The effective nuclear charge (Zeff) generally decreases as you move down a column in the periodic table. This is primarily due to two factors: the increase in the number of electron shells and the shielding effect.
As you go down a group, the number of electron shells increases. For example, lithium (Li) has one electron shell, while cesium (Cs) has six. Even though the nuclear charge increases (more protons in the nucleus), the addition of shells adds more electrons that are located farther from the nucleus. These outer electrons experience a significant amount of shielding from the inner shell electrons, which diminishes the effect of the positive charge from the nucleus on the outermost electrons.
Moreover, the increased distance between the nucleus and the outer electrons also contributes to a reduced attraction. While the actual nuclear charge is higher due to the greater number of protons, the effective nuclear charge that outer electrons feel is less because of the combined effects of distance and shielding. Consequently, even with a larger nuclear charge, the effective nuclear charge perceived by the outermost electrons becomes lower as you descend a group.