The ionization energy of an atom or molecule describes the minimum amount of energy required to remove an electron from the atom or molecule in the gaseous state. Do not confuse with [electron-affinity].

The ionization energy or ionization potential is the energy necessary to remove an electron from the neutral atom. It is a minimum for the alkali metals which have a single electron outside a closed shell. It generally increases across a row on the periodic maximum for the noble gases which have closed shells. For example, sodium requires only $\pu{496 kJ/mol}$ or $\pu{5.14 eV/atom}$ to ionize it while neon, the noble gas immediately preceding it in the periodic table, requires $\pu{2081 kJ/mol}$ or $\pu{21.56 eV/atom}$. The ionization energy is one of the primary energy considerations used in quantifying chemical bonds.

See also:

  1. Wikipedia
  2. Hyperphysics