Question

Nitrogen forms only NCL3 but phosphorous forms PCL3 and PCL5 both. explain

Answer 1

For one thing Nitrogen has only three valence electrons ( Final Electronic Configuration: 2s2 2p3) thus it can only bond with three other species. Also species in the second period have to follow the octet rule when bonding whenever possible. The octet rule is the second period element must have four electron pairs(8 electrons) around it. This won’t be obeyed in NCl5.

However Phosphorous is in the third period and elements from this period and after can exceed the octet rule as they have empty ‘ d’ orbitals( They are present even though electrons may not have filled into them). Thus by way of dative bonds PCl5 can form.