By definition, the atoms in gas molecules that are inert like neon, helium, or argon do not (well nearly never) form stable molecules by chemically bonding with other molecules. It is, however, quite simple to construct a gas discharge tube such as a neon sign for sale that demonstrates that inertness is an issue of degree. You only need to apply an electric voltage of a small amount to electrodes at the ends of a glass tube containing the inert gas and the light starts to light.
It’s a lot easier to explain why neon isn’t inside the discharge tube than it is to explain why it is inert to chemical reactions. The voltage across the discharge tube can accelerate an electron that is free up to a certain amount of kinetic energy. The voltage should be high enough to ensure that the energy is greater than the amount required to “ionize” the atom. A positively charged ion is an atom that has been ionized. That means it’s been able to take an electron from orbital to be “free” of a particle. The electrical current that runs between the electrodes of the tubes’ tubes is transported by the charge of charged electrons and ions.

Photo ( above), shows the gas discharge sign Sam Sampere, Syracuse University created by Sam Sampere, Syracuse University. The sign includes a neon discharge tube (the “Physics” word in orange) and mercury discharge tubes (the “Experience” or the “Experience” word in blue) and an outer frame. The neon sign for sale bottom sculpture represents the electric and magnetic fields of light. The white and yellow sine waves in the sculpture represent fluorescent lights. These fluorescent tubes are mercury discharge tubes that have special coatings on the inside walls. The ultraviolet light produced by the mercury discharge inside a tube is taken up by the coating which subsequently emits light of another hue (and with lower energy). A range of colors is achievable based on the substance of the coating.

Why do gas discharges emit light? In addition to being eliminated by the force of a collision electrons on an atom can be excitation. The electron is believed to have been elevated to an orbital that has more energy. When the electron eases back down to its original orbital, a particle of light (a photon) can remove the excitation energy and the discharge tube glows! The energy of the photon (or its wavelength or color) can be determined by the difference in energy between orbitals. Atoms can emit photons with different energies, which correspond to the different pairs of orbitals. These photon energies, also known as emission lines are spectroscopically unique to the element. As is evident in the sign, the mercury discharge tubes have distinct hues from what the neon discharge tube does. Helium, an inert gas, was discovered in this manner, and observation of the sun revealed a sequence of photon energy levels that were never seen in discharges on the surface of the earth.

It’s more difficult to explain the chemical inertness of some gases. The most common rule is that when two atoms are in proximity, their most energy (or their valence), the orbital of the atoms shifts significantly as the electrons of those atoms reorganize. A chemical bond may form when this reorganization decreases the energy of electrons in total. For normal, non-inert atoms, the electrons are relatively flexible and bonds are often formed. However, the electrons of the inert gas are more insensitive to this effect of proximity and, consequently, do not form molecules.

Another example of a larger phenomenon is the unbearable inertness of matter. This is a contradiction between the inertness (about chemical bonding) of a gas and its vitality in a glow discharge. An atom can be thought of as an inert and unreactive particle as long as the energy that it interacts with other particles (including photons) is small enough so that electrons in the atom don’t get excited. The most patient and laid-back atoms are those made from inert gasses like neon. Still, as interaction energy increases, they lose their inertness and we ultimately get an inert nucleus and electrons in a highly excited plasma. The energy can be increased as well as the nuclei become less inert. Instead, we can drink a mixture of nucleons, similar to neutron stars. Step up the energy some more and we are in the world of quarks. Even nucleons cannot be inert and we’re returning to the primitive, energetic conditions that existed just after the Big Bang.