Across a period
As the nuclear charge increases so the attraction between the outermost electrons and the nucleus increases and therefore so does the ionisation energy.
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General trends:
As the nuclear charge increases so the attraction between the outermost electrons and the nucleus increases and therefore so does the ionisation energy.
On going down a group the number of shells increases so the distance between the nucleus and the outer electrons increases. In addition each complete shell of electrons has a "SHIELDING EFFECT" making it easier to remove the outer electrons and therefore lowering the ionisation