(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy. These orbits are called energy levels. Energy levels in an atom are shown by circles. These orbits are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

Further, the distribution of electrons into different orbits of an atom was suggested by Bohr and Bury. The following rules are the rules followed for writing the number of electrons in different energy levels or shells: (i) The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number or energy level index, 1,2,3,…. Hence the maximum number of electrons in different shells can be written as : first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be = 2 × 42= 32, and so on. The maximum number of electrons that can be accommodated in the outermost orbit is 8. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a stepwise manner.

The maximum number of electrons which can be accommodated in any energy level of the atom of an element is given by 2n² (where n is the number of that energy level). By using this relation we will find that :

(a) the maximum number of electrons which can be accommodated in 1st energy level K shell, where n= 1,

2n² = 2 X (1)² = 2 X 1 = 2. Hence, K shell can have maximum of 2 electrons.

(b) the maximum number of electrons which can be accommodated in 2nd energy level, L shell, where n = 2 , 2n² =2 X (2)² = 2 X 4 = 8. L shell can have maximum of 8 electrons.

(c) the maximum number of electrons which can be accommodated in 3rd energy level M shell, where n = 3, 2n² = 2 X (3)² = 2 X 9 = 18. M shell can have a maximum of 18 electrons.

(d) the maximum number of electrons which can be accommodated in 4th energy level N shell, where n = 4. 2n² = 2 X (4)² = 2 X 16 = 32. N shell can have a maximum of 32 electrons.

The electrons in an atom do not occupy a new shell unless all the inner shells are completely filled with electrons. This means that first all the electrons fill K shell, then L shell, then M shell, then N shell, and so on.