Isomers are compounds with the same molecular formula but different arrangements of atoms. There are two main types of isomerism.Structural isomers have the same molecular formula but different structural formulae. The atoms are bonded together in a different order. Pentane and 2-methylbutane are structural isomers of C5H12.Structural isomerism can arise from a different carbon chain, a different position of the functional group, or a different functional group.Stereoisomers have the same structural formula but a different arrangement of atoms in space. E/Z isomerism and optical isomerism are the two types of stereoisomerism. A bond would need to be broken to turn one stereoisomer into the other.