Section A — Group 16 (Chalcogens)

Ozone (O3): Trigonal electron geometry, angular molecular geometry (sp2), bond angle ~117°. Resonance hybrid (1.5 bond order). Prepared by silent electric discharge through dry O2. Powerful oxidizer via nascent oxygen release: O3 → O2 + [O]. Detected by KI-starch paper (turns blue-black). Oxidizes PbS (black) to PbSO4 (white) — another classic detection reaction.

Sulfur Allotropes: Rhombic (α): stable at room temperature, S8 crown rings, most dense allotrope. Monoclinic (β): stable above 95.6°C, same S8 units but different crystal packing. Transition temperature = 95.6°C. Plastic sulfur: amorphous, quenching artifact.

Sulfur Oxides: SO2 — sp2, angular, acidic, reducing agent. SO3 — sp2, trigonal planar, very reactive with water (forms acid mist — hence oleum in Contact process). SO3 is the immediate precursor to H2SO4.

Contact Process and H2SO4:

• Step 1: S + O2 → SO2
• Step 2: 2SO2 + O2 ⇌ 2SO3 (V2O5, 450°C — reversible, exothermic, rate-yield compromise)
• Step 3: SO3 + H2SO4 → H2S2O7 (oleum) — avoids acid mist
• Step 4: H2S2O7 + H2O → 2H2SO4

H2SO4 properties: diprotic acid, dehydrating agent (chars sugar), oxidizing agent $\frac{hot}{conc. only}$, sulfonating agent.

Section B — Group 17 (Halogens)

Oxidizing Power Trend: F2 > Cl2 > Br2 > I2 (decreases down group). A stronger halogen displaces a weaker one from its salt solution (e.g., Cl2 + 2KBr → 2KCl + Br2).

Hydrogen Halides: Acidic strength HF < HCl < HBr < HI. Thermal stability HF > HCl > HBr > HI. HF is weak acid (strong H-F bond 568 kJ/mol + H-bonding). HF etches glass: 4HF + SiO2 → SiF4↑ + 2H2O — unique to HF.

Chlorine Oxoacids: HOCl (+1, weakest) < HClO2 (+3) < HClO3 (+5) < HClO4 (+7, strongest). More oxygens = better conjugate base stabilization = stronger acid. Fluorine = no oxoacids (no d-orbitals, no positive oxidation states).

Interhalogen Compounds:

• AB: linear (diatomic: ClF, BrCl, ICl)
• AB3: T-shaped, sp3d, 2 lone pairs (ClF3, BrF3)
• AB5: square pyramidal, sp3d2, 1 lone pair (BrF5, IF5)
• AB7: pentagonal bipyramidal, sp3d3, 0 lone pairs (IF7)

Bleaching Powder: CaOCl2. Preparation: Ca(OH)2 + Cl2 → CaOCl2 + H2O. Mixed salt of HCl and HOCl. Bleaches via Cl2 liberation and HOCl/nascent [O].

Section C — Group 18 (Noble Gases)

Xenon Fluorides:

• XeF2: sp3d, linear, 3 lone pairs equatorial in TBP. Hydrolysis: Xe + 2HF + ½O2
• XeF4: sp3d2, square planar, 2 lone pairs trans axial in octahedral. Hydrolysis: disproportionation (Xe + XeO3 + HF + O2)
• XeF6: sp3d3, distorted octahedral, 1 lone pair. Hydrolysis: XeO3 (explosive!) + 6HF

Clathrates: Noble gas atoms physically trapped in cage structures — no chemical bonds. E.g., Ar in ice lattice, Kr in phenol hydroquinone lattices.