Question

Why do Group I metals have lower ionisation enthalpy than corresponding Group II metals?

Answer 1

Because of the large size, these elements have low density which increases down the group from Li to Cs.

Answer 2

Electrons in a similar shell will in general be insufficient at protecting a decidedly changed nucleus from an electron in the valence shell.

In a perfect condition, every electron would shield the nucleus from different electrons, bringing about every valence electron seeing the nucleus having a +1 charge. Not withstanding, as more electrons are added to the valence electron shell, this increases the atomic charge as over +1, making it increasingly hard to evacuate.

As you move to left side of the periodic table, the viable atomic charge turns out to be ever nearer to +1. In the end, in the group 1 elements, there is only one valence electron, bringing about the most minimal positive charge seen by that electron. That makes the group 1 components the most effortless to ionize than the group 2 elements.