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