$He$ has the highest first ionization energy and $Cs$ has the lowest first ionization energy.
Work Step by Step
The first ionization energy depends on two variables: the size of the atom and the effective nuclear charge of the atom on the outermost electrons. Considering the periodic trend of the first ionization energy: - Going through a row, the first ionization energy increases - so the highest is in the noble gas group (as the effective nuclear charge increases and the atom size decreases) - Going through a column, the first ionization energy decreases (though the effective nuclear charge does increases slightly, the atom size increases much more and dominates the increase of the effective nuclear charge) As a result, the element with the highest ionization energy would be in the first row and belongs to the noble gas group. That element is helium $(He)$. The element with the lowest ionization energy would be in the last row and belongs to the 1A group (alkali metals). That element is cesium $(Cs)$. (Actually francium is in the last row, but it is highly radioactive so it is not counted here.)