Question

What is the quantum number set of the ground-state electron that is found in lithium (Li) but not in helium (He)?

Answer 1

The electron configuration of lithium atom is: $Li:[He]2s^1$

The number "2" is the value of the principal quantum number "n". Letter "s" is associated with the value of secondary quantum number "l" and it is equal to zero. The value of "m" (or magnetic quantum number) is zero too. The quantum number set for the highest energy electron will be (2, 0, 0, 1/2).