Writing chemical formulae

To write a formula, swap the numbers across: this is called criss-crossing.

Covalent compounds: write the symbols, write each one's valency underneath, then cross them over as small numbers (subscripts). A 1 is not written.

• H (1) and Cl (1): HCl.
• H (1) and S (2): H₂S.
• C (4) and Cl (1): CCl₄.

Ionic compounds: write the cation first, then the anion, with their charges underneath, and criss-cross the numbers (not the + or – signs):

• Ca²⁺ and Cl⁻: CaCl₂.
• Al³⁺ and O²⁻: Al₂O₃.

Simplify. If both numbers can be divided by the same number, divide them:

• Mg²⁺ and O²⁻ give Mg₂O₂, which is written MgO.
• Ca²⁺ and CO₃²⁻ give CaCO₃.

Check: the charges must balance to zero. In CaCl₂, one Ca²⁺ (+2) is balanced by two Cl⁻ (–1 each).

Brackets go round a polyatomic ion when you need two or more of it:

• Mg(OH)₂, Al(OH)₃ (not AlOH₃), Al₂(SO₄)₃.
• With only one, no brackets: CaCO₃.

Finally, the charges of the ions are not written in the formula of the compound: write CaCl₂, not Ca²⁺Cl₂⁻.