WJEC Chemistry for AS Level Student Book: 2nd Edition

You should be able to demonstrate and apply knowledge and understanding of: ▪ Formulae of common compounds and common ions and how to write formulae for ionic compounds. ▪ Oxidation numbers of atoms in a compound or ion. ▪ How to construct balanced chemical equations, including ionic equations, with appropriate state symbols. Topic contents 11 Formulae of compounds and ions 12 Oxidation numbers 13 Chemical and ionic equations 15 Test yourself Formulae and equations 1.1 In chemistry, each element has a symbol. A symbol is a letter or two letters which stand for one atom of the element. Formulae are written for compounds. Formulae consist of the symbols of the elements present and numbers that show the ratio in which the atoms are present. Using symbols and formulae enables you to write equations for chemical reactions. Atoms are neither created nor destroyed in a chemical reaction, therefore when you write a chemical equation, the same number of atoms of each element must be present on each side of the equation. This is done by balancing the equation. Maths skills • Use ratios to write formulae and construct and balance equations. 10