The concept of valence shell electron pair repulsion provides qualitative but a highly useful idea for predicating geometries of molecules. The essentials of third concept were proposed by Sedgwick and Powell in 1940 but refinements, rationalization and application were provided by Gillespie and Nyholm. According to them, Geometries of molecules can be predicted on the basis of repulsions operating of VSEPR theory are as follows:

(i)    The geometry of a molecule depends upon the total number of valence shell electron pairs (bonded or nonbonded) around the central atom in the molecule.

(ii)    Repulsion between valence shell electron pairs determines molecular shape in order to minimize electron-electron repulsion or to acquire a state of minimum energy or maximum  stability, the valence shell electron Paris keep as far apart as possible or, that maximizes the least distance between any two pairs of electrons.

(iii)    In case more than one geometrical arrangement is possible for a given number f bonded and nonbonded pairs, then the most stable arrangement is consistent with the following hypothesis:

(a) Lone pair-line pairs (   I_P-I_P  ) repulsion is greater than line pair-bond pairs (  I_P-I_P ) repulsion which in turn is greater than bond pairs-bond pairs (  b_p-b_p ) repulsion. That is
                                                         
                                                                  I_P-I_P > I_P-I_P > b_p-b_p

(b) Repulsion between electron pairs at vertices greater than 115⁰  apart can be neglected.

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