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.

Often confused with

Side-by-side differences the NEET paper likes to test.

Group 15 Elements vs Nitrogen vs. Phosphorus

Aspect	Group 15 Elements	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.

Why it is tested: For NEET, understanding the distinct properties and reasons for the anomalous behavior of nitrogen compared to phosphorus is crucial. Questions frequently test these differences, particularly regarding their physical states, allotropy, maximum covalency, and the formation of specific compounds like pentahalides. These comparisons highlight fundamental principles of chemical bonding and periodic trends.