Ionization energy is the amount of energy required to remove the outmost electron. It is closely related to electronegativity.
Period - ionization energy increases as you go from left to right across a period.
Elements on the right of the chart want to take others atom's electron (not given them up) because they are close to achieving the octet. The means it will require more energy to remove the outer most electron. Elements on the left of the chart would prefer to give up their electrons so it is easy to remove them, requiring less energy (low ionization energy).
Group - ionization energy decreases as you go down a group.
The shielding affect makes it easier to remove the outer most electrons from those atoms that have many electrons (those near the bottom of the chart).
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