threevalent

Threevalent is an adjective used in chemistry to describe atoms, ions, or groups that can form three covalent bonds, i.e., have a valence of three. In formal literature, the term trivalent is more common, but threevalent is encountered in some overlapping contexts. Valence denotes the combining capacity of an atom, which can vary with bonding partners, resonance, and lone pairs.

Examples and contexts

- Nitrogen in ammonia (NH3) and most amines is threevalent, forming three sigma bonds and typically possessing one lone pair.

- Phosphorus in phosphorus trihalides (PCl3) is threevalent, forming three bonds to halogens.

- Boron in boron trifluoride (BF3) is threevalent and electron-deficient, often acting as a Lewis acid.

- Aluminum in aluminum trichloride (AlCl3) is commonly described as trivalent, with a +3 oxidation/valence state in many compounds.

- In solid-state chemistry, cations such as Fe3+, Al3+, and Cr3+ carry a +3 charge and are described as threevalent centers within oxides and other lattices.

Geometry and bonding

Threevalent centers can exhibit different geometries depending on electron domains. A three-bond center with no lone pairs, such as BF3, adopts a trigonal planar geometry. If the center bears lone pairs (for example, NH3 has three bonds plus one lone pair), the overall arrangement appears trigonal pyramidal or tetrahedral in electron-domain geometry, influenced by lone-pair repulsion.

Usage and terminology

The term threevalent is largely synonymous with trivalent. In contemporary scientific writing, trivalent is the preferred term, with threevalent appearing mainly in older literature or specific stylistic contexts. See also: valence, trivalent, oxidation state.