Sunday, May 15, 2011

Valence Shell Electron Pair Repulsion (VSEPR) Theory


    Sidgwick & Powell in 1940 formulated a theory based on the repulsions between electron pairs, known as Valence shell electron pair repulsion theory. The various postulates of this theory are as follows;
i).    The unpaired electrons in the valence shell of central atom form bond pairs with surrounding atoms while paired electrons remain as lone pairs.
ii).    The electron pairs surrounding the central atom repel each other. Consequently, they stay as far apart as possible in space to attain stability.
iii).    The geometry and shape of the molecule depends upon the number of electron pairs (bond pair as well as lone pair) around the central atom. The geometrical arrangement of electron pairs with different number of electron pairs around central atom are as follows;
iv).    The force of repulsion is minimum when the electron pairs are far away from one another.
v).    The force of repulsion among the bond pairs and the lone pairs is not same. The order of force of repulsion is as follows;
    lp – lp >lp – bp > bp – bp
    A molecule has Regular geometry if it has no lone pair around it, but if it has lone pairs over it, the molecule has Irregular geometry.

2 comments:

  1. is the geometry and shape of the molecule fixed?

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  2. Definition of spin Multiplicity Rule
    According to spin Multiplicity Rule " transition between states of different multiplicity, S, are forbidden.
    It is a selection rule for electronic transition in complexes. And another selection Rule is Laporte Selection Rule. Which you read in next page of website www.chemistrynotesinfo.blogspot.com

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