Variation along Period
As we move along the period, the atomic size decreases due to increase in nuclear charge. Hence the ionisation energy also increases along the period. Thus alkali metals have smallest Ionisation energy.
Variation down the group
On descending a group, the atomic size increases and the nuclear attraction for the outer electrons decreases. Hence, it becomes easier to remove the outer electrons in a group. Thus, Ionisation energy decreases on descending a group. Thus in group 1, Cs has lowest Ionisation energy in the group as well as in periodic table.
There are some exceptions when we move along the period of the periodic table like for e.g there is a drop in the ionization energy between group2 and group3 and there is also a drop in the ionization energy in between group5 and group6 and the reason behind this is the ELECTRON PAIR REPULSION.
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