3.5 Shapes of 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.

Predict an unfamiliar molecule or ion in a fixed order: identify the central atom, count its bonding pairs and lone pairs, treat each single/multiple bond as one electron domain, choose the electron-domain arrangement, and then name the molecular shape after omitting lone pairs from the visible atom layout.

Use the reference VSEPR arrangements to transfer the method to analogous species. For example, a central atom with five bonding domains and no lone pair gives a trigonal-bipyramidal arrangement, while four bonding domains around carbon give a tetrahedral arrangement. A central atom with three bonds and one lone pair has a pyramidal molecular shape.

Start from the ideal angle for the electron-domain arrangement, then explain deviations: lone pair–lone pair repulsion is strongest, lone pair–bonding pair repulsion is next, and bonding-pair repulsion is weakest. Multiple bonds count as one domain for shape, but their greater electron density can affect repulsion and angle comparisons.

Do not select a shape from the molecular formula alone or treat a lone pair as a bonded atom. The conclusion must follow from the central atom's domain count and the supported VSEPR comparison; do not infer an exact angle when the relevant arrangement or source-supported reference is not established.