An isomer is a molecule that has the same chemical formula as another molecule but differs in the arrangement of its atoms. This means that isomers can have different physical and chemical properties despite having the same number of each type of atom. There are two main types of isomers: structural isomers, which differ in the connectivity of their atoms, and stereoisomers, which have the same connectivity but differ in the spatial arrangement of their atoms. Isomers play a crucial role in chemistry and biology. For example, the isomers of glucose include alpha-glucose and beta-glucose, which have different properties and functions in living organisms. Understanding isomers is essential for fields like pharmacology, where different isomers of a drug can have varying effects on the body.