ValenceShellElectronPairRepulsionModell

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model in chemistry used to predict the three-dimensional arrangements of atoms in molecules and ions. The central premise is that electron pairs in the valence shell of an atom arrange themselves as far apart as possible to minimize repulsion. These electron domains include bonding pairs between atoms and lone pairs of electrons not shared with another atom. Distinguishing between X (bonding pairs) and E (lone pairs) in the AXE notation helps identify the surrounding geometry: A is the central atom, X is the number of atoms bonded to A, and E is the number of lone electron pairs on A.

Common geometries arise from simple AXn categories: linear (AX2), trigonal planar (AX3), tetrahedral (AX4), trigonal bipyramidal (AX5), and octahedral (AX6). Lone pairs reduce the observed bond angles and can alter the molecular geometry compared with the electron-domain geometry. For example, NH3 (AX3E1) is a trigonal pyramidal molecule, while H2O (AX2E2) is bent. Double and triple bonds count as a single electron domain in first-order VSEPR predictions.

History and scope: VSEPR was developed in the mid-20th century and popularized by researchers such as Ronald J. Gillespie. The model provides qualitative predictions that correlate well with the shapes of many main-group molecules, but it has limitations. It may be less reliable for transition-metal complexes, hypervalent species, or cases requiring detailed energetics. It does not directly predict bond lengths or electronic spectra; more rigorous quantum-mechanical methods are used for quantitative results.