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JEE Main levelSingle correct optionDifficulty 2 of 3ph182

Iodine vapour is violet while fluorine is pale yellow. Which explanation is correct?

  1. AThe colour arises from d-d transitions, which are easier in the larger atoms
  2. BHeavier halogens have more unpaired electrons, which absorb visible light
  3. CI2 emits violet light when heated
  4. DThe energy needed to excite valence electrons falls from F2 to I2, so I2 absorbs lower-energy yellow-green light and transmits violet

Answer: D

Halogen colour arises from absorption of visible light, which excites an electron between molecular orbitals. The gap shrinks down the group, so F2 absorbs high-energy violet and looks pale yellow, while I2 absorbs lower-energy yellow-green and looks violet. The halogens have no d-d transitions and no unpaired electrons in the ground state.
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