p-Block Elements: Groups 16-18

Groups 16-18 encompass the chalcogens, halogens, and noble gases — a stretch of the p-block that delivers some of the most frequently tested inorganic reactions in NEET, especially H₂SO₄ manufacture, halogen oxoacid comparisons, and xenon compound geometries.

Group 16 (Chalcogens): Ozone (O₃) is an angular molecule (sp2 hybridized, bond angle ~117 degrees) described as a resonance hybrid. It is a powerful oxidizing agent prepared by passing silent electric discharge through O₂ or by UV irradiation. Ozone detection uses KI-starch paper, which turns blue-black (O₃ liberates I₂ from KI). Sulfur exists as rhombic S₈ (most stable at room temperature, $\alpha$-sulfur), monoclinic S₈ (stable above 95.6 degrees C, $\beta$-sulfur), and plastic sulfur. SO₂ is angular (sp2), acidic, and a reducing agent that decolorizes acidified KMnO₄.

The Contact process for H₂SO₄ proceeds in four critical steps: (1) S + O₂ → SO₂; (2) 2SO₂ + O₂ <=> 2SO₃ (V₂O₅ catalyst, 450 degrees C — key step); (3) SO₃ + H₂SO₄ → H₂S₂O₇ (oleum / fuming sulfuric acid); (4) H₂S₂O₇ + H₂O → 2H₂SO₄. SO₃ is NOT dissolved directly in water because it forms a fine acid mist that is difficult to condense. Instead, it is absorbed in concentrated H₂SO₄ to form oleum. H₂SO₄ is a diprotic strong acid, a powerful dehydrating agent (chars sugar to carbon), a strong oxidizing agent when hot and concentrated, and a sulfonating agent.

Group 17 (Halogens): Oxidizing power decreases down the group: F₂ > Cl₂ > Br₂ > I₂. For hydrogen halides, acidic strength increases as HF < HCl < HBr < HI (bond dissociation energy decreases), but thermal stability follows the opposite order HF > HCl > HBr > HI. HF is a weak acid despite fluorine's high electronegativity because the very strong H-F bond (568 kJ/mol) resists dissociation, and intermolecular hydrogen bonding further stabilizes the undissociated form.

Interhalogen compounds form between different halogens: AB type (ClF, BrCl, ICl), AB₃ (ClF₃ — T-shaped, sp3d), AB₅ (BrF₅, IF₅ — square pyramidal, sp3d2), and AB₇ (IF₇ — pentagonal bipyramidal, sp3d3). The more electronegative halogen is always the outer atom.

Oxoacids of halogens show that acidic strength increases with oxidation state: HOCl (+1) < HClO₂ (+3) < HClO₃ (+5) < HClO₄ (+7). More oxygen atoms stabilize the conjugate base by delocalizing negative charge. Fluorine does not form oxoacids because it has the highest electronegativity and lacks d-orbitals. Bleaching powder (CaOCl₂) is prepared by Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.

Group 18 (Noble Gases): Xenon forms fluoride compounds with distinct VSEPR geometries: XeF₂ (sp3d, linear — 3 lone pairs equatorial in trigonal bipyramidal), XeF₄ (sp3d2, square planar — 2 lone pairs opposite in octahedral), and XeF₆ (sp3d3, distorted octahedral — 1 lone pair). Hydrolysis reactions include: XeF₂ + H₂O → Xe + 2HF + $\frac{1}{2}$ O₂ (partial), and XeF₆ + 3H₂O → XeO₃ + 6HF (XeO₃ is explosive). Clathrate compounds trap noble gas atoms in cage structures of water or organic molecules with no chemical bond.

The key testable concept is the Contact process for H₂SO₄ (specifically that SO₃ is dissolved in H₂SO₄ to form oleum, not directly in water), xenon compound geometries with lone pair positions, and halogen oxoacid strength increasing with oxidation state.