About Valence Shell electron-pair repulsion (VSEPR) Theory

VSEPR Theory

Valence –Shell electron-pair repulsion (VSEPR) Theory Predicts the shapes and geometries of molecules which may pr not obey the octet rule but have only single bonds. The model extensively uses the number of electron pairs surrounding the central atom to predict geometry of individual molecules and icons. It was being developed by Gillespie and Nyholm. VSEPR Theory is usually compared with valence bond theory. Which address molecular shape through orbitals that are energetically accessible for bonding.

Valence Bond theory concerns itself with the formation of s and p bond. Molecule orbital theory is another model for understanding how atoms and electrons are assembled into molecules and poly-atomic ions. VSEPR theory has been criticised for not being quantitative, and therefore, limited to the generation of crude though structurally accurate molecule geometries of contently-bonded molecule.

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The Geometric arrangement of atoms in molecules and icon may be predicted by means of the valence-shell electron-pair repulsion (VSEPR) theory. This theory predicts the shape of molecules which may or not obey the octet rule but have only single bond. VSEPR theory may be summarized as follows:

1.     The Shape of the molecule is determined by the repulsions between all of the electron pair present in the valence shell.

2.     A loan pair of electron takes up more space around the central atom than a bond-pair, since the loan-pair is one occupied by atom while the bond pair is shared by two nuclei. It follows that repulsion between two loan pair is greater than repulsion between a loan pair and a bond pair, which in turn is greater repulsion between two bond pairs. Thus the presence of loan pairs on the central atom causes slight distortion of the bond angle from the ideal shape. If the angle between a loan pair, the central atom and a bond pair in increased, it follow that the actual bond angle between the atom must be decreased.

3.     The magnitude of repulsion between bonded pair of electrons depends on the electro negativity differnce between the central atom and the other atoms.

4.     Triple bond cause more repulsion than double bond and double bond causes more repulsion thansingle bond. With very few exceptions, the prediction based on the VSEPR theory have been shown to be correct.

The AXE Method:

 The “AXE Method” of electron counting is commonly used when applying the VSEPR theory. The A is represents the central atom and always has an implied subscript one. X is represent the number of ligands atoms bonded to A. The E id represent the number of loan electron pair surrounding the central atoms.

Following steps are given to find out the shape of a molecule:

1.     Identify the central atom and count the number of valence electron.

2.     To this add number of other atoms.

3.     If is an icon, add negative charge and subtract positive charges. Represent the total number which we have calculate above as N.

4.     On dividing N by 2 and comparing the result we obtain the shapes of molecule as given  in table.

sTotal N/2	Number of atoms linked to central atom	Shape of moleculer       or ion	Example
2	2	Linear	HgCl2 , BeCl2
3	3	Trigonal Planar	BF3
5	2	Trigonal Bipyramidal	PCl5 , PF5
4	4	Tetrahedral	CH4 , BF4-
4	3	Trigonal Pyramidal	NH3 , PCl3
4	2	Angular	H2O
5	4	Distorted Tetrahedral	SF4 , IF4+