The reactivity of halogens decreases from top to bottom in Group 17 of the periodic table. The order of reactivity is: Fluorine (F) > Chlorine (Cl) > Bromine (Br) > Iodine (I). This decreasing trend is due to increasing atomic size, decreasing electronegativity, and decreasing electron affinity as we go down the group. Fluorine is the most reactive of all non-metals; iodine is the least reactive halogen under normal conditions.
Halogen reactivity order: F > Cl > Br > I (decreases down Group 17).
Reactivity decreases because atomic size increases and electronegativity decreases.
F is the most electronegative element (3.98 Pauling) and the most reactive non-metal.
Displacement reaction evidence: Cl₂ displaces Br⁻ and I⁻; Br₂ displaces I⁻ only; I₂ displaces none.
Oxidising power order: F₂ > Cl₂ > Br₂ > I₂.
Fluorine reacts explosively with most substances; iodine reacts slowly.
F₂ reacts with even noble gases like Xe under certain conditions.
Reducing power of halide ions is reverse: I⁻ > Br⁻ > Cl⁻ > F⁻.
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.
The reactivity of halogens decreases from top to bottom: F > Cl > Br > I. Fluorine is the most reactive halogen (and the most reactive non-metal), while iodine is the least reactive among the four common halogens.
Reactivity decreases down Group 17 because: (1) atomic size increases — the outer shell is farther from the nucleus, so the pull on incoming electrons is weaker; (2) electronegativity decreases; (3) electron affinity decreases; (4) the shielding effect of inner electron shells increases, reducing the effective nuclear charge felt by an incoming electron.
A more reactive halogen displaces a less reactive one from its salt. Cl₂ added to KBr solution liberates Br₂ (Cl₂ + 2KBr → 2KCl + Br₂). Br₂ added to KI solution liberates I₂. But I₂ cannot displace Br⁻ or Cl⁻. This confirms reactivity: Cl > Br > I.
Fluorine is the smallest halogen with the highest electronegativity (3.98) and the strongest pull on electrons. It has the smallest atomic radius, so the nucleus attracts incoming electrons most strongly. It also has the highest oxidising power and reacts spontaneously with almost every element.
Oxidising power decreases down the group: F₂ > Cl₂ > Br₂ > I₂. This is because a stronger oxidising agent more readily gains electrons. Fluorine has the highest tendency to gain electrons, making it the strongest oxidising agent among halogens.
Olfactory Indicators
Olfactory indicators change smell (not colour) in acid or base. Examples: onion, clove oil, vanilla. Used when colour change is hard to detect. NCERT Class 10 Chemistry.
One Mole of Methane — Everything You Need to Know
One mole of methane (CH₄) = 16 g, 6.022×10²³ molecules, 4 moles of H atoms. At STP it occupies 22.4 L. Learn all mole calculations for CH₄ with FAQs.
Difference Between Order and Molecularity
Understand the key differences between the Order of a reaction (an experimental value) and Molecularity (a theoretical concept).
Ostwald Dilution Law — Formula and Derivation
Learn Ostwald's Dilution Law for weak electrolytes. Understand the relationship between degree of dissociation, Ka, and concentration with derivation.
Oxidation Number of Cl (Chlorine) in Different Compounds
Learn the variable oxidation numbers of Chlorine (Cl). Understand why it ranges from -1 to +7 with examples like NaCl, HClO, HClO4.
Turn this guide into revision flashcards, a practice exam, or an AI-generated podcast — free, no signup required.