Bohr to the Rescue

Niels Bohr solved this problem by suggesting that the electrons could only orbit the nucleus in certain ‘allowed’ energy levels. He suggested that an electron may only transfer energy when it moves from one energy level to another. A change from one level to another is called a ‘transition’.

To move up and energy level the electron must gain the exact amount of energy to make the transition.

It can do this by another electron colliding with it or by absorbing a photon of the exact energy.

When moving down a level the electron must lose the exact amount of energy when making the transition.

It releases this energy as a photon of energy equal to the energy it loses.

E₁ is the energy of the level the electron starts at and E₂ is the energy of the level the electron ends at