How do neon lights work?

By definition, the atoms of inert gases such as helium, neon or argon never form stable molecules by chemically bonding with other atoms. One need apply only a modest electric voltage to electrodes at the ends of a glass tube containing the inert gas and the light begins to glow.

The voltage across a discharge tube will accelerate a free electron up to some maximum kinetic energy. An ionized atom has had an electron plucked out of an orbital to make it a "free" particle, and the atom it leaves behind has become a positively charged ion. The resulting plasma of charged ions and electrons carries the electric current between the tube's electrodes.

 These neon lights are mercury discharge tubes with special coatings on their inner walls. The ultraviolet light emitted by the mercury discharge inside a tube is absorbed by the coating. Depending on the exact material of the coating, a whole range of colors can be obtained.

As an alternative to being removed by an energetic collision, an electron on an atom can be excited. One speaks of the electron as having been promoted to an orbital of higher energy. When the electron eases back down to its original orbital, a particle of light  carries away the energy of excitation¿and the discharge tube glows! A photon's energy  depends on the energy difference between orbitals.