A spinning electron also has a spin quantum number that is expressed as 1/2 in units of . However,
that quantum number does not arise from the solution of a differential equation in Schrödinger’s
solution of the hydrogen atom problem. It arises because, like other fundamental particles, the electron has an intrinsic spin that is half integer in units of , the quantum of angular momentum.
As a result, four quantum numbers are required to completely specify the state of the electron in an atom.
The Pauli Exclusion Principle states that no two electrons in the same atom can have identical sets of four
quantum numbers . We will illustrate this principle later.