In which of the following atoms is the 2s orbital closest to the nucleus?

• A. Li
• B. B
• C. C
• D. Be

Answer: (A)

Verified step by step guidance

1. Recall that the 2s orbital is part of the second principal energy level (n=2) and is generally at a similar energy level across different atoms, but its effective nuclear charge (Z_eff) affects how close it is to the nucleus.
2. Understand that the effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in an atom, calculated roughly as \(Z_{eff} = Z - S\), where \(Z\) is the atomic number and \(S\) is the shielding constant representing electron repulsion.
3. Compare the atoms given: Li (Z=3), Be (Z=4), B (Z=5), and C (Z=6). As \(Z\) increases, the nucleus has a stronger positive charge, but the shielding by inner electrons also changes.
4. Recognize that in Li, the 2s electron experiences less shielding and a lower nuclear charge, so it is less tightly held and farther from the nucleus compared to atoms with higher \(Z\). However, the question asks which 2s orbital is closest to the nucleus, so consider how increased nuclear charge pulls the 2s orbital closer.
5. Conclude that the 2s orbital in Li is actually the closest to the nucleus because in atoms with more protons (Be, B, C), the 2s electrons experience more electron-electron repulsion and shielding effects that can cause the orbital to expand slightly, despite the higher nuclear charge.