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VSEPR theory
Valence shell electron pair repulsion (VSEPR) theory is a model used, in chemistry, to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms.〔Jolly, W. L.,''Modern Inorganic Chemistry,'' McGraw-Hill, 1984, p.77-90. ISBN 0-07-032760-2〕 It is also named Gillespie–Nyholm theory after its two main developers. The acronym "VSEPR" is pronounced either "''ves''-per"〔Petrucci R.H., Harwood W.S. and Herring F.G. ''General Chemistry: Principles and Modern Applications'' (Prentice-Hall 8th ed. 2002) p.410 ISBN 0-13-014329-4〕 or "vuh-''seh''-per" by some chemists.
The premise of VSEPR is that the valence electron pairs surrounding an atom tend to repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecule's geometry. Gillespie, now a Professor Emeritus at McMaster University, has emphasized that the electron-electron repulsion due to the Pauli exclusion principle is more important in determining molecular geometry than the electrostatic repulsion.〔R.J. Gillespie (2008), Coordination Chemistry Reviews vol.252, pp.1315-1327, ''Fifty years of the VSEPR model〕
VSEPR theory is based on observable electron density rather than mathematical wave functions and hence unrelated to orbital hybridisation, although both address molecular shape. While it is mainly qualitative, VSEPR has a quantitative basis in quantum chemical topology (QCT) methods such as the electron localization function and the quantum theory of atoms in molecules (QTAIM).〔
==History==
The idea of a correlation between molecular geometry and number of valence electrons (both shared and unshared) was originally proposed in 1939 by Ryutaro Tsuchida in Japan, and was independently presented in a Bakerian Lecture in 1940 by Nevil Sidgwick and Herbert Powell of the University of Oxford.〔http://rspa.royalsocietypublishing.org/content/176/965/153.abstract N.V.Sidgwick and H.M.Powell, Proc.Roy.Soc.A 176, 153–180 (1940) Bakerian Lecture. Stereochemical Types and Valency Groups〕 In 1957, Ronald Gillespie and Ronald Sydney Nyholm of University College London refined this concept into a more detailed theory, capable of choosing between various alternative geometries.〔R.J.Gillespie and R.S.Nyholm, Quart.Rev. 11, 339 (1957)〕〔R.J.Gillespie, J.Chem.Educ. 47, 18(1970)〕
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