Explain the trend of ionisation enthalpy of alkali metals as we move from top to bottom in the group.

Ionisation enthalpy and atomic radius are inversely related.
As we move from top to bottom in a group,the atomic radius increases due to the addition of new shells.
Consequently,the outermost electron is located further away from the nucleus,which increases the shielding effect of the inner electrons.
This increased shielding effect outweighs the increase in nuclear charge,resulting in a weaker electrostatic attraction between the nucleus and the valence electron.
Therefore,less energy is required to remove the valence electron,leading to a decrease in ionisation enthalpy as we move down the group.