Why are halogens coloured.

Most of the halogens are coloured because they absorb radiations which are in the visible region which in turn excite the valence electrons to higher energy levels. As the amount of energy which is required to excite the electrons to a higher level is different for different halogens, each halogen has a different colour.

Why does nitrogen show catenation properties less than phosphorus?

Catenation is the property of bonding of atoms of the same element into a series to form a chain. Catenation is shown by phosphorous more than nitrogen. This is due to the reason that the N - N single bond is relatively weaker as compared to the P - P single bond. The nitrogen atom is small. So, there is greater repulsion of electron density of two nitrogen atoms, thereby weakening the N - N single bond. Phosphorous has a larger atomic size, so it has less repulsion of electron density of two phosphorous atoms. Therefore, the P - P single bond is stronger.