Element

N occurs as gaseous N₂ (which is unreactive due to the high energy required to break the strong triple bond), whereas P occurs in many allotropes, including white phosphorus, P₄, which all contain P-P single bonds. P₄ is very reactive due to the bond strain in the tetrahedral P₄ molecule.

The decreasing preference for low numbers of multiple bonds rather than high numbers of single bonds as the group is descended is a general tendency.

Hydrides

Ammonia, NH₃, is stable, whereas phosphine, PH₃, is thermodynamically unstable. This reflects the lower bond energy of P-H compared to N-H.

PH₃ is not as soluble in water as NH₃. This is due to the fact that NH₃ forms extensive hydrogen bonding with water, and PH₃ does not.

When PH₃ does dissolve in water, it forms a neutral solution, whereas aqueous NH₃ is basic. The enthalpy of hydration of EH₄⁺ is greater for E=N than E=P, and again this reflects the easier formation of hydrogen bonds in the aqueous ammonia system.

In complex formation, PH₃ acts as π-acceptor as well as a σ-donor. This is because P has vacant low energy orbitals, the P 3d-orbitals, available for back donation, whereas N has no such orbitals and so can act as a σ-donor only.

Tri-Halides

PX₃ are made by direct combination of the elements (apart from PF₃).

PCl₃ is reactive, but NCl₃ is endothermic and explosive. This reflects the stronger P-X bonds compared to N-X, due to extra contribution from back donation from occupied p-orbitals on the halide to form P(dπ)-X(pπ) bonds, which cannot form in N-X.

The hydrolysis products of ECl₃ are different: in NCl₃, the fact that B(N-H)>B(N-O), due to the presence of electrostatic repulsion between the non-bonded electrons on N and O, favours formation of ammonia. In PCl₃, the fact that B(P-O)>B(P-H), due to the ability of O to form a bonding pπ-dπ interaction with P which H cannot, favours formation of phosphorus acid.

Penta-Halides:

P forms PX₅: there are no N equivalents.

P is more stable in the high oxidation states.

P forms many complexes with high coordination numbers, eg. six-coordinate PF₆^-, whereas the maximum coordination number of N is four. This tendency for hypervalence is due to the larger size of the P atom compared to N, and the presence of low-lying vacant orbitals which can participate in bonding in P but which are not present in N.

Oxides

Phosphorus oxides are polymeric, made up of PO₄ tetrahedra. Nitrogen oxides are molecular.

This again reflects the greater tendency to form multiple bonds at the top of the group, with B(N=O)>B(P=O), but B(P-O)>B(N-O).