p-Block Elements: Groups 13-15

The p-block spans Groups 13-18, with elements filling the $ns^{2} \; np^{1}$-6 configuration. Groups 13-15 contain some of the most reaction-dense topics in inorganic chemistry, including boron compounds, nitrogen oxides, phosphorus oxoacids, and two major industrial processes.

Group 13 (Boron Family) has the outer configuration $ns^{2} \; np^{1}$. Boron is anomalous — it is a nonmetal that forms electron-deficient covalent compounds and acts as a Lewis acid. Diborane (B₂H₆) features two 3-centre-2-electron (3c-2e) "banana bonds," where each bridging hydrogen shares its electron pair across a B-H-B triangle. Borax (Na₂B₄O₇.10H₂O) contains two BO₄ tetrahedra and two BO₃ triangular units; the borax bead test produces colored metaborates for cation identification. Boric acid (H₃BO₃) is a weak monobasic Lewis acid — it does not donate H+ but accepts OH- from water: H₃BO₃ + H₂O → [B(OH)₄]- + H+. It has a layered structure held by hydrogen bonds. Aluminium chloride (AlCl₃) exists as the dimer Al₂Cl₆, acts as a Lewis acid, and is the key Friedel-Crafts catalyst. The inert pair effect makes the +1 oxidation state stable for thallium (Tl).

Group 14 (Carbon Family) features carbon allotropes: diamond (sp3, hardest natural substance), graphite (sp2, layered conductor due to delocalized electrons), and fullerene (C₆₀, spherical cage). Carbon monoxide (CO) is a neutral ligand in coordination chemistry, a potent poison that binds hemoglobin 200 times more strongly than O₂, and a reducing agent. Carbon dioxide (CO₂) is linear, an acidic oxide, and a greenhouse gas; notably, Mg burns in CO₂ (2Mg + CO₂ → 2MgO + C). Silicon forms silicones (R₂SiO polymers, water-repellent), silicates ($SiO4^{4}$- tetrahedral units), and zeolites (hydrated aluminosilicates used as molecular sieves and ion-exchange catalysts).

Group 15 (Nitrogen Family) features the exceptionally strong N-N triple bond (945 kJ/mol). The Haber process manufactures ammonia: N₂ + 3H₂ <=> 2NH₃ using an Fe catalyst at 450 degrees C and 200 atm, optimized by Le Chatelier's principle (low temperature favors forward, but high temperature needed for rate, so compromise at 450 degrees C). The Ostwald process converts NH₃ to HNO₃ in three steps: (i) 4NH₃ + 5O₂ →(Pt/Rh, 500 degrees C) 4NO + 6H₂O, (ii) 2NO + O₂ → 2NO₂, (iii) 3NO₂ + H₂O → 2HNO₃ + NO.

Oxides of nitrogen range from N₂O (+1, neutral, laughing gas) through NO (+2, neutral, paramagnetic), N₂O₃ (+3, acidic, anhydride of HNO₂), NO₂ (+4, acidic, brown, paramagnetic, dimerizes to N₂O₄), to N₂O₅ (+5, acidic, anhydride of HNO₃). Phosphorus allotropes include white P (P₄ tetrahedra, poisonous, glows in dark), red P (polymeric, stable), and black P (most stable, layered). PCl₅ adopts trigonal bipyramidal geometry (sp3d) with 3 shorter equatorial and 2 longer axial bonds. The basicity of phosphorus oxoacids depends on the number of P-OH bonds: H₃PO₂ (1 P-OH, monobasic), H₃PO₃ (2 P-OH, dibasic), H₃PO₄ (3 P-OH, tribasic).

The key testable concept is the basicity of phosphorus oxoacids determined by P-OH bond count (not total hydrogen count), and the industrial conditions for the Haber and Ostwald processes.