According to Bohr's model of an atom, every atom consists of a small, dense, positively charged nucleus.
The electrons revolve around the nucleus in a definite circular path called orbits or shells.
Each orbit is associated with a definite amount of energy. So the orbits are also called energy shells.
Energy shells are represented by the letters K, L, M, N,…. or the numbers 1,2,3, 4,….. The energy of the orbit is increasing as we go away from the nucleus.
When an electron remains in a particular orbit it does not lose or gain energy. The energy of electrons that remains in a particular orbit is constant.
Energy is gained or emitted whenever an electron moves from one orbit to another orbit.
