Valence Sheel Electron Pair Repulsion Theory (VSEPR Theroy)
Postulates of VSEPR Theory
Valence Sheel Electron Pair Repulsion Theory (VSEPR Theroy) was proposed by Sidwick and Powell in 1940 and later on modified by Gillespie and Nyholm in 1957.
This theory predict the shape of simple molecules and ions on the basis of repulsion of electron pairs present in the valence shell of their central atom. Some Important Postulates of this Theory are given below-
1. Electrons involved in the bond formation is called bonding electrons or bond pair (B.P.) and the rest electrons are called lone pairs (L.P.).
2. Electron pairs in valence shell repel one-another as electron clouds are negatively charged.
3. These electron pairs occupy the space at maximum distance for minimum repulsion.
4. The most stable geometrical arrangement of 2,3,4,5,6 electron pairs is linear, triangular, tetrahedral, triangular bipyramidal and octahedral respectively.
5. The central atom in a molecule is surrounded by only B.P. the molecule has regular or symmetrical geometry but in case of B.P. and L.P. the molecule does not have regular geometry.
6. A lone pair occupies more space than a bond pair because lone pair attached with only one atom. Hence the order of repulsion is-
L.P - L.P. > L.P - B.P. > B.P - B.P.
Greater the repulsion, smaller the bond angle.
7. Multiple bonds does not affect the gross geometry of the molecules rather the geometry is exclusively decided by B.P. and L.P.
8. A lone pair and double bond repulsion is much greater than a lone electron and double bond repulsion.
9. A lone pair and a single bond repulsion is larger than a lone pair and double bond repulsion.
