The fourth quantum number, ms, determines the spin of an electron within an atomic orbital.
Whereas the first 3 quantum numbers describe the atomic orbital where you find the electron, the final quantum number, ms, describes spin the electron possesses.
Concept: Spin Quantum Number Part 12m
The Pauli Exclusion Principle implies that two electrons in the same atomic orbital cannot spin in the same direction so as to avoid having the same four quantum numbers.
Concept: Spin Quantum Number Part 22m
Example: State the electron configuration of boron and list the four quantum numbers of the 1st and the 5th electron.6m