Group 15 Elements — Core Principles

Core Principles

Group 15 elements, or pnictogens, include Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), and Bismuth (Bi). Their general electronic configuration is $ns^2np^3$, giving them five valence electrons.

Key trends include increasing atomic size, decreasing ionization enthalpy and electronegativity down the group. Metallic character increases from N (non-metal) to Bi (metal). They typically exhibit -3, +3, and +5 oxidation states.

The stability of the +3 oxidation state increases down the group due to the inert pair effect, while the +5 state's stability decreases. Nitrogen shows anomalous behavior due to its small size, high electronegativity, and absence of d-orbitals, leading to $p\pi-p\pi$ bonding and limited covalency.

All elements except Bi show allotropy. Their hydrides ($EH_3$) show decreasing thermal stability and basicity, but increasing reducing character down the group. Oxides ($E_2O_3, E_2O_5$) transition from acidic to basic down the group.

Halides ($EX_3, EX_5$) are formed, with nitrogen not forming $NX_5$ due to lack of d-orbitals.

Important Differences

vs Nitrogen vs. Phosphorus

Aspect	This Topic	Nitrogen vs. Phosphorus
Physical State at Room Temp	Gas ($N_2$)	Solid (e.g., $P_4$)
Allotropy	No significant allotropes (exists as $N_2$)	Exhibits multiple allotropes (white, red, black)
Multiple Bonding	Forms stable $p\pi-p\pi$ multiple bonds ($N\equiv N$)	Does not form stable $p\pi-p\pi$ multiple bonds (forms single P-P bonds)
Maximum Covalency	4 (due to absence of d-orbitals)	5 or 6 (due to presence of vacant d-orbitals)
Hydrogen Bonding	Forms strong hydrogen bonds (e.g., in $NH_3$)	Does not form significant hydrogen bonds (e.g., in $PH_3$)
Reactivity with Halogens	Forms only trihalides ($NX_3$), no pentahalides	Forms both trihalides ($PX_3$) and pentahalides ($PX_5$)

Nitrogen and phosphorus, though both non-metals in Group 15, exhibit significant differences primarily due to nitrogen's smaller size, higher electronegativity, and the absence of d-orbitals. Nitrogen exists as a diatomic gas ($N_2$) with a strong triple bond, while phosphorus is a solid with various allotropic forms. Nitrogen's inability to expand its octet limits its maximum covalency to four, preventing the formation of pentahalides like $NCl_5$, which phosphorus readily forms using its vacant d-orbitals. Furthermore, nitrogen's high electronegativity leads to strong hydrogen bonding in its compounds, a feature largely absent in phosphorus compounds.