<Table> <Tr> <Td_colspan="2"> </Td> </Tr> <Tr> <Td_colspan="2"> Pure water (H O) is an example of a compound: the ball - and - stick model of the molecule (above) shows the spatial association of two parts hydrogen (white) and one part (s) oxygen (red) </Td> </Tr> </Table> <Tr> <Td_colspan="2"> Pure water (H O) is an example of a compound: the ball - and - stick model of the molecule (above) shows the spatial association of two parts hydrogen (white) and one part (s) oxygen (red) </Td> </Tr> <P> Any substance consisting of two or more different types of atoms (chemical elements) in a fixed stoichiometric proportion can be termed a chemical compound; the concept is most readily understood when considering pure chemical substances . It follows from their being composed of fixed proportions of two or more types of atoms that chemical compounds can be converted, via chemical reaction, into compounds or substances each having fewer atoms . The ratio of each element in the compound is expressed in a ratio in its chemical formula . A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, and subscripts to indicate the number of atoms involved . For example, water is composed of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H O. In the case of non-stoichiometric compounds, the proportions may be reproducible with regard to their preparation, and give fixed proportions of their component elements, but proportions that are not integral (e.g., for palladium hydride, PdH (0.02 <x <0.58)). </P> <P> Chemical compounds have a unique and defined chemical structure held together in a defined spatial arrangement by chemical bonds . Chemical compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or the subset of chemical complexes that are held together by coordinate covalent bonds . Pure chemical elements are generally not considered chemical compounds, failing the two or more atom requirement, though they often consist of molecules composed of multiple atoms (such as in the diatomic molecule H, or the polyatomic molecule S, etc .). Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): its CAS number . </P>

Properties of a compound are different from its constituents