ICSE Class 9 Answered
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.