The electrons move in definite paths called orbits or shells around a central nucleus. These orbits or shells have different energies and can accommodate different number of electrons in them. The question arises that how are the electrons distributed among these shells? The answer to this question was provided by Bohr and Bury.
According to their scheme, the electron distribution is governed by the following rules:
These orbits or shells in an atom are represented by the letters K, L, M, N or the positive integral numbers, n = 1, 2, 3, 4.
The orbits are arranged in the order of increasing energy. The energy of M shell is more than that of the L shell which in turn is more than that of the K shell.
The maximum number of electrons present in a shell is given by the formula 2n2, where n is the number of the orbit or the shell.
The shells are occupied in the increasing order of their energies.
Electrons are not accommodated in a given shell, unless the inner shells are completely filled.
The maximum number of electrons that can be accommodated in different shells are:
K shell (or n = 1 level) = 2n2 = 2 × (1)2 = 2
L shell (or n = 2 level) = 2n2 = 2 × (2)2 = 8
M shell (or n = 3 level) = 2n2 = 2 × (3)2 = 18 and so on.
The arrangement of electrons in the various shells or orbits of an atom of the element is known as electronic configuration.
Hydrogen (H) atom has only one electron. It would occupy the first shell and electronic configuration of hydrogen can be represented as 1.
The next element, Helium (He) has two electrons in its atom. Since the first shell can accommodate two electrons; hence, this second electron will also be placed in the first shell. The electronic configuration of helium is written as 2.
The third element, Lithium (Li) has three electrons. Now the two electrons occupy the first shell whereas the third electron goes to the next shell of higher energy level, i.e. second shell. Thus, the electronic configuration of Li is 2, 1.