The electronic configuration of an element is defined as follows:
A stepwise distribution of electrons in different orbits (energy levels) of an atom is called its electronic configuration.
To write the electric configuration of an element, one should proceed as follows.
(i) Find out the total number of electrons present in an atom of that element.
(ii) Fill these electrons in the various orbits (shells) one by one in order of increasing energy according to the 2n2 rule, i.e., the shell with the lowest energy is filled first, and then the orbits of increasing energies are filled.
(iii) Then, write the number denoting the number of electrons in various shells starting with the lowest. Each numeral is separated from other by a comma.
For example, lithium (atomic number 3) has been three electrons in its atom. According to the 2n2 rule:
The shell corresponding to n = 1(K-shell) will have two electrons, and the shell corresponding to n=2(L-shell) will have one electron. Therefore, the electronic configuration of lithium is 2, 1. Electronic configuration may also be shown through a diagram.
Distribution of electrons in various orbits and the electronic configurations in the notation form for first 20 elements are presented.