 For example, there was much speculation for the chemical identity of benzene, until the correct structure was described by Friedrich August Kekulé.Likewise, the idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances.For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers.