It is observed that the electron is not only revolving around the nucleus but it is also spinning on its own axis. The spin of the electron produces a small magnetic field as a result of which the electron behaves as a 'tiny magnet'. This quantum number describes the spin orientation of the electron. It is designated by s. Since the electron can spin only in two ways: clockwise and anti-clockwise and, therefore, the spin quantum number can take only two values: + ½ or - ½ .
This quantum number has a value independent of the value of the other three quantum numbers. Instead of giving + ½ and - ½, the two orientations are usually designated by arrows pointing up 
and down 
respectively.
Thus, the four quantum numbers describe position of an electron in an atom by specifying its main energy level (n), sub-level (l) the orientation of the orbital (m) and the direction of its spin (s). In other words, these quantum numbers serve as an address for an electron. The various permitted values of the quantum numbers are summarized below:
| Quantum number | Designation | Values |
|---|---|---|
| Principal | n | 1,2,3,4 any integer |
| Angular | / | 0,1,2,3....(n - 1) |
| Magnetic | m | - 1 .... 0 .... +1 |
| Spin | s | + ½ or - ½ |
