p-Block Elements: Groups 13-18

The p-block elements (Groups 13-18) exhibit rich chemistry driven by their valence configuration $ns^{2}$ np^(1-6). Key themes include inert pair effect, diagonal relationships, anomalous behaviour of first elements, and systematic group trends in oxidation states, bonding, and acid-base character.

Group 13 (Boron Family): B, Al, Ga, In, Tl

Configuration: $ns^{2} \; np^{1}$. Oxidation states: +3 (all), +1 (Tl dominantly due to inert pair effect).

Boron is anomalous: it is a metalloid, forms covalent compounds (electron-deficient), and has diagonal relationship with Si. B₂H₆ (diborane) has unique 3-centre 2-electron (3c-2e) banana bonds. Boron never forms $B^{3}$+ ions — it is always covalent. Boric acid B(OH)₃ is a monobasic Lewis acid (accepts OH^- from water, not a Bronsted acid): B(OH)₃ + H₂O → [B(OH)₄]^- + H⁺.

Aluminium: Amphoteric oxide (Al₂O₃ dissolves in both acid and base). Forms AlCl₃ which dimerises to Al₂Cl₆ (electron-deficient Al completes octet via coordinate bonds). Thermite reaction: 2Al + Fe₂O₃ → Al₂O₃ + 2Fe (highly exothermic, used in welding).

Inert pair effect in Tl: Tl⁺ ($6s^{2}$) is more stable than $Tl^{3}$+ because the 6s electrons are reluctant to participate in bonding (relativistic contraction of s-orbital). TlCl is stable; TlCl₃ decomposes.

Group 14 (Carbon Family): C, Si, Ge, Sn, Pb

Configuration: $ns^{2} \; np^{2}$. Oxidation states: +4 (all), +2 (Sn, Pb dominant due to inert pair effect).

Carbon shows catenation (C-C bonds are strong, 348 kJ/mol) — forms chains, rings, and complex structures. Carbon forms ppi-ppi multiple bonds (C=C, C≡C) unlike heavier members. Allotropes: diamond (sp3, hardest), graphite (sp2, conductor, lubricant), fullerene (C₆₀, sp2 cage), graphene.

Silicon: No catenation (Si-Si bond is weak, 226 kJ/mol). Forms SiO₂ (3D network, very high m.p.) not discrete CO₂-like molecules. Si forms dpi-ppi bonds with O (silicones, silicates). SiCl₄ is hydrolysed (expands octet with d-orbitals); CCl₄ is not (no d-orbitals).

Inert pair effect: $Pb^{2}$+ >> $Pb^{4}$+. PbO₂ is a strong oxidising agent ($Pb^{4}$+ → $Pb^{2}$+). SnCl₂ is a reducing agent ($Sn^{2}$+ → $Sn^{4}$+).

Group 15 (Nitrogen Family): N, P, As, Sb, Bi

Configuration: $ns^{2} \; np^{3}$. Oxidation states: -3, +3, +5 (N also shows -2, -1, +1, +2, +4).

Nitrogen anomalies: (1) Very strong N≡N triple bond (946 kJ/mol) makes N₂ inert. (2) Forms ppi-ppi multiple bonds. (3) Maximum covalence = 4 (no d-orbitals). (4) Small size, high electronegativity.

Phosphorus: Forms P₄ (tetrahedral, 60-degree bond angles cause strain). Exists as P₄O₆ (+3) and P₄O₁₀ (+5). PCl₅ exists but NCl₅ does not (N has no d-orbitals). H₃PO₄ is tribasic; H₃PO₃ is dibasic (one P-H bond, non-ionisable); H₃PO₂ is monobasic (two P-H bonds).

Bismuth: +3 is more stable than +5 (inert pair). Bi₂O₃ is basic. BiCl₃ hydrolyses to BiOCl (bismuthyl chloride). Bi₂O₅ is a strong oxidising agent.

Group 16 (Oxygen Family): O, S, Se, Te, Po

Configuration: $ns^{2} \; np^{4}$. Oxidation states: -2 (all), +2, +4, +6 (S, Se, Te). Oxygen shows only -2, -1 (peroxide), -$\frac{1}{2}$ (superoxide).

Oxygen anomalies: (1) Small size. (2) High electronegativity (2nd highest). (3) No d-orbitals — max covalence = 2. (4) Forms H-bonds (high b.p. of water).

Sulphur: Allotropy — S₈ (rhombic, most stable). Oxyacids: H₂SO₃ (+4), H₂SO₄ (+6). H₂SO₄ is a strong dehydrating agent and oxidising agent (hot conc.). SO₂ is a reducing agent in acidic medium but an oxidising agent with strong reductants. SF₆ exists but OF₆ does not (O has no d-orbitals).

Key p-Block Molecular Structures:

Ozone: | Sulfuric acid: | Nitric acid:

Phosphoric acid: | PCl₅: | SF₆:

Group 17 (Halogens): F, Cl, Br, I

Configuration: $ns^{2} \; np^{5}$. Oxidation states: -1 (all), +1, +3, +5, +7 (Cl, Br, I only — F never shows positive OS).

Fluorine anomalies: (1) Highest electronegativity, only -1 state. (2) F₂ bond is unexpectedly weak (158 kJ/mol vs Cl₂ at 242 kJ/mol) due to lone pair-lone pair repulsion in the small F atom. (3) HF is a weak acid (strong H-bonding). (4) Forms H-bonds (high b.p.).

Bleaching: Cl₂ bleaches by oxidation (permanent). SO₂ bleaches by reduction (temporary).

Interhalogen compounds: XX' type (ClF, BrCl), XX'3 (ClF₃, BrF₃), XX'5 (BrF₅, IF₅), XX'7 (IF₇). The larger halogen is central; geometry predicted by VSEPR.

Oxyacids of Cl: HClO (+1), HClO₂ (+3), HClO₃ (+5), HClO₄ (+7). Acid strength increases with oxidation state: HClO < HClO₂ < HClO₃ < HClO₄. Oxidising power decreases: HClO > HClO₂ > HClO₃ > HClO₄.

Group 18 (Noble Gases): He, Ne, Ar, Kr, Xe, Rn

Configuration: $ns^{2} \; np^{6}$ (He: $1s^{2}$). Generally unreactive due to completely filled valence shell.

Xenon compounds: XeF₂ (linear, sp3d, 3 lone pairs), XeF₄ (square planar, sp3d2, 2 lone pairs), XeF₆ (distorted octahedral, sp3d3, 1 lone pair). Hydrolysis: XeF₂ + H₂O → Xe + HF + O₂; XeF₆ + 3H₂O → XeO₃ + 6HF. XeO₃ is highly explosive. Xe-O bonds exist in XeOF₂, XeOF₄, XeO₃.

Molecular Geometry Diagrams:

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