What does the ending -ate tell you about a compound such as potassium nitrate?

It contains oxygen as well as the other elements

What holds the ions together in a giant ionic lattice?

Strong electrostatic forces between oppositely charged ions

Ions, ionic formulae and ionic lattices

Ionic compounds are made of ions. To write their formulae you need the charges on the ions, including ions made of groups of atoms.

GCSE · 1CH0
International GCSE · 4CH1 · 1.37 to 1.43
About 16 minutes

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Making ions

An ion is an atom or group of atoms with an electrical charge, because electrons have been lost or gained.
Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions. Each gets a full outer shell.
Group 1 → 1+, Group 2 → 2+, Group 6 → 2−, Group 7 → 1−.
Example: Mg (2.8.2) loses 2 electrons → Mg²⁺ (2.8): 12 protons, 10 electrons.
In a dot and cross diagram, show the electrons moved from the metal to the non-metal, with the charges in brackets.

Dot and cross diagram for magnesium oxide: Mg (2.8.2) gives 2 electrons to O (2.6).

Naming and charges

-ide: the compound contains just two elements, for example sodium chloride, magnesium oxide.
-ate: the negative ion also contains oxygen, for example copper sulfate, calcium carbonate.

Ion	Formula
Hydroxide	OH⁻
Nitrate	NO₃⁻
Sulfate	SO₄²⁻
Carbonate	CO₃²⁻
Ammonium	NH₄⁺

Writing formulae and the lattice

The charges must balance to zero.
Sodium sulfate: Na⁺ and SO₄²⁻ → two Na⁺ needed → Na₂SO₄
Calcium hydroxide: Ca²⁺ and OH⁻ → two OH⁻ → Ca(OH)₂ (brackets show two whole hydroxide ions)
Ionic compounds form a giant lattice: a regular 3D arrangement of oppositely charged ions held by strong electrostatic forces in all directions.

Worked example

Write the formula of aluminium oxide. (Al³⁺, O²⁻)

The lowest common multiple of 3 and 2 is 6.
2 × Al³⁺ = 6+; 3 × O²⁻ = 6−.
So the formula is Al₂O₃.

Answer: Al₂O₃

Key idea

Metals lose electrons to form positive ions; non-metals gain them to form negative ions. -ide means two elements; -ate means the ion contains oxygen. Balance the charges to write formulae, using brackets for more than one polyatomic ion.

Key words

Ion: An atom or group of atoms with a charge, because it has lost or gained electrons.
Atom: The smallest particle of an element that can take part in a chemical reaction.
Group: A vertical column of the periodic table. Elements in a group have the same number of outer electrons and similar reactions.
Compound: A substance made of two or more elements chemically combined in fixed proportions.

All GCSE and International GCSE Chemistry key terms and definitions →

Common mistakes

Not “NH₃”. NH₃ is ammonia, a molecule.
Not “NO₃⁻”. That is nitrate.
Not “NH₄⁻”. The ammonium ion is positive.

Quick check

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Extra practice

2 more questions

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Write the formula of potassium carbonate (K⁺, CO₃²⁻). Type numbers as normal, like H2O.
Show the answer
Answer: K2CO3
Two K⁺ balance one CO₃²⁻: K2CO3.
Write the formula of magnesium nitrate (Mg²⁺, NO₃⁻). Type numbers as normal, like H2O.
Show the answer
Answer: Mg(NO3)2
Two NO₃⁻ balance one Mg²⁺. Brackets show there are two nitrate ions: Mg(NO3)2.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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