Writing Chemical Formulae

A chemical formula shows the type and number of atoms in one formula unit of a substance. Getting formulae right is the foundation for writing balanced equations.

Elements and Diatomic Molecules

Some elements exist as single atoms (e.g. Na, Fe, Ar). Others exist as diatomic molecules — pairs of atoms bonded together. You must memorise these seven:

• H₂ — hydrogen
• N₂ — nitrogen
• O₂ — oxygen
• F₂ — fluorine
• Cl₂ — chlorine
• Br₂ — bromine
• I₂ — iodine

Memory trick: Have No Fear Of Ice Cold Beer (H, N, F, O, I, Cl, Br)

Formulae from Valency (Covalent Compounds)

At GCSE, common covalent formulae should be memorised rather than worked out from first principles. Learn these by heart: CO₂ (carbon dioxide), H₂O (water), NH₃ (ammonia), HCl (hydrogen chloride), CH₄ (methane), SO₂ (sulfur dioxide). Reasoning about bond order and double bonds is A-level work — for now, recognise the formula and use it correctly in equations.

Formulae from Ionic Charges (Ionic Compounds)

For ionic compounds, the charges must balance to zero. Sodium (Na⁺) combined with chloride (Cl⁻) gives NaCl — charges cancel. Magnesium (Mg²⁺) combined with oxide (O²⁻) gives MgO. Magnesium (Mg²⁺) combined with chloride (Cl⁻) requires two chlorides to balance: MgCl₂.

The swap-and-drop method: swap the numbers from the charges and drop the sign. Mg²⁺ and Cl⁻ → Mg₁Cl₂ → MgCl₂. Always simplify if possible: Ca²⁺ with O²⁻ → Ca₁O₁ → CaO (not Ca₂O₂).