trigonal planar

Meaning: Three regions of electron density around a central atom repel each other and spread as far apart as possible, which puts them at the corners of an equilateral triangle in one plane. With three bonded groups and no lone pairs (AX₃ in VSEPR notation), the molecular geometry is trigonal planar and the ideal angle is 120°. The central atom is sp² hybridized: three sp² orbitals form σ bonds, and an unhybridized p orbital perpendicular to the plane can form a π bond.

On the MCAT: You are given a structure and asked for geometry, hybridization, or polarity. Typical trigonal planar centers are the carbons of alkenes, carbonyls (C=O), carboxylic acid derivatives, and carbocations, plus BF₃, NO₃⁻, and CO₃²⁻. Because the carbonyl carbon and carbocation are flat, a nucleophile can attack from either face, which explains racemization in SN1 and in additions to ketones. Resonance-delocalized atoms, such as the nitrogen of an amide, are also often planar.

Common confusion: Trigonal planar is not the same as trigonal pyramidal. Both have three bonded atoms, but trigonal pyramidal (NH₃) has a lone pair on the central atom, giving about 107° angles and a sp³ geometry. Count lone pairs first. A double bond counts as one electron group, so H₂C=O is trigonal planar at carbon, with angles near 120°.