CBSE Class 10 Answered

Reason for occurrence of variable valencies:

1) Involvement of available of vacant sub-shell in bonding. (d-orbital) : some of the elements have vacant sub-shell in same energy level. So they expand their valency and exhibit more than 1 valency in its compound. Phosphorus form PCl3 and PCl5 with chlorine. 3 is the normal valency of phosphorus. But in PCl₅, phosphorus forms 5 bond with chlorine atoms by involving its vacant sub-shell (d orbital).Detail of these will be in higher classes.

2) when outermost shell contain more than 8 electrons. Example is Fe: 2,8,8,6,2. Fe loose 2 as well as 3 electrons from its outermost shell. Here it does not follow octet rule.(more in detail you will study in higher classes)