Ionic bonding and ionic formulae

When a metal reacts with a non-metal, the metal atoms lose their outer electrons and the non-metal atoms gain them.

Both end up with a full outer shell, like a noble gas.

Losing negative electrons leaves a positive ion. Gaining them makes a negative ion. The oppositely charged ions attract each other: that attraction is the ionic bond.

Sodium (2,8,1) gives its one outer electron to chlorine (2,8,7). The ions are Na⁺ (2,8) and Cl⁻ (2,8,8). A dot and cross diagram uses dots for one atom's electrons and crosses for the other's, so you can see where each one came from.

Count how far the atom is from a full shell:

• Group 1: lose 1, ion 1+ (Na⁺, K⁺)
• Group 2: lose 2, ion 2+ (Mg²⁺, Ca²⁺)
• Group 3: lose 3, ion 3+ (Al³⁺)
• Group 6: gain 2, ion 2− (O²⁻, S²⁻)
• Group 7: gain 1, ion 1− (Cl⁻, Br⁻)

Non-metal ions take the ending -ide: chlorine becomes chloride, oxygen becomes oxide. Learn four ions made of several atoms too: hydroxide OH⁻, nitrate NO₃⁻, sulfate SO₄²⁻ and carbonate CO₃²⁻.

A Group 6 atom does not lose 6 electrons. It gains 2: the shorter way to a full shell.

Open the periodic table and click sodium, magnesium and aluminium in turn. The detail panel shows Group 1, 2 and 13. This table numbers all 18 columns, so for groups 13 to 18 take away 10 to get the GCSE group: aluminium is Group 3 and forms Al³⁺. Now click oxygen (16, so Group 6) and chlorine (17, so Group 7) and say each ion before you look up the next. Notice the colour key too: the alkali metals all form 1+ ions and the halogens all form 1− ions.

An ionic compound has no overall charge.

Choose the smallest numbers of each ion that make the charges add to zero.

1. Aluminium oxide: Al³⁺ and O²⁻
2. 2 × (3+) = 6+ and 3 × (2−) = 6−
3. 2 aluminium ions and 3 oxide ions: Al₂O₃
4. Electrons transferred: 2 × 3 = 6

Brackets go round an ion of several atoms when you need more than one: calcium hydroxide is Ca(OH)₂, not CaOH₂.