The rare gases, also known as the noble gases, are a group of six gaseous elements found in small amounts in the atmosphere: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe). Collectively they make up about one percent of the earth's atmosphere. The low reactivity of the rare gases is due to the arrangement of electrons in the rare gas atoms. The configuration of electrons in these elements makes them very stable and therefore unreactive. The reactivity of any element is due, in part, to how easily it gains or loses electrons, which is necessary for an atom to react with other atoms. The rare gases do not readily do either.
Most of the rare gases are commercially obtained from liquid air. As the temperature of liquid air is raised, the rare gases boil off from the mixture at specific temperatures and can be separated and purified.
Neon is well known for its use in neon signs. Glass Tubes of any shape can be filled with neon and when an electrical charge is passed through the tube, an orange-red glow is emitted. By contrast, ordinary incandescent light bulbs are filled with argon. Because argon is so inert, it does not react with the hot metal filament and prolongs the bulb's life. Argon is also used to provide an inert atmosphere in welding and high-temperature metallurgical processes. By surrounding hot metals with inert argon, the metals are protected from potential oxidation by oxygen in the air. Krypton and xenon also find commercial lighting applications. Krypton can be used in incandescentlight bulbs and in fluorescent lamps. Both are also employed in flashing stroboscopic lights that outline commercial airport runways. Because they emit a brilliant white light when electrified, they are also used in photographic flash equipment. Due to the radioactive nature of radon, it has found medical applications in radiotherapy.