Why does nitrogen exhibit anomalous behavior compared to other Group 15 elements?

Nitrogen's anomalous behavior stems from its unique characteristics: extremely small atomic size, high electronegativity, and crucially, the absence of vacant d-orbitals in its valence shell. Its small size and high electronegativity lead to strong hydrogen bonding in compounds like ammonia ($NH_3$). The lack of d-orbitals prevents nitrogen from expanding its octet, limiting its maximum covalency to four (e.g., in $NH_4^+$), unlike phosphorus which can form $PCl_5$ by utilizing its 3d-orbitals. Furthermore, nitrogen's small size allows it to form stable $p\pi-p\pi$ multiple bonds, leading to the highly stable diatomic $N_2$ molecule, a feature not observed in heavier pnictogens.

Explain the inert pair effect in Group 15 elements.

The inert pair effect refers to the increasing reluctance of the $ns^2$ valence electrons to participate in chemical bonding as we move down a group in the p-block. For Group 15 elements, this means that for heavier elements like antimony (Sb) and bismuth (Bi), the +3 oxidation state (involving only the $np^3$ electrons) becomes more stable than the +5 oxidation state (involving both $ns^2$ and $np^3$ electrons). This effect is attributed to the poor shielding of the nucleus by the intervening d and f electrons in heavier elements, leading to a stronger attraction of the $ns^2$ electrons towards the nucleus, making them less available for bonding.

Why is $N_2$ a gas at room temperature, while phosphorus exists as a solid?

Nitrogen exists as a diatomic molecule, $N_2$, which contains a very strong triple bond ($N\equiv N$). This triple bond results in a small, nonpolar molecule with very weak intermolecular forces (van der Waals forces). Consequently, very little energy is required to overcome these weak forces, making $N_2$ a gas at room temperature. In contrast, phosphorus exists as discrete $P_4$ tetrahedral molecules (white phosphorus) or polymeric structures (red and black phosphorus), where phosphorus atoms are linked by strong covalent single bonds. These larger, more complex structures lead to stronger intermolecular forces, requiring more energy to break, hence phosphorus is a solid.

Discuss the trend in basicity of Group 15 hydrides ($EH_3$).

The basicity of Group 15 hydrides ($NH_3, PH_3, AsH_3, SbH_3, BiH_3$) decreases down the group. Ammonia ($NH_3$) is a strong Lewis base due to the small size of nitrogen and the high electron density on the nitrogen atom, making its lone pair readily available for donation. As we move down the group, the atomic size of the central atom (E) increases, and its electronegativity decreases. This causes the electron density to become more diffuse over a larger volume, making the lone pair less concentrated and thus less available for donation. Therefore, the basicity order is $NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$.

Why does phosphorus form $PCl_5$ but nitrogen does not form $NCl_5$?

Phosphorus can form $PCl_5$ because it has vacant 3d-orbitals in its valence shell. These d-orbitals can be utilized to expand its octet, allowing phosphorus to accommodate more than eight electrons around it and form five covalent bonds. Nitrogen, on the other hand, belongs to the second period and only has 2s and 2p orbitals in its valence shell. It lacks vacant d-orbitals. Therefore, nitrogen cannot expand its octet beyond four bonds (as seen in $NH_4^+$) and consequently cannot form $NCl_5$ or any other pentahalide.

Group 15 Elements

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Group 15 elements, also known as the pnictogens, occupy the fifteenth column of the periodic table, characterized by their valence shell electronic configuration of $ns^2np^3$ . This group comprises nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). Their chemistry is profoundly influenced by the presence of three half-filled p-orbitals, which allows for a wide range of ox…

Quick Summary

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.

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Key Concepts

Anomalous Behavior of Nitrogen

Nitrogen stands apart from its heavier congeners due to its small size, high electronegativity, and the…

Inert Pair Effect and Oxidation States

The inert pair effect is a phenomenon observed in heavier p-block elements where the $ns^2$ electrons in the…

Allotropy of Phosphorus

Phosphorus exhibits several allotropic forms, with the most important being white phosphorus, red phosphorus,…

Elements — N, P, As, Sb, Bi (

ns^2np^3

configuration). \n- Trends: Atomic size

\uparrow

, Ionization Enthalpy

\downarrow

, Electronegativity

\downarrow

, Metallic Character

\uparrow

(N, P non-metals; As, Sb metalloids; Bi metal). \n- Oxidation States: -3, +3, +5. Stability of +3

\uparrow

down group (inert pair effect); Stability of +5

\downarrow

down group. \n- Nitrogen Anomalies: Small size, high EN, no d-orbitals,

p\pi-p\pi

bonding (

N\equiv N

), max covalency 4. \n- **Hydrides (

EH_3

)**: \n * Thermal stability:

NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3

\n * Basicity:

NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3

\n * Reducing character:

NH_3 < PH_3 < AsH_3 < SbH_3 < BiH_3

\n- Oxides:

E_2O_3, E_2O_5

. Acidic

\rightarrow

Amphoteric

\rightarrow

Basic down group. \n- Halides:

EX_3, EX_5

. Nitrogen forms only

NX_3

. Stability of

EX_5

\downarrow

down group. \n- Phosphorus Allotropes: White (

P_4

, reactive, poisonous, glows), Red (polymeric, less reactive), Black (most stable).

N-P-As-Sb-Bi: New People Always Start Blogging. (For elements of Group 15)