VSEPR geometry

Meaning: VSEPR stands for valence shell electron pair repulsion. Regions of electron density around a central atom, called electron domains, repel one another and arrange themselves as far apart as possible. A lone pair, single bond, double bond or triple bond each counts as one domain. Two domains give linear (180°), three trigonal planar (120°), four tetrahedral (109.5°), five trigonal bipyramidal (90° and 120°), and six octahedral (90°). Lone pairs take up more room than bonding pairs, so they compress bond angles: NH₃ is about 107° and H₂O about 104.5°.

Equation: Steric number = number of atoms bonded to the central atom + number of lone pairs on the central atom (both counts are unitless).

On the MCAT: You are given a formula or Lewis structure and asked for the electron geometry, the molecular geometry (for example SF₄ seesaw, ClF₃ T-shaped, XeF₄ square planar), or a comparison of bond angles. Shape also decides polarity: bond dipoles cancel in linear CO₂, so it is nonpolar, but not in bent H₂O, so it is polar. In a trigonal bipyramid, lone pairs go in equatorial positions.

Common confusion: Electron geometry counts lone pairs; molecular geometry describes only where the atoms are. Water has tetrahedral electron geometry but bent molecular geometry. Also, a double bond is one domain, not two.