Ionization energy is the amount of energy it takes to remove an electron from an atom. First ionization energy is the amount of energy required to remove the first electron. Ionization energy follows periodic trends.
Ionization energy is affected by: nuclear charge, the distance from the nucleus, the shielding effect, and the sublevel effect.
When nuclear charge is increased, the electrons are pulled more tightly into the nucleus, and therefore ionization energy increases.
The distance from the nucleus affects ionization energy because the attraction is stronger when the electrons are closer to the nucleus.
The shielding effect occurs because inner electrons block the electromagnetic attraction between outer electrons and the nucleus. When the shielding effect increases, ionization energy decreases.
The sublevel effect states that full or half-full sublevels are more stable than they normally would be. This causes an increased ionization energy.
Generally, when one goes across a period to the right, ionization energy increases, and when one goes down a group, ionization energy decreases.