Why are halogens coloured?

Explanation

All the halogens are coloured. The colour is due to the reason that their molecules absorb light in the visible region as a result of which their electrons get excited to higher energy levels while the remaining light is transmitted. The colour of the halogens is actually the colour of this transmitted light. The amount of energy needed for excitation decreases progressively from F to I as the size of the atom increases. consequently, the energy of the transmitted light goes on increasing form F to I. In other words, the colour of the halogens deepens from `F_(2)` to `I_(2)`. For example, `F_(2)` absorbs violet light (higher excitation energy) and hence appears pale yellow while iodine absorbs yellow and green light (lower excitation energy) and hence appears deep violet. Similarly, we can account for greenish yellow colour of `Cl_(2)` and orange red colour of bromine.