Chemical Formulae and Equations

This lesson covers how to write chemical formulae and balanced chemical equations — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to be able to write formulae from names, construct word equations and balanced symbol equations, and understand the law of conservation of mass.

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Writing Chemical Formulae

A chemical formula shows the ratio of atoms of each element in a compound. To write a formula, you need to know the charges of the ions involved (for ionic compounds) or the number of bonds each atom forms (for covalent compounds).

Formulae of Ionic Compounds

In an ionic compound, the total positive charge must equal the total negative charge — the compound is electrically neutral overall.

Method: Using Ion Charges

1. Write down the ions and their charges.
2. Work out the ratio needed to balance the charges.
3. Write the formula using subscripts.

Worked Examples

Example 1: Sodium chloride

• Ions: Na⁺ and Cl⁻
• One Na⁺ balances one Cl⁻
• Formula: NaCl