The postulates put forth by Neils Bohr’s about the model of an atom:

(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,….