Isomers are chemical compounds with the same molecular formula, but which are different in some way. The major types are constitutional isomers, stereoisomers, and enantiomers (the last one is actually a subset of the second-to-last one, but we’ll get there in a minute).

Constitutional, or structural, isomers have the same number of atoms, but they are arranged in a different order. For example, four carbon atoms and ten hydrogen atoms can be arranged in two different ways.