 The "first ionization energy" is the energy required to remove the first electron, and generally the "nth ionization energy" is the energy required to remove the atom's "n"th electron, after the ("n"−1) electrons before it has been removed.Trend-wise, ionization energy tends to increase while one progresses across a period because the greater number of protons (higher nuclear charge) attract the orbiting electrons more strongly, thereby increasing the energy required to remove one of the electrons.Ionization energy and ionization potentials are completely different.