Chemical compounds that have the same molecular formulae but different molecular structures or different arrangements of atoms in space. Structural isomers have different molecular structures, i.e. they may be different types of compound or they may simply differ in the position of the functional group in the molecule. Structural isomers generally have different physical and chemical properties. Stereoisomers have the same formula and functional groups, but differ in the arrangement of groups in space. Optical isomers are examples of stereoisomers (see optical activity).