monovalence

Monovalence is the chemical property of an atom, ion, or functional group that gives it the capacity to form a single bond in a chemical species. In classic terms, a monovalent element or group has valence one, meaning it can contribute or accommodate one bonding electron pair to form a single covalent bond. The term derives from Latin mono- “one” and valere “to be strong or to be of value.”

Historically, valence was used to describe the bonding capacity of atoms before modern theories of orbitals and oxidation states were developed. Monovalence is most commonly associated with hydrogen, whose single valence electron forms one covalent bond, and with the alkali metals and halogens, which commonly adopt a +1 or −1 oxidation state and thus participate in one primary bond in many compounds. In covalent compounds, these elements tend to complete their outer shell by forming one bond with another atom.

In contemporary chemistry, the idea of monovalence is treated as a tendency rather than an absolute property. Many elements display variable valence depending on the compound and oxidation state; resonance, coordination, and ionic bonding can blur the notion of a single, fixed valence. Monovalent behavior is still a useful shorthand in describing typical bonding patterns, especially for hydrogen, alkali metals, and halogens, but it does not capture all complexities of chemical bonding.

See also: valence, multivalence, oxidation state, valence electron.