Trends of the periodic table
The period trend
ionization energy increases as you go from left to right across a period because elements on the right of a chart want to take others atom's electrons, because they are too close to achieving the octet. Elements on the left of the chart would prefer to give up their electrons so it is easy to remove them, requiring less energy.
The group trend
Ionization energy decreases as you go down a group, because the shielding effect makes it easier to remove the outer most electrons from those atoms that have many electrons.