AB , AB , AB , AB and AB , the arrangement of electron pairs and the B atoms around the central atom A are : linear, trigonal planar, tetrahedral, trigonal-bipyramidal and octahedral, respectively. Such arrangement can be seen in the molecules like BF (AB ), CH (AB ) and PCl (AB ) as depicted below by their ball and stick models. The VSEPR Theory is able to predict geometry of a large number of molecules, especially the compounds of p -block elements accurately. It is also quite successful in determining the geometry quite-accurately even when the energy difference between possible structures is very small.
The theoretical basis of the VSEPR theory regarding the effects of electron pair repulsions on molecular shapes is not clear and continues to be a subject of doubt and discussion. Fig. . The shapes of molecules in which central atom has no lone pair Table .
Geometry of Molecules in which the Central Atom has No Lone Pair of Electrons Table . Shape (geometry) of Some Simple Molecules/Ions with Central Ions having One or More Lone Pairs of Electrons(E). Theoretically the shape should have been triangular planar but actually it is found to be bent or v-shaped. The reason being the lone pair- bond pair repulsion is much more as compared to the bond pair-bond pair repulsion.
So the angle is reduced to . ° from °. Had there been a bp in place of lp the shape would have been tetrahedral but one lone pair is present and due to the repulsion between lp-bp (which is more than bp-bp repulsion) the angle between bond pairs is reduced to ° from . °.
The shape should have been tetrahedral if there were all bp but two lp are present so the shape is distorted tetrahedral or angular. The reason is lp-lp repulsion is more than lp-bp repulsion which is more than bp-bp repulsion. Thus, the angle is reduced to . ° from .
°. Bent Trigonal pyramidal Bent AB E AB