Alkali Metals
The ionization energy of alkali metals can be explained by this periodic trend. The alkali metals are on the far left side of the periodic table, thus they have the lowest ionization energy. This can be explained by the fact that they all have one valence electron, thus they "want" to get rid of it. Furthermore, an alkali metal's valence electron is relatively far away from the nucleus, making it easier to remove
Alkaline earth metals
The ionization energy of alkaline earth metals is also very low, as it is the second row to the far left. All of these metals have two valence electrons, so they "want" to get rid of them but not as badly as alkali metals. Just like the rest of the periodic table, this group decreases in ionization energy as you go down the group, mainly because as you go down the group, the elements increase in size. And remember, as size increases, ionization energy usually decreases.
halogens
Halogens have one of the highest ionization energies out of all element groups, as they are all the way to the right of the periodic table. They all have seven valence electrons, so they are so close to an outer shell that they don't "want" to move in the other direction. One halogen, Iodine, sometime does actually like to loose an electron to form the ion I+.
Noble Gasses
The noble gasses have the highest ionization energy out of any other element group, thus they are on the far right on the periodic table. Noble gasses all have a full outer shell, so they are very "happy" molecules. They do not want to give up any electrons, as that would make them loose this full outer shell.