Diagram showing how the term 'chemical substances' can be broken down. An arrow from this term is labelled 'classify by structure', and splits into two subterms which are 'molecular' and 'non-molecular'. 'Each of these subterms then has an arrow labelled 'classify by bond type' leading to further subclassifications. The arrow from 'molecular' leads to 'molecular covalent'. The arrow from 'non-molecular' splits into three, which lead to 'non-molecular covalent', 'non-molecular ionic' and 'non-molecular metallic'.