It is already stated that electrons revolve in shells around the nucleus. Electrons are not allowed to occupy all spaces around the nucleus. Only certain orbits are permitted for electrons to revolve around the nucleus. Therefore, the orbits are known as energy levels. Each shell represents an energy level. There are a number of shells in an atom.

Atomic shells are represented by letters. They are labeled as K, L, M, N, etc. They are also designated by the numbers 1, 2, 3, etc. The 1st shell is the K shell, the 2^nd shell is the L shell, the 3^rd shell is the M shell and so on.

Electronic configuration is the arrangement of electrons in the shells of atoms. Only a limited number of electrons can be placed in each shell. The maximum number of electrons that can be accommodated in a shell is based on the principal quantum number (n). It is represented by the formula 2n², where ‘n’ is the shell number.

Note that the maximum number of electrons in the outer shell of an atom is 8, except helium which has 2 electrons. The maximum number of electrons that can be accommodated in a given shell is already given. The M-shell, for instance, can accommodate a maximum of 18 electrons. But when it is an outer shell as in 18Ar, the M-shell is complete or full with only 8 electrons. Elements that have complete shell of electrons, such as He, Ne and Ar are called the noble gases or inert gases.