JEE Main & Advanced Physics Atomic Physics Electronic Configurations of Atoms

The distribution of electrons in different orbitals of an atom is called the electronic configuration of the atom. The filling of electrons in orbitals is governed by the following rules.

(1) Pauli's exclusion principle : "It states that no two electrons in an atom can have all the four quantum number (n, l, \[{{m}_{l}}\] and \[{{m}_{s}}\]) the same."

It means each quantum state of an electron must have a different set of quantum numbers n, l, \[{{m}_{l}}\] and \[{{m}_{s}}\]. This principle sets an upper limit on the number of electrons that can occupy a shell.

\[{{N}_{\max }}\] in one shell \[=2{{n}^{2}};\] Thus \[{{N}_{\max }}\] in K, L, M, N ?. shells are 2, 8, 18, 32,

(2) Aufbau principle : Electrons enter the orbitals of lowest energy first.

As a general rule, a new electron enters an empty orbital for which \[(n+1)\] is minimum. In case the value \[(n+l)\] is equal for two orbitals, the one with lower value of n is filled first.

Thus the electrons are filled in subshells in the following order (memorize)

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, ....

(3) Hund's Rule : When electrons are added to a subshell where more than one orbital of the same energy is available, their spins remain parallel. They occupy different orbitals until each one of them has at least one electron. Pairing starts only when all orbitals are filled up.

Pairing takes place only after filling 3, 5 and 7 electrons in p, d and f orbitals, respectively.