Are There Triangular Molecules?

At a molecular level, atoms are always seeking shapes or combinations with other atoms that generate the least amount of strain and the most stability. Although the triangle is a strong shape in engineering, a molecule formed of three atoms in a triangle is both relatively unstable and uncommon. One such molecule, cyclopropane (C₃H₆), has three carbon atoms arranged in a triangular ring, with pairs of hydrogen atoms attached to each carbon. The angles of the bonds between the three carbons are 60° and do not fully accommodate the electron orbitals of the carbons, causing ring strain. This arrangement also has torsional strain, meaning the molecule would twist if it could. Given that the carbons and their hydrogens would have less strain in another formation, cyclopropane is fairly reactive. (In contrast, a hexagonal benzene ring [C₆H₆] has 120° bond angles, a common arrangement that better accommodates the electron orbitals and is significantly more stable.)

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Other molecules can also be considered triangular, though they do not form triangular rings. For example, boron trifluoride (BF₃), sulfur trioxide (SO₃), formaldehyde (H₂CO), and phosgene (COCl₂) all have one atom at the center and three peripheral atoms at the corners of an equilateral triangle in a single plane. These molecules have what is called trigonal planar molecular geometry. For three-dimensional examples, ammonia (NH₃) has what is called trigonal pyramidal geometry, with three hydrogen atoms supporting a nitrogen atom at the pyramid’s apex. Methane (CH₄) forms a pyramidal (tetrahedral) shape with one central carbon and four hydrogens arranged at 109.5°, and white phosphorus, or tetraphosphorus (P₄), exists as a tetrahedral pyramid of four phosphorus atoms with no central atom.