ChemGem

How to Find the Charge of an Ion from Protons and Electrons (Step-by-Step, with Practice)

An atom that gains or loses electrons becomes an ion: its charges no longer cancel. Charge is protons minus electrons.

Spot it, solve it, practice it: 4 steps, 1 worked example, 5 practice problems. Free, no account.

ChemGemUpdated Section 1.3

The route · 4 steps

  1. Count the protons: each adds +1.
  2. Count the electrons: each adds −1.
  3. Add the two: the result is the charge.
  4. Write it as a superscript, number before sign (Mg2+).
Try one now See a worked example
Spot it

How to recognize ion charge questions

Look for "ion," "charge," "neutral," or electrons gained or lost: the charge comes from protons minus electrons.

Example wording, with the clue words marked:

A neutral atom with 15 protons gains 3 electrons. What charge does it acquire?

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

Step-by-step

Worked example: ion charge

The idea

Ion charge = protons − electrons
charge = p − e
For a single-atom ion. p counts positive protons; e counts negative electrons. The result is a signed charge.
The key facts
  • A neutral atom has equal protons and electrons, so the charges cancel.
  • An ion is an atom or group of atoms with an overall charge.
  • The proton count never changes here. Only electrons move.
A lithium atom and lithium cation share the same 3-proton, 4-neutron nucleus. Losing one electron changes charge, not element or isotope.
Same 3-proton, 4-neutron nucleus. Losing one electron changes the charge, not the element or isotope.
Same protons, different electrons
ParticleProtonsElectronsCharge (p − e)
Li atom330
Li+ (lost 1)32+1
O atom880
O− (gained 1)89−1
O2− (gained 2)810−2
Gaining two electrons takes a neutral atom from 0 to −1 to −2.
Two kinds of ion
Lose electrons
More protons than electrons.
Positive ion (cation).
Example: Mg2+
Gain electrons
More electrons than protons.
Negative ion (anion).
Example: O2−

Why a particular number? Main-group atoms tend to match the electron count of the nearest noble gas (Group 18), which has a full outer shell. Section 2.2 shows how this sets each column’s charge.

Use proton count to check an ion’s charge. Magnesium has atomic number 12: 12 positive protons. With 10 negative electrons, (+12) + (−10) = +2. Write Mg²⁺. The table identifies magnesium. The question must specify the ion or electron count.
The table gives the protons. The question must give the electron count or the ion.

On the periodic table: The atomic number gives the proton count: magnesium (Mg) is 12, the whole number in its square.

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

Worked example · 4 steps

A magnesium ion has 12 protons and 10 electrons. What is its charge?

  1. 1
    Count the positive charges

    Each proton adds +1. On the periodic table: magnesium (Mg) has atomic number 12, so 12 protons.

    12 protons → +12
  2. 2
    Count the negative charges

    Each electron adds −1. Negative numbers drive the total down.

    10 electrons → −10
  3. 3
    Add the two contributions

    Ten negatives cancel ten positives. Two positives remain.

    (+12) + (−10) = +2
    Protons+12Electrons−10Total charge+2
  4. 4
    Write the ion

    2+ means two more positive charges than negative. On the periodic table: Mg is in Group 2, and Group 2 atoms lose 2 electrons.

    Mg2+
  5. ✓
    Answer

    The charge is 2+: Mg2+.

Your turn

Ion charge practice problems

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

Problem 1 of 5 · warm-up

A neutral atom with 15 protons gains 3 electrons. What charge does it acquire?

Hint 1 · the clue

The atom gains electrons.

Hint 2 · the next move

A neutral atom "gains" electrons and its charge is asked: gaining electrons makes a negative ion.

Your path:

  1. Neutral start: electrons = protons
  2. Add the gained electrons
  3. Charge = protons − electrons
Answer

Answer: 3−

Full worked solution

What you know: a neutral atom with 15 protons gains 3 electrons. What you want: its new charge.

  1. Neutral means it starts with 15 electrons and charge 0.
  2. After gaining 3, it has 15 + 3 = 18 electrons. The protons stay the same.
  3. Charge = protons − electrons = 15 − 18 = −3, written 3−.

Check: Electrons now outnumber protons, so the charge must be negative.

Watch out: Gaining electrons makes the charge negative, not positive. Electrons are the negative particles.

Your route: Gain negative electrons → Total charge decreases

15P30.97On the periodic table: An atom with 15 protons is phosphorus (P); you can also read phosphorus’s atomic number, 15, from its box. Phosphorus is in Group 15, so it forms P3− (see 2.2).

Answer: 3−

Problem 2 of 5

A neutral atom with 17 protons gains 1 electron. What charge does it acquire?

Hint 1 · the clue

The atom gains electrons.

Hint 2 · the next move

A neutral atom "gains" an electron and its charge is asked: gaining electrons makes a negative ion.

Your path:

  1. Neutral start: electrons = protons
  2. Add the gained electron
  3. Charge = protons − electrons
Answer

Answer: 1−

Full worked solution

What you know: a neutral atom with 17 protons gains 1 electron. What you want: its new charge.

  1. Neutral means it starts with 17 electrons and charge 0.
  2. After gaining 1, it has 17 + 1 = 18 electrons. The protons stay the same.
  3. Charge = protons − electrons = 17 − 18 = −1, written 1−.

Check: Electrons now outnumber protons, so the charge must be negative.

Watch out: Gaining electrons makes the charge negative, not positive. Electrons are the negative particles.

Your route: Gain negative electrons → Total charge decreases

17Cl35.45On the periodic table: An atom with 17 protons is chlorine (Cl); you can also read chlorine’s atomic number, 17, from its box. Chlorine is in Group 17, so it forms Cl−.

Answer: 1−

Problem 3 of 5

An ion has 20 protons and charge 2+. How many electrons does it have?

Hint 1 · the clue

Proton count and positive ion charge are given.

Hint 2 · the next move

An ion's protons and positive charge are given, electrons asked: a 2+ ion has fewer electrons than protons.

Your path:

  1. Charge = protons − electrons
  2. Positive ion: electrons = protons − charge
  3. Check: protons − electrons = charge
Answer

Answer: 18

Full worked solution

What you know: 20 protons, charge 2+. What you want: the number of electrons.

  1. Charge = protons − electrons, so +2 = 20 − electrons.
  2. A 2+ ion has 2 more protons than electrons. Electrons = 20 − 2 = 18.

Check: 20 − 18 = +2, the given charge.

Watch out: Don't add the charge (20 + 2 = 22). A positive ion has lost electrons, so it has fewer than its protons.

Your route: Positive ion → Fewer electrons than protons

20Ca40.08On the periodic table: An atom with 20 protons is calcium (Ca); you can also read calcium’s atomic number, 20, from its box. Calcium is in Group 2, so it forms Ca2+.

Answer: 18

Problem 4 of 5

An ion has 12 protons and charge 2+. How many electrons does it have?

Hint 1 · the clue

Proton count and positive ion charge are given.

Hint 2 · the next move

An ion's protons and positive charge are given, electrons asked: a 2+ ion has fewer electrons than protons.

Your path:

  1. Charge = protons − electrons
  2. Positive ion: electrons = protons − charge
  3. Check: protons − electrons = charge
Answer

Answer: 10

Full worked solution

What you know: 12 protons, charge 2+. What you want: the number of electrons.

  1. Charge = protons − electrons, so +2 = 12 − electrons.
  2. A 2+ ion has 2 more protons than electrons. Electrons = 12 − 2 = 10.

Check: 12 − 10 = +2, the given charge.

Watch out: Don't add the charge (12 + 2 = 14). A positive ion has lost electrons, so it has fewer than its protons.

Your route: Positive ion → Fewer electrons than protons

12Mg24.31On the periodic table: An atom with 12 protons is magnesium (Mg); you can also read magnesium’s atomic number, 12, from its box. Magnesium is in Group 2, so it forms Mg2+.

Answer: 10

Problem 5 of 5

An ion has 9 protons and 10 electrons. Charge?

Hint 1 · the clue

Count each proton as +1 and each electron as −1. Add those contributions to find the net charge.

Hint 2 · the next move

An ion's protons and electrons are given and "Charge?" is asked: subtract electrons from protons.

Your path:

  1. Protons give + charges
  2. Electrons give − charges
  3. Charge = protons − electrons
Answer

+9 − 10 = −1.

Full worked solution

What you know: 9 protons, 10 electrons. What you want: the charge.

  1. Count the positive charges: 9 protons → +9.
  2. Count the negative charges: 10 electrons → −10.
  3. Add the two contributions: (+9) + (−10) = −1, so the ion is F−.

Check: There is one more electron than protons, so the charge must be negative by 1.

Watch out: Subtract in the order protons − electrons. Doing 10 − 9 gives +1, the wrong sign.

On the periodic table: An atom with 9 protons is fluorine (F), atomic number 9. Fluorine is in Group 17, and Group 17 atoms gain 1 electron (see 2.2), so fluorine forms F−, a −1 charge.

Answer: +9 − 10 = −1.

Watch out

Common ion charge mistakes