氧化數問題................

Since P can have variable oxidation states, due to using expanded d-orbital to form bonds, you can't calculate it directly. The best way to find out its (P) oxidation state is as follows:
P + (-1)x5 = 0(zero) . The oxidation state of P is P +(-5) = 0 . P = 5
Having found out the oxidation state of P, you can found the oxidation state of Cl easily:
5 + (Cl)x5 = 0 ===> 5 Cl = -5 ===> Cl = 5/-5 = -1.
Hence Cl2 (0) --> PCl5 (-1)
I hope you may find my answer helpful.