<h2>Group 15 – Nitrogen Family (N, P, As, Sb, Bi)</h2> <p>General configuration: ns²np³. Nitrogen shows unique properties due to small size, high electronegativity, and absence of d-orbitals.</p> <p><strong>Allotropes of Phosphorus:</strong> White P (most reactive, P₄ tetrahedral, toxic), Red P (polymer chain), Black P (most stable, layered).</p> <p><strong>Oxoacids of P:</strong> H₃PO₃ (phosphorous acid, diprotic, reducing), H₃PO₄ (phosphoric acid, triprotic), H₃PO₂ (hypophosphorous, monobasic, strongest reducing agent).</p> <p><strong>Oxides of Nitrogen:</strong> N₂O (laughing gas), NO (colourless, paramagnetic), NO₂ (brown, paramagnetic), N₂O₅ (anhydride of HNO₃).</p> <h2>Group 16 – Oxygen Family (O, S, Se, Te, Po)</h2> <p><strong>Allotropes of Sulphur:</strong> Rhombic S (stable below 96°C, S₈ crown structure), Monoclinic S (stable above 96°C), Plastic S (amorphous, fibrous).</p> <p><strong>Oxoacids of S:</strong></p> <ul> <li>H₂SO₃ (sulphurous acid)</li> <li>H₂SO₄ (sulphuric acid) — concentrated is dehydrating and oxidising agent</li> <li>H₂S₂O₇ (oleum/pyrosulphuric acid) — fuming sulphuric acid</li> <li>H₂S₂O₈ (Marshall's acid/peroxodisulphuric acid)</li> </ul> <h2>Group 17 – Halogens (F, Cl, Br, I, At)</h2> <p>Most electronegative group. F has no d-orbitals, highest oxidising power, forms only −1 oxidation state.</p> <p><strong>Interhalogen compounds:</strong> XX′ type (e.g., ClF, BrF, ICl). Formed between two different halogens. More reactive than pure halogens.</p> <p><strong>Oxoacids of Cl:</strong> HOCl (hypochlorous, +1), HClO₂ (chlorous, +3), HClO₃ (chloric, +5), HClO₄ (perchloric, +7) — acid strength: HClO₄ > HClO₃ > HClO₂ > HOCl.</p> <p><strong>Pseudohalogens:</strong> CN⁻, OCN⁻, SCN⁻ behave like halide ions.</p> <h2>Group 18 – Noble Gases (He, Ne, Ar, Kr, Xe, Rn)</h2> <p>Complete valence shell (ns²np⁶). Xenon forms compounds with F and O due to large size and low ionisation energy: XeF₂ (linear), XeF₄ (square planar), XeF₆ (distorted octahedral), XeO₃ (pyramidal), XeOF₄ (square pyramidal).</p>