Group 15 Elements Revision Notes, Mnemonics and Trends

Group 15 Elements (Pnictogens): N, P, As, Sb, Bi

This quick revision Notes, Trends and Mnemonics are very important for CSIR-NET, GATE, SLET, NEET, JEE Main/Advanced, IIT-JAM.

Mnemonics: N: Nana | P: Patekr | As: Aishwarya | Sb: Sab | Bi: Bimar

Transition from nonmetals (N, P) to metalloids (As, Sb) to metals (Bi) down the group, with decreasing electronegativity and increasing atomic radius.

1. General Electronic Configuration

ns² np³ (Extra stability due to half-filled p-subshell). This leads to higher Ionization Energy compared to Group 16 (Chalcogens).

2. Key Periodic Trends (Down the Group)

Property	Trend (N → Bi)	Exception/Note
Atomic & Ionic Radius	Increases	N < P < As < Sb < Bi
Ionisation Energy	Decreases	N > P > As > Sb > Bi Exception: N > P (due to stable half-filled p-subshell in N)
Electronegativity	Decreases	N (3.0) > P (2.1) > As > Sb > Bi
Metallic Character	Increases	N, P (non-metal) → As, Sb (metalloid) → Bi (metal)
Melting & Boiling Point	No regular trend	N₂ very low (van der Waals) P₄ high (tetrahedral structure) As, Sb, Bi increase due to metallic nature
Catination	Decreases	P >> As > Sb > N > Bi
Oxidation States	-3, +3, +5 common	+5 stability: N > P > As > Sb > Bi +3 stability: N < P < As < Sb < Bi (inert pair effect)
Basic Nature of Oxides	Increases	N₂O₅ strongly acidic, As₂O₃ and Sb₂O₃ are amphoteric, Bi₂O₃ Basic

3. Important Compounds

Nitrogen (N₂)

• Inert due to triple bond (946 kJ/mol)
• N cannot expand octet (no d-orbitals) → maximum covalency 4 (NH₄⁺, NF₃, etc.)
• N₂ is very stable → high bond energy (941 kJ/mol) → inert at room temp
• N forms pπ–pπ multiple bonds easily (N≡N, C≡N, etc.)
• Only element in group that forms stable N₂O, NO, NO₂, N₂O₃, N₂O₄, N₂O₅

Phosphorus – Allotropic Forms (High Weightage)

• White P: most reactive, poisonous, glows in dark (chemiluminescence), tetrahedral P₄
• Red P: amorphous, less reactive, used in matchsticks
• Black P: most stable, layered structure (like graphite), semiconductor

Trick: "White = Weak & Wicked, Red = Reliable, Black = Best"

Hydrides:
NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ (basicity & stability decreases)

Halides:

• NX₃ (trigonal pyramidal), PX₃ > PX₅ (reactivity)
• PCl₅ (solid: [PCl₄]⁺[PCl₆]⁻, gas: trigonal bipyramidal)
• BiF₅ (most stable halide of Bi)

Oxides:

• N₂O₅ → strongest acid (HNO₃)
• P₄O₁₀ → strongest dehydrating agent
• Bi₂O₅ unstable

Oxyacids:
Acid strength: HNO₃ > H₃PO₄ > H₃AsO₄ > H₃SbO₄

4. Chemical Reactions & Hydrolysis

Hydrolysis of Trihalides (Distinct Mechanisms):
• NCl₃ + 3H₂O → NH₃ + 3HOCl (N is more electronegative, gets H+).
• PCl₃ + 3H₂O → H₃PO₃ + 3HCl$ (P uses d-orbitals, gets OH-).
• AsCl₃ / SbCl₃: Partial hydrolysis to Oxychlorides (SbOCl - Pearl White).

5. Exam Tricks & Mnemonics

Trick for Oxidation State Stability:
“+5 loves Nitrogen, +3 loves Bismuth”
→ +5 state stable for lighter elements (N, P), +3 stable for heavier (Sb, Bi) due to inert pair effect.

Basicity of Hydrides: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃
Mnemonic: Nice Please Avoid Sour Biscuits → decreasing order.

Thermal Stability of Hydrides: Same order as basicity (NH₃ most stable).

Bond Angle in Hydrides: NH₃ (107°) > PH₃ (93°) > AsH₃ > SbH₃
Reason: As we go down, s-character decreases, lone pair in pure s-orbital → bond angle ~90°.

Acidic Nature of Oxides: N₂O₅ > P₄O₁₀ > As₂O₅ > Sb₂O₅ > Bi₂O₃
→ Decreases down the group (non-metallic → metallic character).

Why? Down the group, the atomic size increases and ionization energy decreases. This makes the element more "metallic," meaning it loses electrons more easily. Metallic oxides are characteristically basic, while non-metallic oxides are acidic.

Reactivity of Trihalides (PX₃): PCl₃ > PBr₃ > PI₃
(Opposite to group 17 halides due to poor overlap of 3p-3p, 3p-4p, 3p-5p).

Most Stable Compounds:
• NF₃ (most stable nitrogen halide)
• BiF₅ (only pentahalide of Bi)
• PCl₅ (exists in solid state as ionic)

6. Frequently Asked Exam Questions (Conceptual)

• Why is N₂ inert but P₄ reactive?
• Why does nitrogen show anomalous behavior?
• Why is PH₃ less basic than NH₃?
• Why no hybridization of PH₃?
• Why is Bi₂O₃ amphoteric?
• Order of bond angle in group 15 hydrides?
• Which has maximum catenation? (Phosphorus)
• Strongest oxyacid of phosphorus? (H₃PO₄ is most common, but HClO₄-like strength not applicable)
• P₄O₁₀ is drying agent for which gas? (Not for NH₃, forms adduct)

7. Quick Revision One-Liners

• Nitrogen: smallest size, highest electronegativity, no d-orbital.
• Phosphorus: maximum catenation, P₄O₁₀ strongest dehydrating agent.
• Bismuth: most metallic, strongest inert pair effect, Bi³⁺ more stable than Bi⁵⁺.
• NH₃ is the only hydride used as ligand extensively.
• R₃P=O > R₃N=O stability (phosphorus forms strong pπ-dπ bond).

8. Current Trends & Repeated Questions (2020–2025)

• Comparison of oxides: acidic → amphoteric → basic trend (N₂O₅ → As₂O₃ → Bi₂O₃)
• Hydrides: boiling point order (NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃) due to H-bonding in NH₃
• Maximum covalency of N = 4, P = 5 (PF₅, PCl₅), As = 5, Sb = 5, Bi = 5 (but Bi rarely)
• Thermal stability of hydrides: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃
• Marsh test, Goldschmidt process, matchstick chemistry, fertilisers (superphosphate, DAP)
• Nitrogen cycle, fixation, Ostwald & Haber process flowcharts