Ionization energy is the energy required to remove an electron from an atom or ion. This energy tends to decrease down a group; each element has more occupied energy than the one above. As a result, the outermost electrons are farthest from the nucleus in elements near the bottom of a group. Additionally, as you move down a group, each successive element contains more electrons in the energy levels between the nucleus and outmost electrons. This means that the force of attraction between the positively-charged nucleus and the negatively charged outer electrons becomes weaker, making the outer electron more easily lost. So potassium is more reactive than lithium because the outer electron of a potassium atom is further from its nucleus than
