Electrons in zones close to the center are lower in energy than electrons in zones at greater distances from the center. According to Bohr, the amount of energy needed to move an electron from one zone to another is a fixed, finite amount. At the lowest energy level, the one closest to the atomic center, there is a single 1s orbital that can hold 2 electrons. Each of these orbitals can hold 2 electrons, so a total of 8 electrons can be found at this level of energy. Moving between Levels As Neils Bohr showed, it is possible for electrons to move between energy levels.
Light contains energy.
If a photon of light strikes an atom, it is possible for the energy in the light ray to be transferred to one of the low energy electrons moving around the atomic center. The electron with its extra packet of energy becomes excited , and promptly moves out of its lower energy level and takes up a position in a higher energy level. This situation is unstable, however. Almost immediately the excited electron gives up the extra energy it holds, usually in the form of light, and falls back down to the lower energy level again. Florescence is a phenomenon of moving electrons.