Flexi Says: Because opposite electric charges attract each other, negative electrons are attracted to the positive nucleus. This force of attraction keeps electrons constantly moving through the otherwise empty space around the nucleus. Electrons closer to the nucleus are held by strong forces of attraction by the nucleus. Thus they can only be removed by spending high energy, which means that the electrons tend to stay there and will have low potential energy. On the other hand, electrons away from the nucleus are held by weak forces of attraction by the nucleus and keep moving around thus it requires less energy to remove them. Therefore, it can be said that the electrons farther away from the nucleus have higher energy.
