The energy levels of each atom are quantized according to the number of protons and electrons present.Since each element has a unique set of electrons and protons, each element also has a unique set of energy levels. Usually these energies are measured in relation to the ground state, and a particular excited state for sodium, for example, may be 2.2 eV (electron volts) above the ground state. This means that an atom in the excited state contains 2.2 eV more energy than a ground state atom which, by convention, is ascribed an arbitrary energy of zero. An element may have several electronic energy states.