We now have a model of an atom with a center and electrons orbiting around it at certain energy levels known as shells. The energy shells are separated by forbidden zones ( energy differences). For an electron to bridge the energy difference and go from a low energy shell to a higher one, energy corresponding to the energy difference must be added. An electron that has absorbed energy and has jumped to a higher energy level is called an excited electron. But since the electron wants to have the lowest energy possible it quickly releases the added energy (for example by emitting light) and goes back to its original state, also known as its ground state.