Structural isomerism is a form of isomerism in which molecules with the same molecular formula have atoms bonded together in different orders (as opposed to stereoisomerism).

Chain isomerism

Chain isomers have components of the chain distinctly ordered to create different structures. Here, 3-methylpentane has become 2-methylpentane:

Position isomerism

Positional isomers occur when a functional group changes position on the chain. In the diagram, 2-hydroxypentane has become 3-hydroxypentane:

Functional group isomerism

Functional group isomerism is when a functional group splits up and becomes a different group. Here, butan-1-ol has become ethoxyethane: