The modern theory of atomic structure describes the position of electrons in terms of shells, subshells and orbitals. According to the Pauli exclusion principle, a unique set of four quantum numbers is used to describe the general location and orientation of each electron. These quantum numbers are n (the principle quantum number), l (the angular momentum quantum number), m (the magnetic quantum number) and s (the spin of the electron).
The principal quantum number (n) corresponds to the different orbits allowed for an electron. The principal quantum number can assume integral values 1,2,3, ... . As the orbital becomes larger, the electron spend more time away from the nucleus. The total number of electrons that can occupy an energy shell is given by .
The azimuthal quantum number (l) may attain integer values from 0 to (n -1). This quantum number defines the shape of the orbital. The subshells most commonly referred to are the s, p, d, and f subshells corresponding to the values 0, 1, 2, and 3 for the quantum number l. If an orbital has principal quantum number 2, it will have both an s and p subshell associated with it. The subshells have the following shapes:
