Ionization energy is the energy required to remove an electron from an atom or ion. First ionization energy refers to the energy required to remove an electron from an electrically neutral atom in the gas phase. Subsequent ionization energies reflect the energies needed to strip successive electrons off an increasingly positively charged ion. Arrange the elements in decreasing order of first ionization energy. Rank from highest to lowest first ionization energy. To rank items as equivalent, overlap them.