Explain why halogens are coloured and colour depends on moving down in the group from fluorine to iodine.

Hint: Halogens are the group $17$ elements of p block. Their general electronic configuration is $𝑛𝑠^2 𝑛𝑝^5$
- Fluorine gas is a yellow gas
- Chlorine gas is greenish yellow gas
- Bromine is reddish liquid
- Iodine is violet colour solid

Complete step by step answer: The colour of halogens is due to the absorption of different quanta of radiations in the visible region which results in excitation of outer electrons to higher energy levels, thus different colours are observed. The amount of energy required for excitation for every halogen is different so different colours are observed.
The colour becomes darker and darker on moving down the group as we move down the group the atomic radii increase. Due to this the energy level becomes closer. More shells get added so more energy levels are present. Due to this the high energy wavelengths are absorbed . The colours that are shown are complementary colours and not the absorbed colours.
Halogens have unpaired electrons in their outermost shell. When light of a suitable wavelength falls on them , the electrons get excited from the lower energy region to the higher energy region showing the colour complementary to the colour absorbed.

Note:
In halogens , with increases in atomic number their boiling and melting point steadily increases.
Bromine and iodine are sparingly soluble in water but soluble in solvents like chloroform.
Halogens have high electronegativity . Fluorine is the most electronegative element in the whole periodic table . Down the group electronegativity decreases.
They have very high ionisation enthalpy , thus have little tendency to lose electrons. Down the group ionization enthalpy decreases due to increase in size.