# Why does color of halogen deepens and the boiling point decreases down the group?

Why does the color of the halogens deepens as we go down the group? also why there is a change in physical state of halogens down the group?

\begin{array}{c|c} \mathbf{Gas} & \mathbf{Color} \\\hline \text{fluorine} & \text{very pale yellow (almost invisible to naked eye)}\\ \text{chlorine} & \text{yellow-green} \\ \text{bromine} & \text{red-brown}\\ \text{iodine} & \text{violet-black}\end{array}

• Just a thingy: In case of $\ce{Cl}$ , the color is also almost invisible... You sometimes have to place a white paper in the background to see the slight color. – NeilRoy Jul 11 '15 at 14:10
• BTW a quora link : quora.com/Why-are-halogens-coloured – NeilRoy Jul 11 '15 at 14:12
• Fun Fact : Possible color of $5^{\text{th}}$ halogen : $\ce{At}$ is Black! (black solid) – NeilRoy Jul 11 '15 at 14:15

• As for color. As a guess, I'd say that the active transition is lone pair orbital $\rightarrow$ $\sigma$-antibonding orbital (see any introduction to MO theory for the terminology). The energy of this transition should lower from fluorine to iodine because of great decrease in bonding energy and increase of size of atoms. According to color wheel, the absorption in violet spectrum (with highest energy of photons in visible spectrum) should give yellowish apparent color (case of fluorine and chlorine), red apparent color to adsorption in lower-energy and violet color for adsorption at even lower-energy red spectrum.