3.5.1—The shapes of, and bond angles in, molecules

VSEPR theory predicts molecular shape from the repulsion between electron domains around a central atom. Bonding pairs and lone pairs arrange as far apart as possible in three-dimensional space to minimise repulsion.

Use the central atom to count electron domains: a single, double or triple bond counts as one domain, while a lone pair is also one domain. Identify the electron-domain arrangement, then name the molecular shape after accounting for which domains are lone pairs. Lone pairs repel more strongly than bonding pairs, so they compress the observed bond angles.

Apply the model to the assessed reference cases: BF₃ is trigonal planar (120°), CO₂ linear (180°), CH₄ tetrahedral (109.5°), NH₃ pyramidal (107°), H₂O non-linear (104.5°), SF₆ octahedral (90°), and PF₅ trigonal bipyramidal (90° and 120°).

Do not count the two or three electron pairs in a multiple bond as separate domains, and do not name a shape from the formula alone. Separate the electron-domain arrangement from the molecular shape: lone pairs occupy domains but are not shown as bonded atoms in the molecular shape.