 Ionization energy

The ionization energy is the minimum energy needed to remove an electron from an isolated atom or molecule in a specified state. Ground-state hydrogen has ionization energy $13.6\mathrm{eV}$.

Ionization energy, also known as ionization potential, is the amount of energy required to remove an electron from an atom or a molecule in its gaseous state. It is a measure of how strongly an atom or molecule holds onto its electrons.