Formulae for Ionic Compounds

Ionic compound formula

Shows ratio of positive and negative ions

Overall charge

Must equal zero (charges balance)

Metal written first

Then non‑metal

Swap and drop rule

Use ion charges to determine subscripts

Group 1 metals

Form +1 ions

Group 2 metals

Form +2 ions

Group 3 metals

Form +3 ions

Group 7 ions

–1 charge

Group 6 ions

–2 charge

Group 5 ions

–3 charge

Polyatomic ions

Charged groups of atoms (e.g., SO₄²⁻, NO₃⁻)

Brackets rule

Use brackets when more than one polyatomic ion is needed

Example Na⁺ + Cl⁻

NaCl

Example Mg²⁺ + 2Cl⁻

MgCl₂

Example Al³⁺ + 3F⁻

AlF₃

Example Ca²⁺ + SO₄²⁻

CaSO₄

Example 2NH₄⁺ + SO₄²⁻

(NH₄)₂SO₄

Example Mg²⁺ + 2NO₃⁻

Mg(NO₃)₂

Ionic formula shows

Simplest whole‑number ratio of ions

Charges not shown in final formula

Only subscripts used