Neil Bohr put forward the following postulates about the model of an atom:
(i) Atoms has nucleus in the centre.
(ii) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(iii) While revolving in discrete orbits the electrons do not radiate energy.
(iv) These orbits or shells are called energy levels. Energy levels in an atom are shown in Fig.These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….