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}

  • $\begingroup$ 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. $\endgroup$ – NeilRoy Jul 11 '15 at 14:10
  • $\begingroup$ BTW a quora link : quora.com/Why-are-halogens-coloured $\endgroup$ – NeilRoy Jul 11 '15 at 14:12
  • $\begingroup$ Fun Fact : Possible color of $5^{\text{th}}$ halogen : $\ce{At}$ is Black! (black solid) $\endgroup$ – NeilRoy Jul 11 '15 at 14:15
  • As for the BP: The attraction of covalent molecules is due 'dispersion' interaction. It is a quite complex effect, commonly described as interaction of 'instantaneous dipoles', i.e. interaction of dipoles formed by nucleus and electrons around it. Its efficiency generally increases with number of electrons (due to sheer numbers) and size of electron shell. Since outer electron shells vastly grow in size in fluorine-iodine row, the efficiency of dispersion interactions grow. In fact, close halogen-halogen contacts for heavier halogens (bromine and iodine) are very common in crystal structures of organic compounds.

  • 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.

  • $\begingroup$ The boiling point trend can also be accounted for by stronger halogen bonding going from difluorine to diiodine. Also, I believe the colour is due to a pi* to sigma* transition. $\endgroup$ – Tan Yong Boon Jul 15 '18 at 23:36

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