The total number of electrons that may occupy a subshell is given by 4l+2.
The magnetic quantum number (m) is defined in terms of the azimuthal quantum number. M takes integer values ranging from -l to +l including zero and describes the number and orientation of the orbitals associated with a certain subshell.
Each orbital can contain two electrons. These two electrons have different spins, specified by the spin quantum number (s), which attains the values -1/2 and +1/2. Two electrons occupying the same orbital must have the same first three quantum numbers. However, since the electrons have different spins, the fourth quantum number must be different. This last result is required by the Pauli exclusion principle: only two electrons can occupy an orbital and these electrons must have opposite spins.
Example:
Describe the electron configuration of a neon atom.
Neon has 10 electrons. By the Pauli exclusion principle, each of these electrons must have a different set of quantum numbers. The first step in determining an electron configuration in this case is to assign electrons to shells. Shells correspond to the first quantum number n.
