ionisation potential The energy needed to remove the outermost electron of an atom. The energy needed to remove the second electron is called the second ionisation potential, and so on. The ionisation potential for larger atoms is smaller as the electron is further removed, making the electrostatic attraction due to the nucleus, smaller. This is why the ionisation potential generally decreases as we go down any group in the periodic table. The inner electrons partially shields the outermost electron from the nucleus. For example in alkali metals (group I), the ionisation potential is small. As we go from left to right in any period, the ionisation potential increases because of the increase of nuclear charge without any change in the number of inner shielding electrons.