which is the valency of phosphorus 3,5 / how can we learn valency

Phosphorous shows both the valencies 3 and 5.

Electronic configuration of Phosphorous

P: 1s² 2s² 2p⁶ 3s² 3p³

Phosphorous with valency 3:

With three unpaired electrons in three p orbitals, phosphorous shows valency 3, like in case of compound PCl₃.

 

Phosphorous with valency 5:

In case of PCl₅, one of 3s electron gets promoted to 3d orbital as follows:

P: 1s² 2s² 2p⁶ 3s¹ 3p³3d¹

Thus, phosphorous atom undergoes sp³d hybridisation to form five equivalent sp³d orbitals each containing one unpaired electron.