Oxidation numbers of phosphorus pentachloride
When considering the oxidation state of covalent compounds, we need to pretend they are ionic. When we do so, we assign all of the electrons to the more electronegative element. The electronegativity of phosphorous using the Pauling Scale is 2.19 and the electronegativity of chlorine is 3.16. Since chlorine is the more electronegative element, we assign all electrons in the bonds to the chlorine atoms. Thus $\ce{PCl5}$ is treated as the fictitious ionic compound comprised of $\ce{P^{5+}}$ ions and $\ce{Cl-}$ ions.