Which element has the highest ionization energy?

Helium has the highest first ionization energy of any element.

First ionization energy is the minimum energy needed to remove one electron from a neutral atom in the gas phase. Helium requires about 2,372.3 kJ/mol, or 24.587 eV, to lose its first electron—the highest first-ionization value in the periodic table.

Helium’s exceptional value comes from its compact electron configuration, 1s². Both electrons occupy the atom’s only shell, close to the nucleus, where they experience a strong electrostatic attraction. Helium also has a complete, especially stable shell, so removing an electron is difficult.

Ionization energy generally rises from left to right across a period and falls down a group, although electron configurations create small exceptions. Neon, fluorine, and carbon therefore do not surpass helium. Hydrogen is nearby in the first period, but its single electron experiences a weaker nuclear attraction than helium’s electrons.

The question normally means first ionization energy of neutral atoms. Higher ionization energies can be defined for ions after successive electrons are removed, so specifying “first” makes the comparison precise.