In which of the following atoms is the 3s orbital closest to the nucleus?

• A. Al
• B. Mg
• C. Si
• D. Na

Answer: (B)

Verified step by step guidance

1. Recall that the 3s orbital refers to the principal quantum number n = 3 and the s subshell, which is spherical and closest to the nucleus within that energy level.
2. Understand that the effective nuclear charge (Z_eff) experienced by an electron affects how tightly the electron is held and thus how close the orbital is to the nucleus. A higher Z_eff pulls the orbital closer.
3. Compare the elements given: Na (Z=11), Mg (Z=12), Al (Z=13), and Si (Z=14). As the atomic number increases, the number of protons increases, which generally increases Z_eff for electrons in the same shell.
4. Consider electron shielding: electrons in inner shells shield outer electrons from the full nuclear charge. Since all these elements have electrons in the same 3s orbital, the difference in Z_eff mainly comes from the increasing nuclear charge with relatively similar shielding.
5. Conclude that the 3s orbital in Mg is closest to the nucleus because Mg has a higher effective nuclear charge than Na but fewer electrons than Al and Si, resulting in less electron-electron repulsion and a more tightly held 3s orbital.