What are the main postulates of Bohr's model?

1. In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called as orbits or shells.

2. Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.

3. The energy levels are represented by an integer (n=1, 2, 3 ...) known as the quantum number. This range of quantum number starts from nucleus side with n=1 having the lowest energy level. The orbits n=1, 2, 3, 4 ... are assigned as K, L, M, N ... shells and when an electron attains the lowest energy level it is said to be in the ground state.

4. The electrons in an atom move from a lower energy level to a higher energy level by gaining the required energy and an electron moves from a higher energy level to lower energy level by losing energy.