Isomers are molecules that have the same molecular formula but different arrangements of atoms. This means that isomers have the same number of atoms of each element, but the atoms are arranged differently in space. Isomers can have different physical and chemical properties, such as boiling points, melting points, and reactivity, due to their different structures. There are several types of isomers, including structural isomers, stereoisomers, and geometric isomers.