Isomerism is a phenomenon in chemistry where two or more compounds have the same molecular formula but different structural arrangements. This means that while they contain the same number and types of atoms, their properties can vary significantly due to their different structures. Isomers can be classified into two main types: structural isomers, which differ in the connectivity of atoms, and stereoisomers, which have the same connectivity but differ in the spatial arrangement of atoms. Isomerism is important in various fields, including pharmaceuticals, where different isomers of a drug can have different effects on the body. Understanding isomerism helps chemists design and synthesize compounds with specific desired properties, making it a crucial concept in organic chemistry and materials science.