An ionic equation must pass two tests: the atoms of each element must balance, and the total charge must balance. A correct answer to one test says nothing about the other.
This lesson belongs to conservation reasoning across a reaction. It builds on balancing chemical equations.
How do the two checks work?
Make a small table with atoms in one column and charge in another. Fill in the left side, the right side, then compare.
| Check | What to total | What to compare |
|---|---|---|
| Atoms | Each element on each side | Left = right for every element |
| Charge | Sum of (charge × coefficient) | Left total = right total |
The two checks are independent. An equation can pass one and fail the other, so write both out.
Worked example 1: copper and silver ions
A student writes: Cu + Ag⁺ → Cu²⁺ + Ag
Atoms. Cu: 1 and 1. Ag: 1 and 1. The atoms balance.
Charge. Left: 0 + (+1) = +1. Right: (+2) + 0 = +2. The charge does not balance.
Fix it by putting 2 in front of Ag⁺ and Ag: Cu + 2Ag⁺ → Cu²⁺ + 2Ag.
Recheck. Atoms: Cu 1 and 1, Ag 2 and 2. Charge: left +2, right +2. Both checks now pass.
Worked example 2: iron(III) and tin(II) ions
A student writes: Fe³⁺ + Sn²⁺ → Fe²⁺ + Sn⁴⁺
Atoms. Fe 1 and 1, Sn 1 and 1. Balanced.
Charge. Left: +3 + 2 = +5. Right: +2 + 4 = +6. Not balanced.
Balance the charge with 2Fe³⁺ and 2Fe²⁺: left is 2(+3) + 2 = +8, right is 2(+2) + 4 = +8. 2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺. Fe now 2 and 2, so atoms also pass.
Worked example 3: precipitation
For barium chloride and sodium sulfate, the ionic equation is Ba²⁺ + SO₄²⁻ → BaSO₄. Atoms: Ba 1 and 1, S 1 and 1, O 4 and 4. Charge: left +2 + (−2) = 0, right 0. Both checks pass, because the solid is neutral.
The slip to avoid
The usual slip is to count atoms only and declare the equation done. In worked example 1, the equation looked finished after the atom check, yet it would lose the mark.
Write the charge totals on the page, with the coefficient multiplied in, before you stop. A quick look at redox work in redox and equilibrium shows the same check in action.
Check yourself
Decide whether Zn + Cu²⁺ → Zn²⁺ + Cu passes both checks. Then check Al + 3Ag⁺ → Al³⁺ + 3Ag.
Answer
Zn + Cu²⁺ → Zn²⁺ + Cu: atoms Zn 1 and 1, Cu 1 and 1. Charge left +2, right +2. It passes both checks.
Al + 3Ag⁺ → Al³⁺ + 3Ag: atoms Al 1 and 1, Ag 3 and 3. Charge left 3 × (+1) = +3, right +3. It also passes both checks.
What to study next
Try the cluster practice set to apply both checks on new equations.
If you want a teacher to go through ionic equations with you, see online one-to-one Chemistry tuition.