Hydrogen compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry. Millions of hydrocarbons are known, and they are usually formed by complicated synthetic pathways that seldom involve elementary hydrogen. Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H, and is used when
Chemical formula a different connectivity from other molecules that can be formed using the same atoms in the same proportions (isomers). The formula (CH)CH implies a central carbon atom connected to one hydrogen atom and three CH groups. The same number of atoms of each element (10 hydrogens and 4 carbons, or CH) may be used to make a straight chain molecule, "n"-butane: CHCHCHCH. In any given chemical compound, the elements always combine in the same proportion with each other. This is the law of constant composition. The law of constant composition says that, in any particular chemical compound, all samples of