In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term metamer was formerly used for the same concept. For example, butanol , methyl propyl ether , and diethyl ether have the same molecular formula but are three distinct structural isomers. The concept applies also to polyatomic ions with the same total charge. A classical example is the cyanate ion and the fulminate ion . It is also extended to ionic compounds, so that (for example) ammonium cyanate and urea are considered structural isomers, and so are methylammonium formate and ammonium acetate . Structural isomerism is the most radical type of isomerism. It is opposed to stereoisomerism, in which the atoms and bonding scheme are the same, but only the relative spatial arrangement of the atoms is different. Examples of the latter are the enantiomers, whose molecules are mirror images of each other, and the cis and trans versions of 2-butene. Among the structural isomers, one can distinguish several classes including skeletal isomers, positional isomers (or regioisomers), functional isomers, tautomers, and structural topoisomers. Skeletal isomerism A skeletal isomer of a compound is a structural isomer that differs from it in the atoms and bonds that are considered to comprise the "skeleton" of the molecule. For organic compounds, such as alkanes, that usually means the carbon atoms and the bonds between them. For example, there are three skeletal isomers of pentane: n-pentane (often called simply "pentane"), isopentane (2-methylbutane) and neopentane (dimethylpropane). If the skeleton is acyclic, as in the above example, one may use the term chain isomerism. Position isomerism (regioisomerism) Position isomers (also positional isomers or regioisomers) are structural isomers that can be viewed as differing only on the position of a functional group, substituent, or some other feature on a "parent" structure. For example, replacing one of the 12 hydrogen atoms –H by a hydroxyl group –OH on the n-pentane parent molecule can give any of three different position isomers: Functional isomerism Functional isomers are structural isomers which have different functional groups, resulting in significantly different chemical and physical properties. An example is the pair propanal H3C–CH2–C(=O)-H and acetone H3C–C(=O)–CH3: the first has a –C(=O)H functional group, which makes it an aldehyde, whereas the second has a C–C(=O)–C group, that makes it