Final answer:
Rb has a larger atomic radius than Na because it is in a higher period of the periodic table, which means it has more electron shells and a larger principal quantum number, n. This places Rb's outermost electrons further from the nucleus compared to Na, despite the stronger nuclear charge.
Step-by-step explanation:
The best choice for why Rb has a larger atomic radius than Na is D. Rb is in a higher period of the periodic table. As you go down the groups in the periodic table, the atomic radius increases because there are more electron shells, which means an increase in the principal quantum number, n. Each additional shell adds a layer of electrons further from the nucleus, increasing the size of the atom even though there are more protons in the nucleus of Rb that cause a stronger attraction. This is because the outermost electrons are progressively further from the nucleus and are also shielded by the inner electrons. Therefore, the atomic radius of Rb is larger than that of Na.
When comparing Rb with other elements such as Li, N, F, and I, Rb has the largest atomic radius due to its position in the lower left corner of the periodic table, which typically hosts larger atoms. On the other hand, for the largest first ionization energy among elements like Mg, Ba, B, O, and Te, the element O would have the largest ionization energy because ionization energy tends to increase across a period and decrease down a group, and oxygen is furthest to the right and towards the top among these given elements.