I.

Introduction

Isomer, one of two or more molecules that contain the same atoms but have different arrangements of those atoms. Molecules are groups of atoms bonded together, and a substance made of molecules is called a compound. Two compounds made of isomers may differ in their physical, chemical, and biological properties, even though they contain the same atoms in their molecules. The differences arise from the different arrangement of atoms in the molecules. One compound may be a useful drug, and the other ineffective; or one may be harmless, and the other explosive. Scientists study isomers because these substances each possess unique properties, many of which are essential to living organisms. Other isomers are important in the pharmaceutical, food, and chemical industries. Scientists also study isomers to learn about how the properties of a substance are determined by its molecular structure. Chemists classify isomers as either structural isomers or stereoisomers, depending on how the arrangement of atoms in the molecules differs (See also Atom & Atomic Theory).