How many principal energy levels are occupied by electrons in a neutral helium atom?

• A. 3
• B. 2
• C. 1
• D. 4

Answer: (C)

Verified step by step guidance

1. Recall that the principal energy levels correspond to the main electron shells around the nucleus, labeled by the quantum number \(n = 1, 2, 3, \ldots\).
2. Determine the electron configuration of a neutral helium atom. Helium has 2 electrons, and the electron configuration fills the lowest energy level first.
3. The first principal energy level (\(n=1\)) can hold up to 2 electrons in the 1s orbital.
4. Since helium has exactly 2 electrons, both occupy the \(n=1\) level, and no electrons are in higher energy levels.
5. Therefore, only the first principal energy level is occupied by electrons in a neutral helium atom.