How 3rd period elements can expand their covalence above four?

The elements of 3rd period have vacant sub-shell in same energy level. So they expand their valency by involving the vacant subshell in bonding. For example : 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 ). By involving d-orbital , P can accommodate 5 bond pairs (= 10 electrons) which is beyond the octet rule. In this way the valency of P is expanded from 3 to 5.