Question

Why does heat of atomisation decreases down the group in alkali metal?

Answer 1

Down a group, the atoms have larger and larger atomic radii due to the fact that each next element has more electron shells. Atoms with larger radii attract each other more weakly than atoms with smaller radii do, so it's easier to separate the larger ones, so you need less energy, and therefore the enthalpy of atomization is lower down a group.