arrange the following elements in order of decreasing first ionization energy: O, Cs, Al, Ba, Cd Solution Ionization energy is the energy required to remove an electron from a neutral atom in its gaseous phase. The lower this energy is the easier the atom becomes a cation. Usually atom son the right side of the periodic table have a higher ionization energy. Hence ionization energy increases from left to right on the periodic table. Some elements have several ionization energies which are referred to as first ionization energy, second ionization energy etc. The first ionization energy is the energy required to remove the outermost electron O>Cd>Al>Ba>Cs Cs and Ba are in the 6 th period, Cs has only one valence electron, hence easier to remove it than Ba. Oxygen has the highest ionization energy as it is on the right of the periodic table. It is smaller than the rest and hence the electrons are held strongly by the positive nucleus. .