VSEPR theory - VALENCE SHELL ELECTRON PAIR REPULSION THEORY

 VALENCE SHELL ELECTRON PAIR REPULSION THEORY (VSEPRT)

It was described by Sidgwick and Powel in 1940 and further developed by Gillespie and Nyholm in 1957

    ·       Shape of molecule depends upon the no of bonded or non-bonded electron pairs around  the central atom

    ·       These electron pairs having negative charge repel each other and tend to occupy such position in space that minimize repulsion and thus maximize distance between them

    ·        If the central atom possesses only bonded pairs of electrons along with identical atoms then shape of the compound is symmetrical and according to Sidgwick & Powel.

Eg.        

CO2 — 180° — linear

 BF3 — 120° — triangular

CH4 — 109° 28' — tetrahedral

PCl5 - 120° and 90°   - Trigonal bipyramidal

 If the central atom possesses bonded pair of electrons as well as lone pair of electrons, then shape of the molecule will be unsymmetrical ie. the original bond angle will be disturbed due to repulsion between lone pair of electrons.

 Similarly, on having different type of side atoms, molecule becomes unsymmetrical due to unequal force of repulsion between e– .

  Order of repulsion is -  

  lp – lp > lp – bp > bp – bp

 Bond angle α     1/ no of lone pair

By increasing no. of lone pair of electrons, bond angle is decreased approx. by 2.5°.

 eg.:-        

CH4       NH3      H2O --------> sp3

109°      107°       105°              hybridization

d). The VSEPR model considers double and triple bonds to have slightly greater repulsive effects than single bonds because of the repulsive effect of a electrons.

For example, the H3C-C-CH3 angle in (CH3)2C=CH2 is smaller and H3C-C=CH2 angle is larger than the trigonal 120°

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