Structure of the Atom — Science STD 9 — Question

Two postulates given by sir niel's bohr are:

1. The electrones move in certain stationary orbits in an atom and these orbits are called energy levels or orbits or shells. 
2. In a particular shell an electron neither loses nor gain energy it is the ground state of an atom.

In short: he defined the stability of an atom and he also concluded that an electron cannot lose its energy as long as it revolves in a particular orbit and hence it cannot fall into the nucleus by loosing energy.