Group 15 Elements: The Nitrogen Family
Group 15 of the periodic table includes Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), and Bismuth (Bi). These elements are collectively known as Pnictogens. As we move down the group, there is a transition from non-metallic to metallic character.
1. Electronic Configuration
The general electronic configuration is $ns^2 np^3$. The s-orbital is completely filled, and the p-orbital is exactly half-filled, imparting extra stability to these elements.
- N (7): $[He] 2s^2 2p^3$
- P (15): $[Ne] 3s^2 3p^3$
- As (33): $[Ar] 3d^{10} 4s^2 4p^3$
- Sb (51): $[Kr] 4d^{10} 5s^2 5p^3$
- Bi (83): $[Xe] 4f^{14} 5d^{10} 6s^2 6p^3$
2. Periodic Trends
- Atomic Radius: Increases down the group. ($N < P < As < Sb < Bi$)
- Ionization Enthalpy: Decreases down the group due to increased size. Group 15 has higher IE than Group 14 (due to smaller size) and Group 16 (due to stable half-filled configuration).
- Electronegativity: Decreases down the group. ($N > P > As > Sb > Bi$)
- Metallic Character: Increases down the group.
N, P: Non-metals
As, Sb: Metalloids
Bi: Typical Metal
3. Oxidation States
Common oxidation states are $-3, +3$, and $+5$.
Inert Pair Effect
The tendency to show -3 oxidation state decreases down the group.
The stability of +5 oxidation state decreases down the group, while that of +3 increases due to the Inert Pair Effect (reluctance of s-electrons to participate in bonding).
Bi(V) is a strong oxidizing agent because it wants to revert to Bi(III).
Nitrogen exhibits oxidation states from -3 to +5 (e.g., $NH_3$ (-3), $N_2H_4$ (-2), $NH_2OH$ (-1), $N_2$ (0), $N_2O$ (+1), $NO$ (+2), $HNO_2$ (+3), $NO_2$ (+4), $HNO_3$ (+5)).
4. Anomalous Properties of Nitrogen
Nitrogen differs from the rest of the group due to:
- Small size and high electronegativity.
- High Ionization Enthalpy.
- Absence of d-orbitals: It cannot expand its covalency beyond 4 (e.g., $NCl_5$ does not exist, but $PCl_5$ does).
- Ability to form $p\pi-p\pi$ multiple bonds (exists as diatomic gas $N \equiv N$, while others are polyatomic solids like $P_4$).
5. Hydrides ($EH_3$)
All group 15 elements form hydrides of the type $EH_3$ (e.g., $NH_3, PH_3, AsH_3, SbH_3, BiH_3$).
| Property | Trend Down the Group ($NH_3 \to BiH_3$) |
|---|---|
| Thermal Stability | Decreases (Bond length increases) |
| Reducing Character | Increases ($BiH_3$ is strongest reducer) |
| Basic Character | Decreases ($NH_3$ is most basic due to small size) |
| Bond Angle | Decreases ($107.8^\circ \to \approx 90^\circ$) |
6. Important Compounds
A. Dinitrogen ($N_2$)
Very inert at room temperature due to high bond dissociation energy of the triple bond ($N \equiv N$).
B. Ammonia ($NH_3$)
- Preparation (Haber's Process): $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ ($\Delta H = -46.1$ kJ/mol). Favorable conditions: High Pressure, Low Temp (optimum ~700K), Catalyst (Fe oxide + $K_2O/Al_2O_3$).
- Properties: Trigonal pyramidal, Lewis base. Forms complex with $Cu^{2+}$ (Deep blue).
C. Nitric Acid ($HNO_3$)
Prepared by Ostwald's Process (Catalytic oxidation of $NH_3$). It is a strong oxidizing agent.
D. Phosphorus Allotropes
- White Phosphorus ($P_4$): Discrete tetrahedral units. Highly reactive, glows in dark (chemiluminescence), insoluble in water but soluble in $CS_2$. Bond angle $60^\circ$ (high strain).
- Red Phosphorus: Polymeric structure ($P_4$ chains). Less reactive, non-poisonous. Obtained by heating white P at 573K in inert atmosphere.
- Black Phosphorus: Most stable thermodynamically. Layered structure.
E. Phosphine ($PH_3$)
Prepared by reaction of Calcium Phosphide with water: $Ca_3P_2 + 6H_2O \rightarrow 3Ca(OH)_2 + 2PH_3$. Used in Holme's signals.
F. Halides ($PCl_3, PCl_5$)
$PCl_5$ in solid state exists as an ionic solid: $[PCl_4]^+ [PCl_6]^-$. $PCl_3$ is pyramidal, $PCl_5$ is trigonal bipyramidal (gas/liquid).
Group 15 Quiz
Test your mastery of the Nitrogen Family. 10 MCQs with explanations.
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