Ionization potential is a periodic property. Generally in a group with the increase of atomic number the ionization potential of elements decreases from top to bottom. This is due to the fact that with the increase of atomic number in a group the number of electron shell increases and hence the atomic radius also increases.
So, the outermost electron is situated at more and more distance from the nucleus. Hence the attraction of the nucleus on the outermost electron decreases. Then it is comparatively easy to remove the outermost electrons to form positive ion. For this reason, the ionization potential decreases lion, top to bottom in a group. In any group the ionization potential of first clement is the highest and that of last element is the lowest when one goes from top to bottom.