Energy Levels in Atoms

We can think of an atom as consisting of a positive nucleus (protons and neutrons) surrounded by negative electrons. The electrons can be thought of as “orbiting” the nucleus, but are only allowed in certain orbits (or energy levels). A photon with exactly the right energy can excite the electron from one level to another. The electron will drop back to the “ground state”, and emit photons with specific energies as it does so.