Going down a group, the ionisation energy decreases. This is due to the shielding or screen effect of the outer electrons from the nucleus and so the attraction is weaker and they are more easily removed.

Why does ionization energy decrease down a group?

The greater the ionization energy, the more difficult it is to remove an electron. ... Conversely, as one progresses down a group on the periodic table, the ionization energy will likely decrease since the valence electrons are farther away from the nucleus and experience greater shielding.

Why does ionization potential decrease in group and increase in period?

The ionization energy of the elements within a period generally increases from left to right. This is due to valence shell stability. The ionization energy of the elements within a group generally decreases from top to bottom. This is due to electron shielding.