Phosphorus cannot form `PH_(5)` with its outer electronic configuration as `3s^(3)3p^(3)` because

Explanation

Though phosphorus exhibts +3 and +5 oxidation states, it cannot form `PH_5` . This is because of high enthalpy of atmoisation `(Delta_alphaH) (435.89 KJ "mol"^(-1))` of dihydrogen and slightly negative electron gain enthalpy, `Delta_(eg) H (-72.8 KJ "mol"^(-1))` of hydrogen , it acts only as a weak oxidising agent. therefore , it can oxidise P to +3 oxidation state but not to its highest oxidation state of +5. Thus, P forms `PH_3` but not `PH_5`