Ionization energy decreases from top to bottom, and increases from left to right.
Ionization energy is the energy is takes for an atom to lose an electron, thus becoming a cation (since its now more positive than negative). So, the larger an atom (high atomic radius, more electrons) is the easier it is for them to lose electrons, which means the energy required (ionization energy) is lower. So as you go down a group, ionization energy decreases. As you go across a period, left to the right the ionization energy increases because the elements are getting closer to filling their electron shell. This is shown by the right most column 18, the noble gases, who have 8 valence electrons and hold onto them tightly and thus do not react often with other elements. This is why the noble gases have the highest ionization energy.
