The p-Block Elements
⚗️ Chapter 7: The p-Block Elements
1. Introduction
The p-Block elements are those in which the last electron enters the p-orbital of the outermost shell. This chapter (Class 12) focuses on Group 15 (Nitrogen family), Group 16 (Oxygen family), Group 17 (Halogens), and Group 18 (Noble gases), covering their properties, important compounds, and industrial processes like the manufacture of ammonia and nitric acid.
2. Group 15: The Nitrogen Family
Elements: N, P, As, Sb, Bi
General electronic configuration: ns² np³
Key trends: Atomic/ionic radii increase down the group; ionisation enthalpy decreases; metallic character increases down the group (N, P non-metals; As, Sb metalloids; Bi metal).
(a) Ammonia — Haber's Process
Industrial synthesis of ammonia from nitrogen and hydrogen:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92.4 kJ/mol
Optimum conditions: pressure ~200 atm, temperature ~700 K, iron catalyst with molybdenum promoter (based on Le Chatelier's principle — since forward reaction is exothermic and involves decrease in moles, high pressure and moderately low temperature favour higher yield).
(b) Nitric Acid — Ostwald's Process
Industrial manufacture of HNO₃ from ammonia:
- 4NH₃ + 5O₂ →(Pt catalyst, 500K, 9 bar) 4NO + 6H₂O
- 2NO + O₂ → 2NO₂
- 3NO₂ + H₂O → 2HNO₃ + NO
Concentrated HNO₃ is a strong oxidising agent and acts as aqua regia when mixed with HCl (3:1) to dissolve noble metals like gold and platinum.
(c) Phosphorus Compounds
Allotropes of phosphorus: White phosphorus (P₄, tetrahedral, reactive, poisonous, glows in dark), red phosphorus (polymeric, less reactive, non-poisonous), black phosphorus (most stable).
Phosphine (PH₃): Colourless, poisonous gas with a garlic-like smell; prepared by heating white phosphorus with concentrated NaOH.
3. Group 16: The Oxygen Family
Elements: O, S, Se, Te, Po
General electronic configuration: ns² np⁴
Sulphuric Acid — Contact Process
Industrial manufacture of H₂SO₄:
- S + O₂ → SO₂ (burning sulphur/roasting pyrites)
- 2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅ catalyst, ~720 K, 2 bar pressure)
- SO₃ + H₂SO₄ → Oleum (H₂S₂O₇), then diluted with water to get H₂SO₄
💡 Note: SO₃ is absorbed in concentrated H₂SO₄ (not directly in water) to avoid the formation of an acid mist, then the oleum formed is diluted carefully with water.
Uses of sulphuric acid: Manufacture of fertilisers, detergents, dyes, pigments; used as an electrolyte in car batteries; a powerful dehydrating and oxidising agent.
4. Group 17: The Halogens
Elements: F, Cl, Br, I, At
General electronic configuration: ns² np⁵
Key trends: Most electronegative group in the periodic table; F is most reactive; oxidising power decreases down the group (F₂ > Cl₂ > Br₂ > I₂); all exist as diatomic molecules.
Interhalogen Compounds
Formed between two different halogens; the larger halogen (less electronegative) is central atom bonded to smaller ones. General formulas: XX′, XX′₃, XX′₅, XX′₇ (e.g. ClF, ClF₃, IF₅, IF₇).
Oxoacids of Halogens
Acid strength increases with increasing oxidation state of the halogen: HOX < HOXO < HOXO₂ < HOXO₃ (i.e., hypohalous < halous < halic < perhalic acid).
5. Group 18: The Noble Gases
Elements: He, Ne, Ar, Kr, Xe, Rn
General electronic configuration: ns² np⁶ (except He: 1s²) — completely filled outer shells make them chemically almost inert.
Xenon compounds: Xenon (large size, low ionisation enthalpy) is the only noble gas to show significant chemistry — forms compounds with highly electronegative elements like F and O, e.g. XeF₂, XeF₄, XeF₆, XeOF₄, XeO₃.
| Compound | Preparation |
|---|---|
| XeF₂ | Xe + F₂ (1:5 ratio, 673K, Ni vessel, sunlight) |
| XeF₄ | Xe + F₂ (1:5 ratio, 873K, 7 bar pressure) |
| XeF₆ | Xe + F₂ (1:20 ratio, 573K, 60-70 bar pressure) |
6. Important Formula/Process Summary
| Process | Key Conditions |
|---|---|
| Haber's process (NH₃) | ~200 atm, ~700K, Fe catalyst + Mo promoter |
| Ostwald's process (HNO₃) | Pt catalyst, 500K, 9 bar |
| Contact process (H₂SO₄) | V₂O₅ catalyst, 720K, 2 bar |
| Oxidising power of halogens | F₂ > Cl₂ > Br₂ > I₂ |
| Acid strength of oxoacids | Increases with oxidation state of halogen |
7. Solved Examples
✏️ Example 1: Why does NH₃ act as a Lewis base while PH₃ is a weaker one?
Solution: Nitrogen is smaller and more electronegative than phosphorus, so the lone pair on N is more concentrated and available for donation, making NH₃ a stronger Lewis base than PH₃.
✏️ Example 2: Why is only XeF₂ found to be linear while ClF₃ is bent (T-shaped)?
Solution: XeF₂ has 3 lone pairs and 2 bond pairs on Xe (sp³d hybridisation, trigonal bipyramidal electron geometry), placing lone pairs in equatorial positions, resulting in a linear molecular shape. Similarly, ClF₃ has 2 lone pairs and 3 bond pairs, giving a T-shaped (bent) structure.
8. Quick Revision Points
- Haber's process is exothermic — moderate temperature (not too high) and high pressure give the best yield.
- In the Contact Process, SO₃ is absorbed in conc. H₂SO₄ (never directly in water) to avoid acid mist.
- Oxidising power of halogens decreases down the group: F₂ > Cl₂ > Br₂ > I₂.
- Acid strength of oxoacids of a halogen increases with the oxidation state of the halogen atom.
- Xenon is the only noble gas showing significant chemical reactivity, forming compounds with F and O.
- Interhalogens are always more reactive than the parent halogens (except F₂) due to weaker X–X′ bond compared to X–X.
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