Both nitrogen and phosphorous are placed in group VA as they some similarities between them which are discussed below:
1. Electronic Structure: N and P both contain 5 electrons in their outermost energy shells.
N (7) – 1s2 2s2 2p3
P (15) -1s2 2s2 2p6 3s2 3p3
2. Bond and Valency: N and P both usually form covalent compounds. In these covalent compounds, usual valency 3, e.g. – NH3, PH3, PCI3. In case of P, however, the covalency can also be 5, e.g. – PCI5. But NCI5 is not possible. In this case, an electron from the 3s2 orbital of P promotes to the 3d orbital, and thus 5 unpaired electrons exist in the outer layer.
3. Hydride formation: Both elements form stable hydrides with Hydrogen, such as NH3 and PH3. Both hydrides are alkaline, but ammonia is more alkaline than phosphine PH3.
4. Nature: N and P are both non-metals and they both exhibit allotropism.
From the above points it is clearly understood that nitrogen and phosphorous can be placed in the same group.