Question

what is drago rule? can one know the hybridisation of PH3?

Answer 1

 Almost all groups,the formula for calculating the number of hybrid orbitals is group number of central element+no.of atoms surrounding the central element with an exception of oxygen-charge on the central element  whole divided by 2.
The hybridisation in pH3 molecule is a situation in which pH3 bonds are made by pure p orbital not by hybridised orbitals.According to the radial distance data if central atom & surrounded atom has larger atomic radii overlapping does not takes place & hence such molecules has no well defined hybridization.