IMAT Chemistry Practice Exam 2026 – Complete Study Resource

Trigonal planar geometry arises when a central atom has three regions of electron density arranged in one plane, about 120° apart. This arrangement is described by sp2 hybridization: mixing one s and two p orbitals to form three equivalent sp2 hybrids. These three hybrids lie in a plane and point 120° from each other, forming the three sigma bonds to surrounding atoms. The remaining p orbital stays unhybridized and can participate in pi bonding if the molecule has a double bond, but it does not affect the in-plane arrangement of the three sigma bonds. That’s why sp2 is the correct description for trigonal planar shapes, as opposed to sp3 (four bonds in a tetrahedral arrangement), sp (two bonds in a linear arrangement), or dsp2 (square planar geometries more common in certain transition-metal complexes).