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My book says

Alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light.

Why do they develop colour when exposed to light? It is important to note that alkyl fluorides, and alkyl chlorides are not said to develop colour.

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This is because of spontaneous breakdown in case of iodides and bromides which have a lower bond-dissociation enthalpy as compared to fluorides and chlorides. This low enthalpy means that even at normal temperature or on exposure to sunlight, the halides (iodides and bromides) decompose to a certain extent forming free iodine and bromine, both of which are coloured and hence impart colour to the original halides.

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  • $\begingroup$ @Satwikpasani.Hi Pasani, why do free iodine and bromine (radicals) impart color, whereas bonded don't? $\endgroup$
    – Sensebe
    Nov 9, 2013 at 8:42
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    $\begingroup$ @CURIE Colour is a property of the electronic distribution of the chemical species. Ion and its atom in a bonded molecule state are completely different chemical species, and therefore there is no correlation in their chemical porperties. $\ce{Cl-}$ is essential for life while $\ce{Cl2}$ is poisonous. The same is true for $\ce{Na}$. $\ce{Cu^++}$ is blue while metallic copper is pink. The same holds true for $\ce{Fe^++,Fe^3+}$. $\endgroup$
    – stochastic13
    Nov 9, 2013 at 10:04
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This is because of the fact that in presence of light iodine and bromine absorbs light and excitation of the electron from the ground state to the excited state takes place which on returning back to the ground state radiates energy. This radiating energy comes under the visible region. Therefore they are co lured.

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    $\begingroup$ which on returning back to the ground state radiates energy. So this is fluorescence?! Are you sure? I don't believe that is right... $\endgroup$
    – G M
    May 7, 2014 at 19:21
  • $\begingroup$ The color one sees with a white light source is the complement of the absorbed light [that is also the reradiated light] The color is the light that is NOT absorbed. $\endgroup$
    – jimchmst
    Nov 19, 2022 at 0:54

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