Question

why phosphorus has valency 3 and 5?​

Answer 1

Answer:

Valency of 3 is due to requirement of 3 more electrons to complete it's octet. The valency of 5 is due to exitation of an electron from 3s orbitals to 3d orbital . ... It contains empty 3d orbitals so phosphorus can expand its octet by exciting on electron from 3s to 3d . Hence phosphorus can show valency 3 and 5 both.

Answer 2

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In this case, The outer orbit must have eight electrons for stability in combining with other elements, also known as valency. So one can either add 3 electrons to the outer orbit or take away 5 electrons with equal ease. So Phosphorus has a valency of 3 or 5.