Among the following halogens
F₂, Cl₂, Br₂ and I₂
Which can undergo disproportionation reactions?

Solution

Chemistry ma’am · 4 steps

To disproportionate, a halogen must go both up and down from zero, and fluorine has no way up.

1. What disproportionation needs. In disproportionation the same element is both oxidised and reduced. Starting from 0 in X₂, one atom must drop to -1 while another rises to a positive oxidation state.
2. Why fluorine fails. Fluorine is the most electronegative element, so it never shows a positive oxidation state. It also has no d orbitals to expand its valence. It can only go from 0 to -1, so it can only be reduced.
3. Chlorine, bromine, iodine. These can show +1, +3, +5 and +7. In alkali they split into -1 and a positive state, for example 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O, where Cl goes from 0 to -1 and to +5. Bromine and iodine react the same way.
4. So. Only Cl₂, Br₂ and I₂ can disproportionate, which is option (b).

The idea to keep. Disproportionation needs one element to move both up and down in oxidation state. Fluorine only shows -1 and 0, so it is the one halogen that never disproportionates.

Answer from the NTA final answer key. Solution by PrepWiser.