Trigonal

Trigonal is an adjective used in science to denote triangular or threefold symmetry. It can describe shapes, arrangements, or symmetry that derive from triangles or a threefold axis, and it is used across geometry, chemistry, and crystallography.

In chemistry, trigonal geometry appears in several common molecular shapes. Trigonal planar describes a central atom bonded to three atoms in a single plane with about 120-degree bond angles (for example, BF3). Trigonal pyramidal describes three bonds plus a lone pair, giving a roughly pyramidal shape with bond angles near 107 degrees, as seen in ammonia. Trigonal bipyramidal describes five substituents arranged with three in a plane and two above and below, yielding 90-degree and 120-degree angles as in PCl5 and related compounds.

In crystallography, trigonal refers to a crystal system or symmetry class characterized by a threefold rotational axis. Crystals in the trigonal system may display rhombohedral symmetry, and the system is treated as distinct from the hexagonal system in modern classifications. Minerals such as calcite crystallize in the trigonal (rhombohedral) system, often forming elongated rhombohedra or scalenohedra.

In mineralogy and materials science, the term trigonal is used to describe crystals or motifs that exhibit threefold symmetry or rhombohedral geometry. The concept is foundational to describing crystal habit and lattice symmetry, and it serves as a counterpart to other geometric and crystal systems in scientific nomenclature.