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.

Explanation:

4d^55s^1 ( third choice)