Which main group atom would be expected to have the lowest second ionization energy?

The trend in the size of atomic radii across the periodic table is that the atomic radius increases as we go from right to left in the periodic table. The atomic radius increases going from top to bottom (increases in n energy levels and so the distance from the nucleus).

The atoms with a greater radius are present in the lower left side corner. They require lower energy because they have a greater radius and so, have the largest distance of the electron from the nucleus as a result of which electrons are less tightly bound by the nucleus. So, require less first ionization energy.

Hence, elements of group 1 have the lowest first ionization energy.

Since the elements lost 1 electron during the first ionization energy. for second ionization energy, group 2 has 1 electron left, losing this valence electron will make elements in group 2 more stable. Hence, group 2 atoms (alkaline earth elemental atoms) have the lowest second ionization energy.