Final answer:
Rubidium (Rb) has more shielding than Sodium (Na) due to the presence of additional electron shells, which effectively shield the outer electrons from the nuclear charge, resulting in a lower nuclear attraction for Rb's outer electrons.
Step-by-step explanation:
Rubidium (Rb) has more shielding than Sodium (Na). This occurs because Rb is located below Sodium on the periodic table, which means it has more electron shells. As we move down a group in the periodic table, each element has an additional electron shell compared to the one above it. These extra shells of core electrons are very effective at shielding the outermost electrons from the nuclear charge. In contrast, electrons in the same principal shell do not shield each other very well.
As a result, since Rb has more inner shells compared to Na, its outer electrons experience less nuclear attraction and are held less tightly. This greater amount of shielding in Rb also contributes to its lower first ionization energy when compared to Na.