The electrotopological state formalism is developed further in conjunction with atom classification. The classification scheme is based on the characteristics of hydride groups: (1) atomic number of an atom as element identifier, (2) a valence state designation consisting of valence and simple connectivity <5 values (for each atom together with its bonded hydrogen atoms, as in -CH3 or -NH-), and (3) an aromaticity indicator. This scheme may be viewed as a three-dimensional array. In a few cases, bonded neighbor analysis is also required. The scheme is developed and illustrated. For each atom type in a molecule, the electrotopological state indices are summed. TTiese atom type E-state indices, based on a summation of E-state values, are useful for database characterization, molecular similarity analysis, and QSAR. A QSAR/ QSPR example is given for boiling point for a set of 245 alkanes and alcohols for which the standard error is 8.0 °C.

Fifteen physicochemical descriptors of side chains of the 20 natural and of 26 non‐coded amino acids are compiled and simple methods for their evaluation described. The relevance of these parameters to account for hydrophobic, steric, and electric properties of the side chains is assessed and their intercorrelation analyzed. It is shown that three principal components, one steric, one bulk, and one electric (electronic), account for 66% of the total variance in the available set. These parameters may prove to be useful for correlation studies in series of bioactive peptide analogues.

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