Equivalent weight is the mass of one equivalent’s valency. Its definition varies depending on which kind of particle is referred to. The equivalent weight of an element is the atomic weight divided by its chemical valency. It indicates the amount of mass that can combine with one gram atom of hydrogen (= 1.008 grams) or that could replace it in chemical compounds. For example, the equivalent weight of oxygen is 16/2 = 8, of aluminum 26.97/3 = 8.99. The equivalent weight of an ion is the ion mass divided by the electronic charge of an ion (specified in units of elementary charge). If a type of ion occurs in several valence states, e.g. as it is the case with iron (Fe2+ and Fe3+), it also has several equivalent weights. The equivalent weight of a chemical compound is the quotient of the molecular mass and the number of equivalents of the substance the compound reacts with. The equivalent weight of radicals is the sum of the atomic masses of the atoms of which the radical consists of, divided by the valence of the radical.