Question

why halogens are coloured explain from MO diagram

Answer 1

The origin of the colour of the halogens stems from the excitation between the highest occupied π* MO and the lowest unoccupied σ* MO. The energy gap between the HOMO and LUMO decreases according to F2 > Cl2 > Br2 > I2.  The amount of energy required for excitation depends upon the size of the atom. Fluorine is the smallest element in the group and the force of attraction between the nucleus and the outer electrons is very large. As a result, it requires a large excitation energy and absorbs violet light (high energy) and so appears pale yellow. On the other hand, iodine needs significantly less excitation energy and absorbs yellow light of low energy. Thus it appears dark violet. Using similar arguments, it is possible to explain the greenish yellow color of chlorine and the reddish brown color of bromine.  The halogens show a variety of colours when dissolved in different solvents. Solutions of iodine can be bright violet in CCl4, pink or reddish brown in aromatic hydrocarbons and deep brown in alcohols for example. This can be explained by weak donor-acceptor interaction and complex formation. The presence of charge-transfer bands further supports this since they are thought to be derived from interaction with the HOMO σu* orbital.

Answer 2

"The halogens exert color form the HOMO (π*) to LUMO (σ*)

         The energy gap difference between HOMO and LUMO is in the decreasing order from F2>Cl2>Br2>I2. Here the atom of fluorine is small. so the electrons are bound very strongly towards the nucleus. Hence the energy required for the excitation of the electrons are high and it absorbs violet light and appears as pale yellow.

         Moreover, Iodine requires very low energy for the excitation of the electrons. It absorbs yellow light at low energy and appears dark violet.Similarly, greenish yellow for chlorine and reddish-brown color for bromine  When dissolved in solvents halogens show variety of colors.

          For e.g. Iodine in$CCl_4$ is bright violet and Iodine in aromatic hydrocarbon is reddish brown and iodine in alcohol is deep brown. This color change is due to weak donor electron acceptor interaction due to complex formation.The presence of charge-transfer bands further supports this, because they are derived from interaction with the HOMO σu* orbital.

The MO diagram of fluorine is given below,"