Question

According to VSEPR theory, what is the shape of the `PH_3` molecule best described as?

Answer 1

The shape of phosphine is `"pyramidal"`; i.e. it is an analogue of ammonia.

Explanation:

There are `5+3` valence electrons for which to account; and this gives 4 electron pairs arranged the central phosphorus atom. These assume a tetrahedral geometry, however, one of the arms of the tetrahedron is a lone pair, and geometry descends to trigonal pyramidal with respect to phosphorus.

The `/_H-P-H` `~=` `94""^@`, whereas that for `/_H-N-H~=105""^@`. This difference in bond angles between the homologues is not readily rationalized, and beyond the scope of even a third year inorganic chemist. This bond angle discrepancy is also observed for `H_2S` versus `H_2O`.

So, the shape is PYRAMIDAL :

Hope it Helps :)