ionization energy is the energy needed to remove the most loosely attached electron from an atom. the atom is not connected to any other atoms. the chemical elements to the left of the periodic table have a much lower ionization energy. the ones to the right have a much higher ionization energy. the chemical elements down the periodic table have a much lower ionization energy (due to electrons being farther away from the atom with increasing atomic radius). the ionization energy increases as each electron is removed.