OH, is hydroxide, OH−.
  2. Read the counts. Ca has an understood 1. The 2 outside the parentheses counts whole OH− groups.
  3. So there are 1 Ca2+ and 2 OH−.

Check: (+2) + 2(−1) = 0. The formula is neutral.

Watch out: The outside 2 is not calcium's charge and does not mean O2. It multiplies both O and H.

On the periodic table: Calcium (Ca) is in Group 2, so it forms Ca²⁺.

That’s right: 1 Ca2+ and 2 OH−

YOUR PERIODIC-TABLE REFERENCE

8OOxygen16.00Group 16 · Period 2
20CaCalcium40.08Group 2 · Period 4

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 12

Identify ions and counts in Al2O3.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: 2 Al3+ and 3 O2−

On the periodic table: Aluminum (Al) is in Group 13 and forms Al³⁺. Oxygen (O) is in Group 16 and forms O²⁻.

Explain the worked problem to me

What you know: The formula Al2O3. What you want: Which ions, and how many of each.

  1. Recognize the ions. Al is a fixed-charge metal: Al3+. O as a single-atom anion is oxide: O2−.
  2. Read the subscripts as counts: 2 aluminum ions and 3 oxide ions.
  3. Answer: 2 Al3+ and 3 O2−.

Check: 2(+3) + 3(−2) = +6 − 6 = 0.

Watch out: Subscripts are counts, not charges. The "2" on Al does not mean Al2+. Use the known ion charges.

On the periodic table: Aluminum (Al) is in Group 13 and forms Al³⁺. Oxygen (O) is in Group 16, so oxide is O²⁻.

That’s right: 2 Al3+ and 3 O2−

YOUR PERIODIC-TABLE REFERENCE

8OOxygen16.00Group 16 · Period 2
13AlAluminum26.98Group 13 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 13

Name NaCl.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Sodium chloride

On the periodic table: Na is sodium and Cl is chlorine.

Explain the worked problem to me

What you know: The formula NaCl. What you want: Its name.

  1. Choose the rule. Na is a metal and Cl is a nonmetal, so use ionic naming.
  2. Name the cation first. Sodium is a Group 1 fixed-charge metal (Na+), so no Roman numeral is needed.
  3. Name the single-atom anion with -ide: chlorine → chloride. Name: Sodium chloride.

Check: (+1) + (−1) = 0, matching the 1:1 formula.

Watch out: Do not add prefixes like "monosodium monochloride". Prefixes are only for molecular compounds.

On the periodic table: Na is sodium, in Group 1, so it forms Na⁺. Cl is chlorine, in Group 17, so it forms chloride, Cl⁻.

That’s right: Sodium chloride

YOUR PERIODIC-TABLE REFERENCE

11NaSodium22.99Group 1 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 14

Name FeCl3.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Iron(III) chloride

Explain the worked problem to me

What you know: The formula FeCl3. What you want: Its name.

  1. Choose the rule. Fe is a metal and Cl a nonmetal, so use ionic naming. Iron is a variable-charge metal, so it needs a Roman numeral.
  2. Find iron's charge from the anions. Three Cl− give 3(−1) = −3, so iron must be +3 to reach zero.
  3. Write the charge as III, then name the anion with -ide: Iron(III) chloride.

Check: (+3) + 3(−1) = 0.

Watch out: The Roman numeral gives iron's charge, not the number of chlorides or iron atoms. Do not write "iron trichloride".

On the periodic table: Chlorine (Cl) is in Group 17, so each chloride is Cl⁻. Fe is the symbol for iron.

That’s right: Iron(III) chloride

YOUR PERIODIC-TABLE REFERENCE

26FeIron55.85Group 8 · Period 4
17ClChlorine35.45Group 17 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 15

Write the formula for copper(II) oxide.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: CuO

Explain the worked problem to me

What you know: The name copper(II) oxide. What you want: Its formula.

  1. Read the ions. The Roman numeral II means Cu2+. Oxide is O2−.
  2. Balance charges. One of each works: (+2) + (−2) = 0.
  3. Crisscross would give Cu2O2; reduce 2:2 to 1:1. Subscripts of 1 are not written: CuO.

Check: Reverse it: one O2− requires copper +2, which matches (II).

Watch out: Do not read II as "two copper atoms". Cu2O is copper(I) oxide.

On the periodic table: Copper’s symbol is Cu. Oxygen (O) is in Group 16, so oxide is O²⁻.

That’s right: CuO

YOUR PERIODIC-TABLE REFERENCE

8OOxygen16.00Group 16 · Period 2
29CuCopper63.55Group 11 · Period 4

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 16

Write the formula for calcium nitrate.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Ca(NO3)2

Explain the worked problem to me

What you know: The name calcium nitrate. What you want: Its formula.

  1. Read the ions. Calcium is Group 2: Ca2+. Nitrate is the polyatomic ion NO3−.
  2. Balance charges. One Ca2+ needs two nitrates: (+2) + 2(−1) = 0.
  3. Put the nitrate group in parentheses with 2 outside, keeping its 3 inside: Ca(NO3)2.

Check: Atoms: 1 Ca, 2 N, 6 O.

Watch out: CaNO3 leaves a +1 charge. Without parentheses the 2 would not multiply the whole nitrate group.

On the periodic table: Calcium (Ca) is in Group 2, so it forms Ca²⁺.

That’s right: Ca(NO3)2

YOUR PERIODIC-TABLE REFERENCE

7NNitrogen14.01Group 15 · Period 2
20CaCalcium40.08Group 2 · Period 4

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 17

Name N2O4.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Dinitrogen tetroxide

Explain the worked problem to me

What you know: The formula N2O4. What you want: Its name.

  1. Choose the rule. N and O are both nonmetals, so use molecular (prefix) naming.
  2. First element: 2 nitrogen atoms → di- + nitrogen = dinitrogen.
  3. Second element: 4 oxygen atoms → tetra- + oxide. The "a" drops before "o", giving tetroxide. Name: Dinitrogen tetroxide.

Check: Read the name back: di = 2 N, tetra = 4 O. That gives N2O4.

Watch out: Do not reduce N2O4 to NO2. That is a different molecule. Molecular formulas keep actual atom counts.

On the periodic table: N is nitrogen and O is oxygen.

That’s right: Dinitrogen tetroxide

YOUR PERIODIC-TABLE REFERENCE

7NNitrogen14.01Group 15 · Period 2
8OOxygen16.00Group 16 · Period 2

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 18

Write the formula for sulfur dioxide.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: SO2

Explain the worked problem to me

What you know: The name sulfur dioxide. What you want: Its formula.

  1. Choose the rule. Sulfur and oxygen are nonmetals, so read the prefixes as atom counts.
  2. "Sulfur" has no prefix. On the first element that means one S.
  3. "Di" on oxide means 2 O. Formula: SO2.

Check: Name it back: 1 S (no mono-) and 2 O (dioxide). It matches.

Watch out: Do not crisscross charges here. Sulfur and oxygen share electrons; the prefixes alone give the formula.

On the periodic table: Sulfur’s symbol is S and oxygen’s symbol is O.

That’s right: SO2

YOUR PERIODIC-TABLE REFERENCE

8OOxygen16.00Group 16 · Period 2
16SSulfur32.06Group 16 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 19

Name HCl(aq).

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Hydrochloric acid

Explain the worked problem to me

What you know: HCl(aq), meaning HCl dissolved in water. What you want: Its acid name.

  1. Choose the rule. The (aq) and the H in front mean acid naming.
  2. Check for oxygen. HCl has only H and Cl, so it is a binary acid.
  3. Use hydro- + root + -ic acid: hydro- + chlor- + -ic acid = hydrochloric acid.

Why it matters: The same formula has two names. HCl gas is hydrogen chloride; HCl(aq) is hydrochloric acid.

Watch out: Do not call it "chloric acid". Without hydro- that name points to a different, oxygen-containing acid.

On the periodic table: Cl is the symbol for chlorine, which gives the root chlor-.

That’s right: Hydrochloric acid

Practice 20

Name HNO3(aq).

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Nitric acid

Explain the worked problem to me

What you know: HNO3(aq). What you want: Its acid name.

  1. Choose the rule. H in front plus (aq) means acid naming.
  2. Find the anion. After removing H+, NO3− remains. That is nitrate.
  3. Change the ending: -ate becomes -ic acid. nitrate → nitric acid.

Check: One H+ balances one NO3−: (+1) + (−1) = 0.

Watch out: Do not add hydro-. Hydro- is only for binary acids with no oxygen. Do not use -ous; that comes from -ite (nitrite → nitrous acid).

That’s right: Nitric acid

YOUR PERIODIC-TABLE REFERENCE

8OOxygen16.00Group 16 · Period 2

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 21

Write the strong-base hydroxide of Ba2+.

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: Ba(OH)2

On the periodic table: Barium (Ba) is in Group 2, so it forms Ba²⁺.

Explain the worked problem to me

What you know: The cation Ba2+ and the hydroxide ion OH−. What you want: The neutral strong-base formula.

  1. Balance charges. One Ba2+ needs two OH−: (+2) + 2(−1) = 0.
  2. Put the whole hydroxide group in parentheses with 2 outside: Ba(OH)2.
  3. Confirm it is a strong base. Ba is in the Ca, Sr, Ba set on the "lowercase b" list.

Check: Atoms: 1 Ba, 2 O, 2 H. It dissociates as Ba(OH)2 → Ba2+ + 2OH−.

Watch out: BaOH2 means one O and two H. Parentheses are needed to show two whole OH groups.

On the periodic table: Barium (Ba) is in Group 2, so it forms Ba²⁺.

That’s right: Ba(OH)2

YOUR PERIODIC-TABLE REFERENCE

8OOxygen16.00Group 16 · Period 2
56BaBarium137.33Group 2 · Period 6

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Practice 22

Does NaOH dissolved in water remain as neutral NaOH molecules?

Give me a hint

First decide whether this asks for compound type, ion charge, formula, or name, then use the matching route.

Check the answer

Answer: It dissociates into Na+ and OH− ions.

On the periodic table: Sodium (Na) is in Group 1, so it forms Na⁺.

Explain the worked problem to me

What you know: NaOH is dissolved in water. What you want: Whether it stays as neutral NaOH molecules.

  1. Decide the type. Na is a Group 1 metal (Na+), and OH− is the polyatomic hydroxide ion. So NaOH is ionic, not molecular.
  2. Ionic compounds separate into their ions when they dissolve. NaOH is a strong base, so it separates completely.
  3. The O and H inside hydroxide stay bonded together. So: It dissociates into Na+ and OH− ions.

Check: NaOH → Na+ + OH−. Charges: 0 = (+1) + (−1).

Watch out: Do not split hydroxide into O and H. OH− stays together.

On the periodic table: Sodium (Na) is a Group 1 metal, so it forms Na⁺ and NaOH is ionic.

That’s right: It dissociates into Na+ and OH− ions.

YOUR PERIODIC-TABLE REFERENCE

1HHydrogen1.008Group 1 · Period 1
8OOxygen16.00Group 16 · Period 2
11NaSodium22.99Group 1 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Skill check: the chapter’s key skills

Try each fresh question without the worked examples. Record your first answer and whether you used a hint. Explain the route as well as the answer.

Skill check · 6 questions · tap to open

What this set is for: one question on each key skill from this chapter. Try each on your own, without hints, then mark how it went in the tracker below. A miss tells you which section to review.

Skill 1: Recognize ionic and molecular compounds and explain what holds each together.

Compare NaCl and CO2: which is ionic and which is molecular? Why?

Give me a hint

Locate each element on the periodic table relative to the staircase line. Metal + nonmetal suggests ions held together by opposite charges; nonmetals only suggests a molecule held together by shared electrons.

Check the answer

NaCl is ionic: positive/negative ions attract. CO2 is molecular: atoms share electrons.

Explain the worked problem to me

What you know: The formulas NaCl and CO2. What you want: Which is ionic, which is molecular, and why.

  1. NaCl: Na is a metal (left of the staircase) and Cl is a nonmetal. Metal + nonmetal → ionic.
  2. What holds NaCl together: Na+ and Cl− attract because their charges are opposite. (+1) + (−1) = 0.
  3. CO2: C and O are both nonmetals. Two nonmetals → molecular. Its atoms share electrons in covalent bonds. So NaCl is ionic: positive/negative ions attract. CO2 is molecular: atoms share electrons.

Why it matters: NaCl's formula is an ion ratio. CO2's formula counts the actual atoms in one molecule.

That’s right: NaCl is ionic: positive/negative ions attract. CO2 is molecular: atoms share electrons.

On the periodic table: Sodium (Na) is a Group 1 metal and chlorine (Cl) is a nonmetal, so NaCl is ionic. Carbon (C) and oxygen (O) are both nonmetals, so CO₂ is molecular.

YOUR PERIODIC-TABLE REFERENCE

6CCarbon12.01Group 14 · Period 2
11NaSodium22.99Group 1 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Skill 2: Use the ion reference and periodic table to identify common charges.

Give the usual ions of K, Mg, and Cl; name NO3−.

Give me a hint

Use each element’s group: Group 1 metals form +1 ions, Group 2 metals form +2 ions, and Group 17 nonmetals form −1 ions. Polyatomic ions such as NO3− come from the ion reference list.

Check the answer

K+, Mg2+, Cl−; nitrate, charge −1.

On the periodic table: Potassium (K) is in Group 1, magnesium (Mg) in Group 2, and chlorine (Cl) in Group 17, giving K⁺, Mg²⁺, and Cl⁻.

Explain the worked problem to me

What you know: The elements K, Mg, Cl and the ion NO3−. What you want: Usual ion charges and the name of NO3−.

  1. K is in Group 1. Group 1 metals lose 1 electron: K+.
  2. Mg is in Group 2. Group 2 metals lose 2 electrons: Mg2+.
  3. Cl is in Group 17. It gains 1 electron: Cl−.
  4. NO3− is a common polyatomic ion, nitrate. So: K+, Mg2+, Cl−; nitrate, charge −1.

Check: Each ion matches a noble-gas electron count: K+ has 19 − 1 = 18 (argon), Mg2+ has 12 − 2 = 10 (neon), and Cl− has 17 + 1 = 18 (argon).

Watch out: Nitrate's −1 belongs to the whole group, not to one atom.

That’s right: K+, Mg2+, Cl−; nitrate, charge −1.

On the periodic table: Potassium (K) is in Group 1 (K⁺), magnesium (Mg) is in Group 2 (Mg²⁺), and chlorine (Cl) is in Group 17 (Cl⁻). Nitrate’s −1 charge belongs to the whole NO₃⁻ group; the periodic table does not give it.

YOUR PERIODIC-TABLE REFERENCE

12MgMagnesium24.31Group 2 · Period 3
17ClChlorine35.45Group 17 · Period 3
19KPotassium39.10Group 1 · Period 4

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Skill 3: Write a neutral ionic formula, identify its ions, and read subscripts and parentheses.

Write the formula from Al3+ and OH−, then identify ions/counts and atom counts.

Give me a hint

Identify whether the compound is ionic or molecular. For ionic compounds, use known ion charges and require a zero total charge; keep polyatomic groups intact. For names, follow the rules for that compound type.

Check the answer

Al(OH)3: 1 Al3+ and 3 OH−; 1 Al, 3 O, 3 H. +3 + 3(−1) = 0.

On the periodic table: Aluminum (Al) is in Group 13 and forms Al³⁺.

Explain the worked problem to me

What you know: Ions Al3+ and OH−. What you want: The formula, the ion counts, and the atom counts.

  1. Balance charges. One Al3+ needs three OH−: (+3) + 3(−1) = 0.
  2. Put hydroxide in parentheses with 3 outside: Al(OH)3. That is 1 Al3+ and 3 OH−.
  3. Count atoms. The outside 3 multiplies both O and H: 3 × 1 O = 3 O, 3 × 1 H = 3 H. So Al(OH)3: 1 Al3+ and 3 OH−; 1 Al, 3 O, 3 H. +3 + 3(−1) = 0.

Check: 7 atoms total: 1 + 3 + 3.

Watch out: AlOH3 would mean 1 O and 3 H. Parentheses are required.

That’s right: Al(OH)3: 1 Al3+ and 3 OH−; 1 Al, 3 O, 3 H. +3 + 3(−1) = 0.

On the periodic table: Aluminum (Al) is in Group 13 and forms Al³⁺.

YOUR PERIODIC-TABLE REFERENCE

1HHydrogen1.008Group 1 · Period 1
8OOxygen16.00Group 16 · Period 2
13AlAluminum26.98Group 13 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Skill 4: Name an ionic compound and write its formula from a name.

Name CuCl2, then write calcium carbonate.

Give me a hint

Identify whether the compound is ionic or molecular. For ionic compounds, use known ion charges and require a zero total charge; keep polyatomic groups intact. For names, follow the rules for that compound type.

Check the answer

Copper(II) chloride; CaCO3, from Ca2+ and CO32−.

On the periodic table: Calcium (Ca) is in Group 2, so it forms Ca²⁺.

Explain the worked problem to me

What you know: The formula CuCl2 and the name calcium carbonate. What you want: A name and a formula.

  1. CuCl2 is ionic. Copper has variable charge, so find it: two Cl− give −2, so copper is +2. Write it as (II).
  2. Name the anion with -ide: copper(II) chloride.
  3. Calcium carbonate: calcium is Group 2, Ca2+. Carbonate is CO32−. (+2) + (−2) = 0, so one of each, and a single group needs no parentheses. So: Copper(II) chloride; CaCO3, from Ca2+ and CO32−.

Check: CaCO3 has 1 Ca, 1 C, 3 O.

Watch out: The II is copper's charge, not a count. Do not write Ca2(CO3)2; reduce to 1:1.

That’s right: Copper(II) chloride; CaCO3, from Ca2+ and CO32−.

On the periodic table: Cu is the symbol for copper, and chlorine (Cl) is in Group 17, so chloride is Cl⁻. Calcium (Ca) is in Group 2, so it forms Ca²⁺.

YOUR PERIODIC-TABLE REFERENCE

6CCarbon12.01Group 14 · Period 2
20CaCalcium40.08Group 2 · Period 4
29CuCopper63.55Group 11 · Period 4
17ClChlorine35.45Group 17 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Skill 5: Read a molecular formula and use prefixes to connect formulas and names.

Name N2O3, then write sulfur trioxide.

Give me a hint

Prefixes give the number of each atom: di- = 2, tri- = 3. The first element keeps its name; the second ends in -ide.

Check the answer

Dinitrogen trioxide; SO3. Prefixes give the actual atom counts.

Explain the worked problem to me

What you know: The formula N2O3 and the name sulfur trioxide. What you want: A name and a formula.

  1. Each compound contains only nonmetals (N and O; S and O), so use molecular prefixes. Prefixes give actual atom counts.
  2. N2O3: 2 N → dinitrogen; 3 O → trioxide. Name: dinitrogen trioxide.
  3. Sulfur trioxide: no prefix on sulfur means 1 S; tri- means 3 O. So: Dinitrogen trioxide; SO3. Prefixes give the actual atom counts.

Check: Read each back: di = 2, tri = 3, no prefix on the first element = 1.

Watch out: Do not crisscross charges for these molecules, and do not reduce counts.

That’s right: Dinitrogen trioxide; SO3. Prefixes give the actual atom counts.

On the periodic table: N is nitrogen, O is oxygen, and S is sulfur.

YOUR PERIODIC-TABLE REFERENCE

16SSulfur32.06Group 16 · Period 3

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Skill 6: Recognize aqueous acids and common strong bases, and choose the acid naming route.

Trace the extra H in H3O+ from HCl; write the strong-base hydroxide of Ca2+; name HNO2 as an acid.

Give me a hint

An acid gives H+ to water. A common strong base pairs a Group 1 metal ion, or Ca2+, Sr2+ or Ba2+, with OH−. An -ite ion becomes an -ous acid.

Check the answer

HCl donates H+ to H2O, making H3O+ and Cl−. Ca(OH)2 is calcium hydroxide, a strong base. HNO2 is nitrous acid: nitrite → -ous acid.

On the periodic table: Chlorine (Cl) is in Group 17, so it forms Cl⁻. Calcium (Ca) is in Group 2, so it forms Ca²⁺.

Explain the worked problem to me

What you know: HCl in water, the ion Ca2+, and HNO2. What you want: Where H3O+ gets its extra H, a hydroxide formula, and an acid name.

  1. An acid donates H+ to water: HCl + H2O → H3O+ + Cl−. The third H came from HCl.
  2. Ca2+ needs two OH−: (+2) + 2(−1) = 0. Formula Ca(OH)2, calcium hydroxide, on the strong-base list.
  3. HNO2 contains NO2−, nitrite. The -ite ending becomes -ous acid. So: HCl donates H+ to H2O, making H3O+ and Cl−. Ca(OH)2 is calcium hydroxide, a strong base. HNO2 is nitrous acid: nitrite → -ous acid.

Check: Atoms: 1 H from HCl + 2 H from H2O = 3 H in H3O+. Charges: the reactants are neutral, and the products give (+1) + (−1) = 0.

Watch out: HNO2 is not nitric acid. Nitric acid is HNO3, from nitrate.

That’s right: HCl donates H+ to H2O, making H3O+ and Cl−. Ca(OH)2 is calcium hydroxide, a strong base. HNO2 is nitrous acid: nitrite → -ous acid.

On the periodic table: Chlorine (Cl) is in Group 17, so it forms Cl⁻. Calcium (Ca) is in Group 2, so it forms Ca²⁺.

YOUR PERIODIC-TABLE REFERENCE

1HHydrogen1.008Group 1 · Period 1
8OOxygen16.00Group 16 · Period 2
17ClChlorine35.45Group 17 · Period 3
20CaCalcium40.08Group 2 · Period 4

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

What to do next

Mark each skill: “I can do it independently,” “I needed help,” or “I need another explanation.” Revisit the section with the same goal for any skill needing help, then try a different question. A successful first attempt is evidence of progress; one answer alone does not establish lasting mastery.

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