# Why do alkyl bromides and iodides develop colour when exposed to light?

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.

• @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 $\ce{Fe^++,Fe^3+}$. – Satwik Pasani Nov 9 '13 at 10:04