价层电子对互斥理论VSEPR theory

价层电子对数 = 成键原子数 + 孤电子对数（双键、三键按一对计）。2、3、4、5、6 对电子分别呈直线、平面三角形、四面体、三角双锥和八面体排布。排斥强弱为：孤对–孤对 > 孤对–成键对 > 成键对–成键对，所以 NH₃ 键角（107°）和 H₂O 键角（104.5°）都小于 109.5°。分子形状只看原子位置，孤对不计入形状名称。

The steric number is the number of bonded atoms plus lone pairs, with a double or triple bond counted once. Two to six pairs adopt linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral arrangements. Repulsion runs lone pair–lone pair > lone pair–bond pair > bond pair–bond pair, which is why the bond angles of NH₃ (107°) and H₂O (104.5°) are below 109.5°. The shape name describes only the atom positions, not the lone pairs.

1940 年西奇威克与鲍威尔指出，分子形状取决于中心原子价层电子对的总数。1957 年吉莱斯皮与尼霍姆系统化了这一思想，引入孤对排斥更强等规则，形成今天的 VSEPR 理论。吉莱斯皮此后数十年不断完善并推广了该模型。

In 1940 Sidgwick and Powell pointed out that molecular shape depends on the total number of electron pairs in the central atom's valence shell. In 1957 Gillespie and Nyholm systematised the idea, adding rules such as the stronger repulsion of lone pairs, which produced the modern theory. Gillespie went on refining and promoting the model for decades.

VSEPR 对过渡金属配合物（受 d 电子和晶体场影响）、重碱土金属卤化物（如气态 CaF₂、BaF₂ 为弯曲形）以及某些 AX₆E 型分子（如 [SeCl₆]²⁻ 为正八面体）预测不准。吉莱斯皮后来提出配体密堆积模型作为补充。电子定位函数（ELF）等量子化学方法为电子对区域提供了理论依据。

VSEPR fails for transition-metal complexes, where d electrons and ligand-field effects dominate, for heavy alkaline-earth dihalides (gaseous CaF₂ and BaF₂ are bent), and for some AX₆E species ([SeCl₆]²⁻ is a regular octahedron). Gillespie later proposed the ligand close-packing model as a complement. Quantum-chemical tools such as the electron localisation function (ELF) give the electron-pair domains a theoretical footing.