The p-block elements of Groups 16, 17, and 18 — the chalcogens, halogens, and noble gases — represent one of the most densely tested segments of inorganic chemistry in NEET. Mastery of this chapter demands precise knowledge of molecular geometries, reaction mechanisms, and the reasoning behind physical and chemical trends.
Group 16 — Chalcogens: Oxygen and sulfur are the two most important chalcogens for NEET. Ozone (O3), the triatomic allotrope of oxygen, is a resonance hybrid molecule with sp2 hybridization. Its central oxygen bears one lone pair and forms two bonds, giving an angular (V-shaped) molecular geometry with a bond angle of approximately 117°. The angular shape and aromatic resonance make O3 a powerful oxidizing agent. Upon decomposition, it releases nascent (atomic) oxygen [O], which is far more reactive than molecular O2. The laboratory detection of ozone uses KI-starch paper — ozone oxidizes from KI to liberate I2, which forms a deep blue-black complex with starch.
Sulfur exists in several allotropic forms. At room temperature, rhombic sulfur (α-sulfur, S8 rings in an orthorhombic crystal) is thermodynamically the most stable allotrope. Above 95.6°C — the transition temperature — monoclinic sulfur (β-sulfur, also S8 but in a monoclinic crystal lattice) becomes the stable form, persisting up to sulfur's melting point of 119°C. Plastic sulfur, an amorphous, rubber-like form, results from rapid quenching of boiling sulfur in cold water.
Sulfur dioxide (SO2) has sp2 hybridization and an angular shape. Crucially, SO2 is an acidic gas and a prominent reducing agent — it decolorizes acidified KMnO4 by reducing Mn(VII) to Mn(II). Sulfur trioxide (SO3), by contrast, is trigonal planar (sp2, no lone pair on S) and is the intermediate in H2SO4 manufacture.
The Contact process for sulfuric acid production is, without doubt, the highest-yield industrial chemistry topic in NEET. It proceeds in four steps: (1) Combustion of sulfur: S + O2 → SO2; (2) Catalytic oxidation (the key equilibrium step): 2SO2 + O2 ⇌ 2SO3, using V2O5 catalyst at 450°C — a temperature selected as the optimal compromise between thermodynamic yield (exothermic reaction, low T favors product) and kinetic rate; (3) Absorption of SO3 in concentrated H2SO4 to form oleum (H2S2O7): SO3 + H2SO4 → H2S2O7; (4) Dilution of oleum with water: H2S2O7 + H2O → 2H2SO4. The critical NEET fact is that SO3 is NEVER dissolved directly in water — it forms a fine H2SO4 mist that cannot be condensed and collected. Oleum is the solution. Sulfuric acid itself is a diprotic strong acid, a powerful dehydrating agent (chars sugar to carbon), an oxidizing agent when hot and concentrated (reacts with Cu to give SO2), and a sulfonating agent.
Group 17 — Halogens: The oxidizing power of halogens decreases down the group: F2 > Cl2 > Br2 > I2, reflecting decreasing electronegativity and electron affinity. For hydrogen halides, the acidic strength trend is the reverse of bond strength: HF < HCl < HBr < HI. HF, despite fluorine's extreme electronegativity, is a weak acid because the H-F bond is the strongest H-X bond and resists dissociation. Intermolecular hydrogen bonding between HF molecules further stabilizes the undissociated form. In contrast, thermal stability of HX parallels bond strength: HF > HCl > HBr > HI.
The oxoacids of chlorine show increasing acidic strength with increasing Cl oxidation state: HOCl (+1) < HClO2 (+3) < HClO3 (+5) < HClO4 (+7). This is because more oxygen atoms withdraw electron density from the O-H bond (inductive effect) and better stabilize the conjugate base through charge delocalization. Fluorine uniquely forms no oxoacids because it lacks d-orbitals and cannot exhibit positive oxidation states.
Interhalogen compounds are formed between two different halogens, with the less electronegative (larger) halogen as the central atom. Types: AB (ClF — linear), AB3 (ClF3 — T-shaped, sp3d, 2 lone pairs), AB5 (BrF5 — square pyramidal, sp3d2, 1 lone pair), AB7 (IF7 — pentagonal bipyramidal, sp3d3, 0 lone pairs). Bleaching powder (CaOCl2) is prepared by Ca(OH)2 + Cl2 → CaOCl2 + H2O and is a mixed salt of HCl and HOCl.
Group 18 — Noble Gases: Xenon forms a remarkably stable series of fluorides, all predictable by VSEPR. XeF2 (sp3d, 5 electron pairs: 2 bond + 3 lone) is linear because the 3 lone pairs occupy equatorial positions in the trigonal bipyramidal arrangement, placing F atoms in the axial positions at 180°. XeF4 (sp3d2, 6 electron pairs: 4 bond + 2 lone) is square planar because the 2 lone pairs occupy opposite (trans) axial positions in the octahedral arrangement. XeF6 (sp3d3, 7 electron pairs: 6 bond + 1 lone) is distorted octahedral due to the single lone pair disrupting the regular geometry. Hydrolysis of XeF6 with excess water gives the explosive XeO3 and HF: XeF6 + 3H2O → XeO3 + 6HF. Noble gas clathrate compounds physically trap noble gas atoms in cage-like lattice structures of water or organic molecules without any chemical bonding.