ChemGem

How to Use Periodic Trends: Atomic Radius and Electronegativity (Step-by-Step, with Practice)

The periodic table predicts which atoms are larger and which pull hardest on shared electrons. Ions need one extra check first. These trends are general patterns, not rules without exceptions.

Spot it, solve it, practice it: 3 steps, 1 worked example, 7 practice problems. Free, no account.

ChemGemUpdated Section 3.4

The route · 3 steps

  1. Decide: neutral atom or ion?
  2. Neutral atoms: same period or same group? Apply the size or electronegativity trend.
  3. Ions with equal electrons: the one with more protons is smaller.
Try one now See a worked example
Spot it

How to recognize periodic trend questions

Look for "larger", "smaller", "radius", or "more electronegative" comparing two atoms or ions.

Example wording, with the clue words marked:

Which neutral atom is generally larger, Na or K?

Not sure it’s this type? Compare the neighbors:

Step-by-step

Worked example: periodic trend

The idea

First decide: neutral atom or ion?
Neutral atoms
Use periodic position.
Size generally grows down a group and to the left across a period.
Ions with equal electron counts
Compare protons instead.
More protons pull the same electrons in tighter: a smaller ion.
See the atomic-size trend on the whole table. Neutral atomic size generally increases LEFT across a period and DOWN a group. Read the highlighted third row to compare Na, Mg, and Cl. Ion sizes need a separate charge/electron check.
Atomic size: neutral atoms are generally larger down a group and to the left across a period. In row 3, Na is larger than Mg, and Mg larger than Cl.
Periodic table with an arrow pointing up the side and an arrow pointing right along the bottom, both marking higher electronegativity. The shaded upper-right group of N, O, F, Cl and Br marks the most electronegative elements, with fluorine (F) highest.
Electronegativity: how strongly an atom pulls on shared bonding electrons. It generally rises up a group and to the right, peaking at fluorine (F). Noble gases are usually left out.
How an ion compares with its parent atom
ChangeExampleProtonsSize
Loses electrons (cation)Mg → Mg2+Unchanged, 12Smaller
Gains electrons (anion)Cl → Cl−Unchanged, 17Larger
Magnesium loses two electrons to form a smaller Mg²⁺ ion; chlorine gains one electron to form a larger Cl⁻ ion. The proton count stays unchanged in each comparison.
Mg loses two electrons and shrinks; Cl gains one and grows. The proton count stays the same.
Na⁺ and Mg²⁺ both have ten electrons; magnesium has twelve protons versus sodium’s eleven, so Mg²⁺ has the smaller electron cloud. Cloud sizes are schematic.
Same electron count, different proton count: 12 protons hold 10 electrons tighter than 11 do. Cloud sizes are schematic.

On the periodic table: Size: read position. Larger down a group and to the left across a period.

On the periodic table: Electronegativity: higher up and to the right, toward fluorine (F).

On the periodic table: Group number gives the main-group ion: Group 1 forms 1+ (Na+), Group 2 forms 2+ (Mg2+).

On the periodic table: The atomic number gives the proton count: Na 11, Mg 12.

Worked example · 3 steps

Na+ and Mg2+ both have 10 electrons. Which ion is smaller?

Periodic table with arrows pointing down a group and left across a period, both marking larger atomic size. Group 1 (H to Fr) and period 3 (Na to Ar) are shaded.
Atoms get larger down a group and to the left across a period.
  1. 1
    Check the common feature

    Equal electron counts let us compare nuclear pull. On the periodic table: Na is Group 1, so it forms Na+; Mg is Group 2, so it forms Mg2+. Each loses its valence electrons.

    Na+: 11 − 1 = 10 electrons    Mg2+: 12 − 2 = 10 electrons
  2. 2
    Compare the protons

    On the periodic table: the atomic numbers are 11 and 12.

    IonAtomic number = protonsElectronsAtomic mass (u)Position
    Na+111022.99Group 1 · Period 3
    Mg2+121024.31Group 2 · Period 3
  3. 3
    Choose the stronger pull

    One more proton attracts the same 10 electrons. More protons generally pull the cloud in tighter.

    Na+
    11 protons pull on 10 electrons
    Mg2+
    12 protons pull on 10 electrons: smaller
  4. ✓
    Answer

    Mg2+ is smaller. Decide with proton attraction, not the charge label alone.

Your turn

Periodic trend practice problems

Answer first, then check. Hints open one at a time; the answer and full solution open after you try.

Problem 1 of 7 · warm-up

Which neutral atom is generally larger, Na or K?

Hint 1 · the clue

On the periodic table: Sodium (Na) and potassium (K) are both in Group 1. K is one row lower, in Period 4.

What does moving down a group add to an atom?

Hint 2 · the next move

"Neutral atom" and "larger" for two elements in the same group signal the down-a-group size trend.

Your path:

  1. Na and K are in one group
  2. Down a group adds a shell
  3. Lower atom is larger
Answer

Answer: K

Full worked solution

What you know: Na and K are neutral atoms in the same group. What you want: Which is generally larger.

  1. Same group, so use the down-a-group trend.
  2. Going down a group adds an occupied shell. The outer electrons sit farther from the nucleus, and inner electrons shield them.
  3. K is one row below Na, so K is larger.

Why it matters: Neutral atomic size grows down and to the left. K is lower, so it should be bigger.

Watch out: K has more protons, but more protons does not make it smaller here. The extra shell wins down a group.

On the periodic table: Sodium (Na) is in Period 3 and potassium (K) is in Period 4, both in Group 1, so K has one more occupied shell.

Answer: K

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.

Problem 2 of 7

Which is generally smaller, Na or Cl?

Hint 1 · the clue

On the periodic table: Sodium (Na, Group 1) and chlorine (Cl, Group 17) are both in Period 3. Cl is farther right.

Across a period, protons are added but the outer shell stays the same. How does that change the pull on the outer electrons?

Hint 2 · the next move

"Smaller" for two atoms in the same period signals the across-a-row size trend.

Your path:

  1. Na and Cl share a period
  2. Across a row, more protons pull harder
  3. Atom farther right is smaller
Answer

Answer: Cl

Full worked solution

What you know: Na and Cl are neutral atoms in the same period. What you want: Which is generally smaller.

  1. Same period, so use the across-a-row trend.
  2. Across a period, protons are added while electrons go into the same main shell. Shielding barely changes.
  3. The stronger pull draws the electrons in. Cl is farther right, so Cl is smaller.

Why it matters: Size grows down and to the left; Na is at the left edge, so it should be the larger one.

Watch out: More electrons does not mean bigger across a row. Cl has more electrons but is smaller.

On the periodic table: Sodium (Na) and chlorine (Cl) are both in Period 3, and Cl is farther right, so Cl is generally smaller.

Answer: Cl

YOUR PERIODIC-TABLE REFERENCE

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.

Problem 3 of 7

Compare neutral potassium and bromine, both in period 4. Which generally has the larger atomic radius?

Hint 1 · the clue

On the periodic table: Potassium (K) is in Group 1 at the far left of Period 4. Bromine (Br) is in Group 17, near the right end of Period 4.

The comparison is between neutral atoms in one period.

Hint 2 · the next move

"Both in period 4" and "larger atomic radius" signal the across-a-row trend for neutral atoms.

Your path:

  1. Same period → use across-row trend
  2. Moving right, size shrinks
  3. Atom farther left is larger
Answer

Answer: potassium

Full worked solution

What you know: K and Br are neutral atoms in Period 4. What you want: Which is generally larger.

  1. Same period, so use the across-a-row trend.
  2. Moving right, protons increase while electrons go into the same main shell, so the pull strengthens and size shrinks.
  3. K is at the far left; Br is near the right. So potassium is larger.

Why it matters: Size grows down and to the left. Both are in row 4, so the one farther left wins.

Watch out: Br has more electrons, but more electrons in the same shell does not make it bigger.

Your route: Same period → Size generally decreases to the right.

On the periodic table: Potassium (K, Group 1) and bromine (Br, Group 17) are both in Period 4. K is farther left, so it is generally larger.

Answer: potassium

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.

Problem 4 of 7

Within one periodic-table group, compare neutral magnesium and barium. Which generally has a larger radius?

Hint 1 · the clue

On the periodic table: Magnesium (Mg) and barium (Ba) are both in Group 2. Mg is in Period 3 and Ba is in Period 6.

The comparison is down a group.

Hint 2 · the next move

"Within one periodic-table group" and "larger radius" signal the down-a-group size trend.

Your path:

  1. Same group → use down-group trend
  2. Lower atom has more occupied shells
  3. Lower atom is larger
Answer

Answer: barium

Full worked solution

What you know: Mg and Ba are neutral atoms in Group 2. What you want: Which is generally larger.

  1. Same group, so use the down-a-group trend.
  2. Mg is in Period 3 and Ba is in Period 6, so Ba has three more occupied shells.
  3. Its outer electrons are much farther from the nucleus, so barium is larger.

Why it matters: Size grows down and to the left. Ba is lower, so it should be bigger.

Watch out: Ba has many more protons, but down a group the extra shells win.

Your route: Same group → More shells down the group.

On the periodic table: Magnesium (Mg) is in Period 3 and barium (Ba) is in Period 6 of Group 2, so Ba has more occupied shells.

Answer: barium

YOUR PERIODIC-TABLE REFERENCE

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.

Problem 5 of 7

Ca2+ and K+ each contain 18 electrons. Which ion is generally smaller?

Hint 1 · the clue

On the periodic table: Calcium (Ca) has atomic number 20 and potassium (K) has atomic number 19, the whole numbers in their boxes.

The ions are isoelectronic.

Hint 2 · the next move

Two ions "each contain" the same electron count: that means compare protons, and more protons means smaller.

Your path:

  1. Equal electron counts → compare protons
  2. Find each element's proton count
  3. More protons → smaller ion
Answer

Answer: Ca2+

Full worked solution

What you know: Ca2+ and K+ both have 18 electrons. What you want: Which ion is smaller.

  1. Equal electron counts (isoelectronic), so compare protons.
  2. Ca has 20 protons. K has 19 protons.
  3. 20 protons pull the same 18 electrons in more tightly, so Ca2+ is smaller.

Why it matters: Electron counts: Ca 20 − 2 = 18; K 19 − 1 = 18. They match, so the proton comparison is fair.

Watch out: Don't use the atom trend here. Neutral Ca is smaller than neutral K too, but for ions the proton count is the reason.

Your route: Equal electrons → Compare proton counts.

On the periodic table: Calcium (Ca) has atomic number 20 and potassium (K) has atomic number 19, so Ca2+ has 20 protons and K+ has 19.

Answer: Ca2+

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.

Problem 6 of 7

Which is more electronegative, N or P?

Hint 1 · the clue

Both are in Group 15. Which is higher in the column?

Hint 2 · the next move

"More electronegative" for two elements in one group signals the trend: electronegativity rises up a group.

Your path:

  1. N and P are in Group 15
  2. Electronegativity increases up a group
  3. Higher element is more electronegative
Answer

Answer: N

Full worked solution

What you know: N and P are both in Group 15. N is above P. What you want: Which is more electronegative.

  1. Electronegativity is how strongly an atom pulls on shared bonding electrons.
  2. It generally increases up a group and to the right, toward F.
  3. N is higher in the group, so N is more electronegative.

Why it matters: N's bonding electrons are in shell 2, closer to the nucleus than P's shell-3 electrons, so they are pulled harder.

Answer: N

Problem 7 of 7

Which is smaller: Na+ or Mg2+? Both have 10 electrons.

Hint 1 · the clue

On the periodic table: Sodium (Na) has atomic number 11 and magnesium (Mg) has atomic number 12, the whole numbers in their boxes.

The electron counts match, so compare protons. More protons attract those electrons more strongly, making the ion smaller.

Hint 2 · the next move

"Which is smaller" for two ions with "10 electrons" each means compare protons: more protons pull tighter.

Your path:

  1. Equal electron counts → compare protons
  2. Find each element's proton count
  3. More protons → smaller ion
Answer

Mg2+: 12 protons attract the same 10 electrons more strongly than 11 protons.

On the periodic table: Magnesium (Mg) has atomic number 12 and sodium (Na) has atomic number 11, the whole numbers in their boxes.

Full worked solution

What you know: Na+ and Mg2+ both have 10 electrons. What you want: Which ion is smaller.

  1. Equal electron counts, so compare proton counts.
  2. Na has 11 protons. Mg has 12 protons.
  3. 12 protons pull the same 10 electrons in more tightly, so Mg2+ is smaller.

Check: Electron counts: 11 − 1 = 10 and 12 − 2 = 10. They match, so the comparison is fair.

Watch out: Use the proton count, not just the bigger charge label, as the reason.

On the periodic table: Sodium (Na) has atomic number 11 and magnesium (Mg) has atomic number 12, so Na+ has 11 protons and Mg2+ has 12.

Answer: Mg2+: 12 protons attract the same 10 electrons more strongly than 11 protons.

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.

Watch out

Common periodic trend mistakes