trigonal bipyramidal

Meaning: Trigonal bipyramidal is the electron-domain geometry when a central atom has five regions of electron density, which VSEPR places as far apart as possible. The hybridization is sp³d. Three equatorial positions lie in one plane at 120° to each other. Two axial positions lie above and below that plane, 90° from the equatorial positions and 180° from each other. The two kinds of position are not equivalent. Lone pairs occupy equatorial positions first, because there they have fewer 90° repulsions.

Equation: Steric number = bonded atoms + lone pairs on the central atom = 5 (a dimensionless count).

On the MCAT: You are given a formula, draw the Lewis structure, count domains, and name the electron geometry and then the molecular shape. With 0 lone pairs the shape is trigonal bipyramidal (PCl₅), with 1 it is seesaw (SF₄), with 2 it is T-shaped (ClF₃), and with 3 it is linear (XeF₂). You may then predict bond angles or polarity. PCl₅ is nonpolar because its bond dipoles cancel by symmetry, while SF₄ and ClF₃ are polar.

Common confusion: Students mix up electron geometry and molecular shape. SF₄ is trigonal bipyramidal in electron geometry but seesaw in shape, because only atoms count when naming shape. Also, the angles are not all equal: 90°, 120°, and 180° all occur. The central atom must be in period 3 or lower to expand its octet.