Answer
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Work Step by Step
In the hydrogen atom, the 2s and 2p orbitals have the same energy.
Hydrogen has only one electron, so there's no electron-electron repulsion. The energy levels depend only on the principal quantum number (n), not the orbital shape (s, p, d, etc.).
This means all orbitals with the same n (like 2s, 2p, 2d) are degenerate — they have equal energy.
In helium, the 2p orbital is higher in energy than the 2s orbital.
Helium has two electrons, so electron-electron repulsion and shielding effects come into play.
The 2s orbital penetrates closer to the nucleus than the 2p orbital, meaning it experiences less shielding and is lower in energy.
The 2p orbital is more shielded and thus feels a weaker effective nuclear charge, making it higher in energy.
