Question

Another student makes the observation that the ground state of an electron in a hydrogen atom is a "1s" state and that the two ground-state electrons in a helium atom occupy the 1s state also. Extending this reasoning, the student concludes that the ground state of a neutral lithium atom (with its 3 electrons) would be for all 3 electrons to be in the 1s state. Do you agree or disagree with this conclusion regarding lithium? Explain why or why not based on physical principles.

Answer 1

Disagree. The maximum number of electrons that a s subshell can accomodate is 2. So the first two electrons of Li will fill the 1s state and next electron will go to the next which is 2s. Since the no of electrons an orbit can accommodate is , maximum no of electrons in first orbit is 2. So the third electron will go to next shell which is the second orbit.

This is also in accordance with Paulis Exclusion Principle. That is no two electron can have same set of quantum numbers. Since there are only two possible values for spin quantum number, it is not possible to accommodate a third electron in the s subshell.