: 250
Group 17 (F, Cl, Br, I) has ns^{2np}^5. Fluorine's anomalies are critical: highest EN (only -1 state, never positive), F-F bond anomalously weak (158 kJ/mol, lone pair repulsion), HF is weak acid (very strong H-F bond resists dissociation, 568 kJ/mol), and HF etches glass (SiO2 + 4HF to SiF4 + 2H2O).
Oxidising power: F2 > Cl2 > Br2 > I2. Each displaces those below it. Bond energy: Cl-Cl > Br-Br > F-F > I-I. Acid strength of HX: HI > HBr > HCl >> HF. Reducing power of HX: HI > HBr > HCl > HF.
Oxyacids of chlorine show a paradox: acid strength increases (HClO < HClO4) but oxidising power decreases (HClO > HClO4). More O atoms stabilise the conjugate base (stronger acid) but also stabilise the compound itself (weaker oxidiser).
Interhalogen compounds (XX', XX'3, XX'5, XX'7) are more reactive than parent halogens. Geometries: linear, T-shaped, square pyramidal, pentagonal bipyramidal. Only IF7 exists as XX'7.
Cl2 disproportionation: cold NaOH gives NaOCl (bleach), hot NaOH gives NaClO3 (chlorate). Aqua regia (3HCl:1HNO3) dissolves Au and Pt by combined oxidation and complexation.
Bleaching powder Ca(OCl)2 is made from Ca(OH)2 + Cl2.