bond angles

Meaning: Electron groups around a central atom repel each other and spread as far apart as they can (VSEPR). The number of groups sets the hybridization and the ideal angle.

Four groups, sp³: about 109.5°, tetrahedral (CH₄, alkane carbons). Three groups, sp²: about 120°, trigonal planar (alkene carbons, carbonyl carbons). Two groups, sp: 180°, linear (alkynes, nitriles, CO₂).

A lone pair takes up more space than a bonding pair, so it squeezes the bonds together. NH₃ is about 107° and H₂O about 104.5°.

On the MCAT: You are given a structure and asked to count electron groups (bonded atoms plus lone pairs) to assign hybridization, then predict the angle, shape, or whether atoms lie in one plane. Passages also use small rings: cyclopropane has C–C–C angles of 60°, far from 109.5°, so it has angle strain and reacts readily in ring-opening reactions. Another common setup compares angles across a series, such as CH₄, NH₃ and H₂O.

Common confusion: Students count bonded atoms and forget lone pairs, or count a double bond as two groups. A double or triple bond counts as one electron group. So the carbon in formaldehyde (H₂C=O) has three groups and angles near 120°. Also, water's oxygen has tetrahedral electron geometry but a bent molecular shape, which is why its angle is near 104.5°, not 180°.