How does shielding compete with nuclear charge with respect to atomic mass, and the Periodic Table?
Two properties work in opposition: increased nuclear charge; shielding by other electrons. Shielding wins down a Group.
Going down a Group, a column on the Periodic Table, the valence electrons are still the same. As we descend the column, while the nuclear charge
The result? Decreased ionization enthalpies down a Group. As physical scientists we should look at actual data: see this site.