The ionization potential is one of these and may be defined as the energy required to remove an electron from an atom or ion. The term, ionization energy, is sometimes used in place of ionization potential. The trends in ionization energy are just the reverse of those for atomic radii.
- Within Group – Ionization reduces the motion of the item group (column) from top to bottom. This is because the outermost electron’s key quantity increases going down the group. There are more protons in atoms that pass down a group (greater positive charge), yet the effect is to draw in the shells of electrons, make them smaller and shield the outer electrons from the nucleus attractive power.
- Across Periods – The energy of ionization generally increases the movement of elements from left to right (row). This is because the atomic radius generally decreases moving over a period of time, resulting in a greater effective attraction between the negatively charged electrons and the positively charged nucleus. Ionization is the minimum value for the alkali metal on the left side of the table and the maximum value for the noble gas on the far right side of the table. The noble gas has a shell packed with valence, so it resists the removal of electrons.