The gas atoms inert like neon, helium, and argon can not (or nearly never) make stable molecules through chemical bonding to other elements. It is very easy to construct an inert gas discharge tube such as a custom neon sign. This proves that inertness can be relative. You only need to apply the smallest amount of electric current to the electrodes located at the ends of a glass tube filled with the inert gas, and the light begins to glow.
It is much easier to explain the reason why neon does not change into inert during a discharge tube than to explain why it is inert during chemical reactions. The voltage across a discharge tube will accelerate an electron that is free up to the maximum energy of its kinetic. The voltage must be large 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. It means that it has received an electron from the orbital to be “free” of particles. The resultant plasma of charged electrons and charged ions carry the electric current between the electrodes of the tube.

Photo ( above) of a gas discharge sign that Sam Sampere, Syracuse University developed. The sign includes the neon discharge tube (the “Physics” word in orange) and mercury discharge tubes (the “Experience” word in blue, and the “Experience” word in blue), and an outer frame. The sign’s bottom artifact represents the electric and magnetic field of light. The sculpture’s white and yellow sine waves are composed of fluorescent lamps. The fluorescent tubes are mercury discharge tubes that have special coatings on their inner walls. The coating absorbs light emanating from the mercury discharge within the tube and releases light that has a lower power (and a different color) Depending on the exact material of the coating, various colors can be obtained.

So why do these gas discharges emit lights? As an alternative to being removed by an energetic collision, electrons within an atom could be stimulated. The electron is thought to be elevated to an orbital with higher energy. When the electron eases back to its orbital the light particle (a photon) carries away the energy generated by excitation and the discharge tube glows! A photon’s energy (its wavelength or color) is determined by the energy difference between orbitals. Atoms emit photons with different energy levels based on the orbitals it is in. The photon energy spectrum is the emission lines of spectroscopists exclusive to an atom. As can be seen in the image, mercury discharge tubes display distinct hues from the neon discharge tube. The inert gas helium was discovered this way, and observation of sunlight revealed a series of photon energy levels that had never before been observed in discharges on the planet.

It is harder to understand the chemical inertness that is present in some gases. When two atoms are nearby and have the highest energy or valence, orbitals on the atoms change substantially and the electrons of the two atoms are reorganized. If this restructuring decreases the energy of all electrons involved and a chemical bond may develop. The electrons of normal, non-inert molecules are very flexible, and bonds can form. However, electrons from gases that are inert are more insensitive to this effect of proximity and thus, they do not create molecules.

The apparent contradiction between the inertness of gas (about chemical bonding) and its liveliness (in glow discharges) is an illustration of a wider phenomenon we might call the unbearable inertness of matter. An atom can be thought of as an inert, non-reactive particle, as long as the energy of its interaction with other particles (including photons) is small enough so that the atom’s electrons don’t get excited. The calmest and most relaxed particles are made up of inert gases such as neon. Yet, as interaction energy increases, they lose their inertness, and eventually, we get a soup of inert nuclei and electrons in a highly excited plasma. The energy could be increased as well as the nuclei become less inert. We get instead a brew of nucleons as in a neutron star. You can increase the energy to the point that you are in the realm of quarks. Even nucleons cannot be inert and we are back to the primordial energetic conditions that existed shortly after the big bang.