divalent

Divalent is a term used in chemistry to describe a species that has a valence of two, meaning it can form two covalent bonds or carry a two-unit electric charge. In practice, it is most often applied to ions that carry a 2+ or 2− charge: divalent cations with a +2 oxidation state and divalent anions with a −2 charge.

Common examples of divalent cations include calcium (Ca2+), magnesium (Mg2+), iron(II) (Fe2+), and zinc (Zn2+). Divalent anions include carbonate (CO3^2−), sulfate (SO4^2−), and oxalate (C2O4^2−). Some elements can form divalent ions in certain chemical environments, while others exhibit multiple oxidation states and are not always divalent.

The term is also used for ligands or functional groups capable of forming two bonds or coordinating with two partners, and it is often interchangeable with bivalent in older or different literature. Divalent ions play important roles in biology, geochemistry, and materials science; for example, Ca2+ serves as a cellular second messenger and contributes to the hardness of water alongside Mg2+.

In coordination chemistry, many transition-metal ions are observed in the +2 oxidation state and form complexes with various ligands; the exact valence depends on the chemical context. Distinctions from monovalent (one valence) and multivalent (three or more valence) terms reflect the typical bonding behavior of the species involved.