It has already been discussed that in isolated atom, each orbit has fixed amount of energy associated with it. The electrons moving in a particular orbit possess the energy of that orbit. The larger the orbit, the greater is its energy. It becomes clear that outer orbit electrons possess more energy than the inner orbit electrons.

A convenient way of representing the energy of different orbits of an isolated atom is shown in fig (b). This is known as energy level diagram. The first orbit represents the first energy level, the second orbit indicates the second energy and so on. The larger the orbit of an electron, the greater is its energy and higher is the energy level.