Isomers, which are monomers of the same elemental composition, are in different forms. For example, carbon isomers of graphite, diamond, oxygen isomers of oxygen, ozone

And compounds with the same molecular formula, but has a different structure of the phenomenon is called isomerism; with the same molecular formula but the structure of the different compounds are isomers of each other, for example, pentane C5H12, there are three isomers of ortho-pentane, iso-pentane, neo-pentane