<P> An isomer (/ ˈaɪsəmər /; from Greek ἰσομερής, isomerès; isos = "equal", méros = "part") is a molecule with the same molecular formula as another molecule, but with a different chemical structure . Isomers contain the same number of atoms of each element, but have different arrangements of their atoms . Isomers do not necessarily share similar properties, unless they also have the same functional groups . There are two main forms of isomerism (/ ˈaɪsəməˌrɪzəm, aɪ ˈsɒ - /): structural isomerism (or constitutional isomerism) and stereoisomerism (or spatial isomerism). </P> <P> In structural isomers, sometimes referred to as constitutional isomers, the atoms and functional groups are joined together in different ways . Structural isomers have different IUPAC names and may or may not belong to the same functional group . This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; position isomerism, which deals with the position of a functional group on a chain; and functional group isomerism, in which one functional group is split up into different ones . </P>

Species with different arrangements of atoms but having no structural differences