1. An atom consists of a dense positively charged central part called as Nucleus.
2. The electrons revolve around the nucleus in fixed circular paths are called orbit or shell. The electrostatic force of attraction between nucleus & electron balanced by the centrifugal force. Hence the electrons do not fall into the nucleus and therefore atom remains stable.
3. Electron can rotate only in certain permitted orbits known as stationary state.
4. Each stationary state is having definite amount of energy hence called as energy level.
5. Electrons in the energy level nearest to the nucleus have lower energy while those are at greater distance from the nucleus have higher energy.
6. As long as the electron stays in the same energy level, the energy remains constant. The energy of an electron can change only when it moves from one level to another.
7. When the excited electron jumps from lower to higher energy level, it absorbs or gain energy. When the excited electron jumps from higher to lower energy level, it emits or loses energy.
8. The angular momentum of an electron (mvr) must be an integral multiple of h/2π. Hence mvr = nh/2π.