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Chemistry · Grades 8–11

Ionic formulas and charge balance

Find neutral ratios without assuming every metal has only one charge.

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Learn the idea

Neutral formula

For a neutral ionic compound, total positive and negative charge sum to zero.

Ratios

Use the smallest whole-number ion ratio, not an arbitrary count.

Variable charge

A Roman numeral in a name such as iron(III) specifies the oxidation state in that compound.

Polyatomic ions

Treat a named polyatomic ion as a group; use brackets when more than one group is needed.

Worked examples

Which neutral formula matches Mg²⁺ and Cl⁻?

One +2 ion requires two −1 ions.

MgCl₂

Which neutral formula matches Al³⁺ and O²⁻?

Two Al ions give +6 and three oxide ions give −6.

Al₂O₃

Check your understanding

  1. How many chloride ions balance one Fe³⁺ ion?

  2. In iron(III) chloride, what does III identify?

    1. The third isotope.
    2. The iron oxidation state +3.
    3. Three iron atoms in every formula.

  3. Which formula matches Ca²⁺ and nitrate NO₃⁻?

    1. Ca₂NO₃
    2. Ca(NO₃)₂
    3. CaNO₃

Answers and marking notes

Answers and marking notes
  1. 3

    Three charges of −1 balance +3.

  2. The iron oxidation state +3.

    The numeral specifies the oxidation state, not the atom count.

  3. Ca(NO₃)₂

    Two nitrate ions are needed to balance one calcium ion.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.