The atoms inert gas atoms like neon helium, argon, and helium do not (well, rarely!) form stable molecules through chemical bonding with other elements. It is, however, quite simple to construct 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 located at the end of the glass tube containing the inert gas and the light begins to glow.
It’s much simpler to explain why neon doesn’t get inert when it is discharged in a tube than to explain why it becomes 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 must be large enough so that this energy is greater than the amount needed to “ionize” the atom. The atom that has been ionized has had an electron taken out of an orbital to make it a “free” particle and the atom it has left behind has been transformed into positively charged ions. The electrical current that runs between the tubes’ electrodes is carried by the charge of charged electrons and ions.

Photo ( above) of a gas discharge sign Sam Sampere, Syracuse University created by Sam Sampere, Syracuse University. This sign incorporates the neon discharge tube (the orange word “Physics”) and mercury discharge tubes (the blue word “Experience” and the frame on the outside). The bottom of the sign’s sculpture symbolizes the magnetic and electric fields of light. The sculpture’s yellow and white sine waves are composed of fluorescent light bulbs. These are mercury discharge tubes with special coatings on the inside walls. The light that is emitted by the mercury discharge inside the tube is absorbed by the coating, which subsequently emits light of different hues (and with less photon energy). A range of colors is possible depending on the substance of the coating.

They emit light. So why is this? An electron can be excited so that it is possible to remove it from an atom. The electron is believed to have been elevated to an orbital that has higher energy. The electron then returns to its orbital, and the particle of light (a photon), carries away the energy excitation. The discharge tube is lit. The energy of a photon (its wavelength or color) depends on the energy differences between orbitals. An atom can emit photons of different energies based on the orbitals it has. The photon energy spectrum is the emission lines that are unique to a particular atom. The mercury discharge tubes have distinct from the neon discharge tubes, as is evident from the custom neon sign. This is how the inert gas Helium was discovered. Observations of sunlight exposed a range of photon energies that were not seen before in Earth discharges.

The chemical inertness that is present in certain gases is subtler to explain. When two atoms come into proximity with the greatest energy or valence, the orbitals of the atoms shift dramatically and the electrons on the two atoms reorganize. If this process reduces the energy total of the electrons involved and a chemical bond may develop. For normal, non-inert atoms, the electrons are relatively pliable and bonds often form. However, electrons in inert gas are more insensitive to this effect of proximity and so they rarely make molecules.

Another example of a more significant phenomenon is the inexplicable inertness of matter. This paradox is caused by the inertness (about chemical bonding) of a gas and its dynamism in a glow discharge. An atom may be considered an inert and unreactive particle so long as the amount of its interactions with other particles (including photons) is low enough that electrons of the atom do not get excited. The most patient and laid-back particles are made up of inert gasses like a custom neon sign. But, the interaction energies rise and nuclei lose their inertness. The result is an amalgamation of electrons and inert nuclei, a highly excited plasma. The energy could be increased, and nuclei will become less inert. We get instead a brew of nucleons, similar to the neutron star. You can increase the energy even further and you enter the world of quarks. Even nucleons can no longer be inert, and we’re returning to the primitive, energetic conditions that existed just after the Big Bang.