Short Answer

Reactivity of halogens decreases down the group because the atomic size increases, and the valence electrons are farther from the nucleus.

• Larger atomic radius reduces nuclear attraction for incoming electrons, making it harder to gain an electron.
• As a result, fluorine is the most reactive, while iodine and astatine are less reactive.

Detailed Explanation :

Electronic Configuration and Reactivity

Halogens are elements of Group 17, including fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).

• General electronic configuration: ns²np⁵.
• Each halogen needs one electron to complete its octet, which drives its chemical reactivity.

Factors Causing Decreased Reactivity Down the Group

1. Increase in Atomic Size:
   • Atomic radius increases from F → At due to additional electron shells.
   • Outer electrons are farther from the nucleus, decreasing attraction for incoming electrons.
2. Decrease in Effective Nuclear Charge:
   • Inner shell electrons shield the nucleus, reducing pull on valence electrons.
   • Down the group, shielding effect increases, lowering reactivity.
3. Electron Affinity Trend:
   • Electron affinity decreases down the group.
   • Fluorine has a high tendency to gain an electron → highly reactive.
   • Iodine and astatine have lower electron affinity → less reactive.
4. Bond Dissociation Energy:
   • Halogen molecules are diatomic (X₂).
   • Bond strength decreases slightly down the group, but larger size reduces nuclear attraction for added electrons, making reaction slower.
5. Electronegativity:
   • Decreases down the group → reduced tendency to attract electrons.
   • Fluorine is highly electronegative → easily gains electrons, while astatine is less electronegative.

Reactivity Trend Across Halogens

1. Fluorine (F₂):
   • Small size, high electronegativity → strongest oxidizing agent → most reactive.
2. Chlorine (Cl₂):
   • Larger size than fluorine → slightly less reactive but still very reactive.
3. Bromine (Br₂):
   • Even larger atomic size → reduced reactivity compared to Cl₂.
4. Iodine (I₂):
   • Large atomic radius → weak nuclear pull → less reactive.
5. Astatine (At₂):
   • Very large atom, low electron affinity → least reactive halogen.

Chemical Evidence

1. Displacement Reactions:
   • More reactive halogens displace less reactive halogens from their compounds.
   • Example: Cl₂ + 2KBr → 2KCl + Br₂
   • Demonstrates reactivity decreases down the group.
2. Reactions with Metals:
   • Fluorine reacts violently with metals → produces halide salts.
   • Iodine reacts slowly due to lower reactivity.
3. Hydrogen Halides:
   • Fluorine reacts directly with hydrogen at room temperature → HF formation.
   • Iodine reacts with hydrogen only at high temperature → HI formation.

Conclusion

Reactivity of halogens decreases down the group due to increased atomic size, greater shielding, lower electron affinity, and decreased electronegativity. Fluorine is the most reactive halogen, while iodine and astatine are much less reactive. This trend explains displacement reactions, chemical reactivity with metals, and bond formation tendencies across Group 17 elements, providing insight into halogen chemistry.