Fluorine anomalies: (1) Highest electronegativity → only -1 state (never positive OS). (2) F-F bond $\frac{158 kJ}{mol}$ is weaker than Cl-Cl $\frac{242 kJ}{mol}$ due to lone pair-lone pair repulsion in small F2. (3) HF is weak acid (Ka = 6.6 × 10^-4) despite F being most electronegative — the very strong H-F bond $\frac{568 kJ}{mol}$ resists dissociation. (4) HF etches glass: SiO2 + 4HF → SiF4 + 2H2O. Bond energy order: Cl-Cl > Br-Br > F-F > I-I (F-F anomalously low). Acid strength of HX: HF << HCl < HBr < HI (increasing size → weaker H-X bond → easier dissociation). Oxidising power: F2 > Cl2 > Br2 > I2 (F2 is the strongest oxidising agent, reduces everything).