Oxidation States and Trends in Physical and Chemical Properties — Core Principles

Core Principles

Group 15 elements (N, P, As, Sb, Bi) have a valence shell configuration of $ns^2 np^3$, giving them 5 valence electrons. Their characteristic oxidation states are -3, +3, and +5. Nitrogen, due to its small size and lack of d-orbitals, exhibits a wide range of oxidation states but cannot form pentavalent compounds like $PCl_5$.

The stability of the +5 oxidation state decreases down the group, while the +3 oxidation state stability increases, a phenomenon attributed to the inert pair effect, especially prominent for Sb and Bi.

Physically, atomic size, metallic character, and density increase down the group, while ionization enthalpy and electronegativity decrease. Melting and boiling points show a more complex trend, peaking around Arsenic.

Chemically, the thermal stability of hydrides ($EH_3$) decreases, but their reducing character and acidic nature of oxides ($E_2O_3, E_2O_5$) decrease down the group, reflecting the transition from non-metallic to metallic character.

Important Differences

vs Nitrogen vs. Phosphorus (Group 15)

Aspect	This Topic	Nitrogen vs. Phosphorus (Group 15)
Physical State (Room Temp)	Nitrogen (N): Gas ($N_2$)	Phosphorus (P): Solid ($P_4$ or polymeric)
Allotropy	Nitrogen (N): No allotropes (exists as $N_2$)	Phosphorus (P): Exhibits several allotropes (white, red, black)
Maximum Covalency	Nitrogen (N): 4 (due to absence of d-orbitals)	Phosphorus (P): 5 (due to presence of vacant d-orbitals)
Formation of Pentahalides	Nitrogen (N): Does not form pentahalides (e.g., $NCl_5$ does not exist)	Phosphorus (P): Forms pentahalides (e.g., $PCl_5$)
pπ-pπ bonding	Nitrogen (N): Forms strong pπ-pπ multiple bonds (e.g., $N equiv N$)	Phosphorus (P): Does not form strong pπ-pπ bonds; prefers single bonds
Basicity of Hydride	Nitrogen (N): $NH_3$ is strongly basic	Phosphorus (P): $PH_3$ is weakly basic

Nitrogen and Phosphorus, despite being in the same group, exhibit significant differences primarily due to Nitrogen's small size, high electronegativity, and the absence of vacant d-orbitals. Nitrogen exists as a diatomic gas with strong pπ-pπ bonding, limiting its maximum covalency to four and preventing pentahalide formation. Phosphorus, a solid, can expand its octet using d-orbitals, forming pentahalides and exhibiting allotropy. These differences highlight the anomalous behavior of the first element in a group.