why ionisation energy decrease on moving down the group
The amount of energy required to remove the outermost electron from an isolated, neutral gaseous atom to form a cation is known as ionisation energy.
Ionisation energy decreases as we proceed down the group. This is because the size of the atom increases as we move down the group, and therefore the attraction between the nucleus and the outermost electron decreases. Because of this, it becomes easier for atom to lose the valence electrons. For example, the ionisation energy of potassium is more than that of sodium.
Ionisation energy increases as we move from left to right across a period. Thus the ionisation energy of carbon is more than that of lithium.