Ionization Energy Calculator

The Ionization Energy Calculator lets you look up the energy required to remove one electron from a neutral gaseous atom in its ground state. Ionization energy (IE) is a key periodic property: the higher it is, the more tightly an atom holds its electrons. The first ionization energy is always lower than the second because removing a second electron from an already positively charged ion requires overcoming greater electrostatic attraction.

Ionization energy increases across a period (left to right) as nuclear charge grows while shielding remains roughly constant, and decreases down a group as valence electrons move farther from the nucleus and are shielded by inner shells. The highest first ionization energy belongs to helium at 2372 kJ/mol, while cesium has the lowest among stable elements at 376 kJ/mol. Successive ionization energies show dramatic jumps when a core electron is removed.

Enter an element symbol or atomic number to retrieve its first, second, and higher ionization energies in kJ/mol or eV. Applications include predicting metallic versus nonmetallic character, understanding oxidation states, and explaining why certain ions form preferentially. For example, the large jump between the 2nd and 3rd IE of magnesium confirms it forms Mg²⁺ rather than Mg³⁺.