Valence

The combining capacity of atoms or groups of atoms. For example, potassium (K+) and sodium (Na+) are monovalent, while calcium (Ca2+) and magnesium (Mg2+) are divalent. Some elements may have more than one valence state, such as iron, which can be either divalent or trivalent. This change from one valence state to another involves the transfer of an electron.