VSEPR Theory VSEPR Theory Valence shell electron pair repulsion (VSEPR) rules are a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers. The acronym "VSEPR" is sometimes pronounced "vesper" for ease of pronunciation; however, the phonetic pronunciation is technically more correct. The premise of VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry. The number of atoms bonded to a central atom plus the number of lone pairs of its nonbonding valence electrons is called its steric number. VSEPR theory is usually compared and contrasted with valence bond theory, which addresses molecular shape through orbitals that are energetically accessible for bonding. Valence bond theory concerns itself with the formation of sigma and pi bonds. Molecular orbital theory is another model for understanding how atoms and electrons are assembled into molecules and polyatomic ions.
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VSEPR theory has long been criticized for not being quantitative, and therefore limited to the generation of "crude", even though structurally accurate, molecular geometries of covalent molecules. However, molecular mechanics force fields based on VSEPR have also been developed. VSEPR Theory Definition :This theory is very useful in predicting the geometry or shape of a number of polyatomic molecules or ions of non-transition elements. This theory was proposed for the first time by Sidgwick and Powell in 1940 and developed by Gillespie and Nyholm in 1957. According to this theory “The shape of a given species (molecule or ion) depends on the number and nature of electron pairs surrounding the central atom/ion of the species.� 1. Spatial arrangement of electron pairs around the central atom/ion of a given molecule/ion The electrons already present in the valence shell of the central atom/ion of a given species(=a) plus the electrons acquired by the central/ion as a result of bonding with the other atoms (=b), i.e, (a+b)/2, gives the number of electron pairs present in the valence shell of the central atom/ion. This theory assumes that these electron pairs occupy localized orbitals which they arrange themselves in space in such a way that they keep apart from one another as far as possible so that they may experience minimum electrostatic repulsion between them and hence may give minimum energy and maximum stability to the species. 2. Regular and Irregular geometry of molecule/ion
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The electron pairs surrounding the central atom/ion are either only bonding pairs or, some of them are bonding pairs and the remaining are lone pairs. If the central atom/ion is surrounded only by the bonding pairs, then the species has a regular geometry, i.e, there is no distortion in the shape of the species. If however, the central atom/ion is surrounded by the bonding orbitals and lone pairs, the bond angle gets altered from the value expected for a particular geometry of the molecule or ion. The change in the bond angle changes the geometry of the molecule too, thus distorting the shape of the molecule.
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