Ionization is the process in which an electron is pulled completely away from an atom, leaving an ion with positive charge. The energy necessary to accomplish this process is called the ionization energy or the ionization potential of the atom.

The first ionization potential of an element is the amount of energy required to remove one electron from the ground state orbital of highest energy, e.g., the valence shell, of the neutral atom. An ion with unit positive charge results. The second ionization potential is the energy needed to completely remove an electron from the valence shell of the ion that results from the first ionization. Similar definitions may be given for the third, fourth, and higher ionization potentials.

Ionization is closely related to electronegativity. The electronegativity of an atom is a measure of its ability to attract an electron and hold on to it. Its value...