Atoms, elements and compounds: write formulae and balanced chemical equations for the reactions in this specification.

The idea

A chemical formula shows the elements in a substance and the number of atoms of each, using symbols and subscript numbers, for example H₂O or CO₂. A balanced chemical equation uses these formulae to show reactants and products with the same number of each type of atom on both sides. You balance by adjusting the large numbers in front of formulae, called coefficients, never by changing the subscripts inside a formula. For example, hydrogen burning in oxygen is 2H₂ + O₂ → 2H₂O. State symbols (s), (l), (g) and (aq) may be added to show physical states. This skill applies to all reactions named in the specification, including neutralisation, displacement, combustion and reactions of acids with metals and carbonates.

What earns the marks

• Formulae are written correctly from the names or ions given, using the correct symbols and subscripts.
• The equation has the same number of atoms of each element on both sides.
• Coefficients are placed in front of whole formulae and are used to balance, not subscripts.
• State symbols, where required, are correct for the reaction conditions described.
• Ionic formulae use the correct charges, for example Na⁺ and O²⁻, so that the compound is electrically neutral.
• The final equation is checked atom by atom to confirm it is balanced.

Common mistakes and how to put them right

• Changing a subscript to balance, such as turning H₂O into H₂O₂.

  Put it right by changing only the coefficient in front of the formula.

• Forgetting to multiply through when a coefficient is added, for example writing 2H₂O but counting only one oxygen.

  Put it right: Recount all atoms after each change.

• Using incorrect ion charges in a formula, such as writing sodium oxide as NaO.

  Put it right by balancing charges to give Na₂O.

Exam tips

• Count atoms element by element and write the tally beside the equation until it balances.
• Balance formulae containing several elements last, and leave hydrogen and oxygen until near the end where possible.
• Check that every formula is a valid substance before balancing, because a wrong formula cannot be fixed by coefficients.