Ionization energy is the amount of energy required to remove an electron from an atom or ion in its gaseous state. This process results in the formation of a positively charged ion. Generally, elements with higher atomic numbers have higher ionization energies because their electrons are held more tightly by the nucleus. The ionization energy can vary across the periodic table. It tends to increase from left to right across a period, as the increasing nuclear charge attracts electrons more strongly. Conversely, ionization energy decreases down a group, as the outer electrons are farther from the nucleus and experience more shielding from inner electrons.