Neon Lights: How They Produce Different Colors and Shapes
Gas atoms that are inert like neon helium, argon, and helium do not (well most of the time!) form stable molecules through chemical bonding with other elements. It is very easy to construct gas discharge tubes, such as neon lamps. This is proof that inertness is not absolute. The light will shine after applying a small electrical voltage to the electrodes located on the sides of the glass tube, which has the inert gases.
It’s a lot easier to explain why the neon isn’t inert within a discharge tube than to clarify why it’s inert to chemical reactions. The discharge tube can accelerate free electrons to a specific maximum energy of kinetic. The voltage must be large enough that the energy is more than that needed to “ionize” the atom. A positively charged ion is an atom that has been “ionized. That means it’s had an electron taken from an orbital to render it “free” of a particle. The resulting plasma composed of charged electrons and ions is the conduit for the electric current between the electrodes of the tube.
The photo ( above) illustrates a sign for gas discharge created by Sam Sampere of Syracuse University. The sign is made up of a custom neon sign discharge tube (the “Physics” word written in orange) and mercury discharge tubes (the “Experience” word in blue, and the “Experience” word in blue), and an outer frame. The sculpture at the bottom of the sign depicts the magnetic and electric fields of light. The white and yellow sine waves that are visible in the sculpture are fluorescent lights. These fluorescent tubes are mercury discharge tubes with special coatings on the interior walls. The coating absorbs ultraviolet light that is emitted by the mercury discharge inside the tube and releases light that has a lower power (and with a different hue) Based on the substance of the coating various colors can be obtained.
Why do the gas discharges emit light? Electrons can be stimulated to make it possible to remove them from an atom. The electron is thought to have been elevated to orbits with higher energy. As the electron slows to its orbital, a particle of light (a photon) carries away the energy of excitation and the discharge tube glows! The energy of a photon (or its wavelength or color) is determined by the difference in energy between its orbitals. An atom may emit photons with different energies corresponding to its different orbital pairs. The photon energy spectrum is the emission lines of spectroscopists unique to a particular atom. The mercury discharge tubes are distinct from the neon discharge tubes as can be seen in the custom neon sign. This is the reason why Helium, an inert gas, was discovered. In the light of day, observations revealed a variety of photon energy levels that had never been previously seen in earth discharges.
The chemical inertness of certain gases is more difficult to explain. When two atoms meet with the greatest energy or valence, the orbitals of the atoms change substantially and the electrons on the two atoms are reorganized. If this restructuring decreases the energy total of the electrons the chemical bond could form. For ordinary, non-inert atoms electrons are pliable and bonds often form. The electrons in inert gases, however, are resistant to this proximity effect which is why they rarely form bonds to form molecules.
The apparent contradiction between the inertness of gas (about chemical bonding) and its vivacity (in a glow discharge) is an example of a wider phenomenon that could be described as the inexplicable inertness of matter. An atom can be considered an inert and non-reactive particle if the energy involved in its interactions is not enough to prevent electrons from being excited. The most patient and laid-back atoms are those made of inert gases like neon. However, interaction energies increase and nuclei lose their inertness. We end up with an amalgamation of electrons and inert nuclei. It is a very charged plasma. The more energy you add (actually quite a bit), the nuclei become no longer so inert either. Instead, we get a cocktail of nucleons, similar to neutron stars. The energy is increased more, and we enter the realm of quarks. Even nucleons can no longer be inert and we are returning to the primitive, energetic conditions that were present shortly after the Big Bang.