Substituent Atom (Group) An atom (group) that replaces one or more hydrogen atoms attached to a parent structure or characteristic group except for hydrogen atoms attached to a chalcogen atom. Ref: IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook . doi:10.1351/goldbook.S06076 PDF Document Rearrangement A reaction which preserves the number of rings and number of skeletal atoms and in which the molecule rearranges so that one or more atoms or groups of atoms move from one atom to another.