Reactivity Gradation in Halogens — Order and Reasons Explained

Reactivity order: F > Cl > Br > I (decreasing reactivity down Group 17)

Key data: • Fluorine (F): most reactive non-metal, electronegativity 3.98 (Pauling scale) • Chlorine (Cl): very reactive gas, electronegativity 3.16 • Bromine (Br): moderately reactive liquid, electronegativity 2.96 • Iodine (I): least reactive halogen (solid), electronegativity 2.66

Atom properties down Group 17: • Atomic radius: increases (F < Cl < Br < I) • Nuclear charge: increases • Number of electron shells: increases • Electronegativity: decreases • Electron affinity: generally decreases (F is exception due to small size) • Oxidising power: decreases (F > Cl > Br > I)

Reason 1 — Atomic size increases: • As we go from F to I, atomic radius increases • Larger atom → outer electron shell is farther from the nucleus • Weaker attraction for incoming electrons • Harder to gain an electron → less reactive

Reason 2 — Electronegativity decreases: • F is the most electronegative element (3.98) • Down the group, less ability to attract electrons • Less tendency to form negative halide ions

Reason 3 — Electron affinity decreases: • Energy released when an electron is gained decreases down the group • Less energetic advantage to gaining an electron

Reason 4 — Shielding effect increases: • More inner electron shells shield the nucleus • Effective nuclear charge felt by incoming electron is reduced • Weaker pull on the electron → less reactive

A more reactive halogen displaces a less reactive halogen from its salt solution:

F₂ + 2KCl → 2KF + Cl₂ (F displaces Cl) Cl₂ + 2KBr → 2KCl + Br₂ (Cl displaces Br) Cl₂ + 2KI → 2KCl + I₂ (Cl displaces I) Br₂ + 2KI → 2KBr + I₂ (Br displaces I) I₂ + 2KBr → No reaction (I cannot displace Br)

Summary: • F₂ displaces Cl⁻, Br⁻, I⁻ • Cl₂ displaces Br⁻, I⁻ (not F⁻) • Br₂ displaces I⁻ only • I₂ cannot displace any halide ion

This shows that oxidising power order: F₂ > Cl₂ > Br₂ > I₂ Fluorine is the strongest oxidising agent among halogens.