By definition, the atoms of inert gases such as helium, neon sign or argon never (well, almost never) form stable molecules by chemically bonding with other atoms. But it is pretty easy to build a gas discharge tube�such as a neon light�which reveals that inertness is a relative matter. One need apply only a modest electric voltage to electrodes at the ends of a glass tube containing the inert gas and the led opti neon sign begins to glow.

It's much easier to explain why neon isn't inert in a discharge tube than it is to explain why it is inert to chemical reactions. The voltage across a discharge tube will accelerate a free electron up to some maximum kinetic energy. The voltage must be large enough so that this energy is more than that required to "ionize" the atom. 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.