Question

Phosphorus exhibits +3 and +5 oxidation states. However, it forms PH₃ only, not PH₅. This is because the exhibition of highest oxidation state (+5) of P is not favoured by high Δ_aH value of dihydrogen and Δ_egH value of hydrogen.
what does Δ_aH and Δ_egH mean in here?

Answer 1

Δ_aH value - enthalpy of atomisation and Δ_egH electron gain enthalpy. If the formation of compound isinvolved with high enthalpy of atomisation and high electron gain enthalpy it will make the compound unstable. Because lots of energy needed to form the compound. That is why PH₅ formation does not occur that much easy compare to PH₃.