Chemical substance 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. An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it
Magic cube classes not enough to qualify for the next class. <br>The smallest normal simple magic cube is order 3. Minimum correct summations required = 3"m" + 4 Each of the 3"m" planar arrays must be a simple magic square. The 6 oblique squares are also simple magic. The smallest normal diagonal magic cube is order 5.<br> These squares were referred to as ‘Perfect’ by Gardner and others! At the same time he referred to Langman’s 1962 pandiagonal cube also as ‘Perfect’.<br> Christian Boyer and Walter Trump now consider this "and" the next two classes to be "Perfect". (See "Alternate Perfect" below).<br>A. H.