Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of theperiodic table and is a noble gas. Argon is the third most abundant noble gas in Earth's crust. Argon is produced industrially by the fractional distillation of liquid air. Argon is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning. Argon is also used inincandescent, fluorescent lighting, and other gas discharge tubes. Argon makes a distinctiveblue-green gas laser. Argon is also used in fluorescent glow starters.

Xenon is used in a wide variety of applications. It is a component of excimer laser mixes to produce certain wavelengths. It is a required component in the gas mixes used to produce 282nm (XeBr), 308nm (XeCl), and 351nm (XeF). It is used in sputter deposition, especially when depositing coatings with higher molecular weights. Xenon is more expensive, than commonly used argon, but a higher yield and better coating is obtained when coating with heavier atoms. Xenon is often used for sputter coating heavier coatings including those used to make hard disks. Xenon has long been used as a propellant for satellites and other spacecraft. Xenon is a preferred propellant for electric propulsion whether with Hall effect thrusters or ion thrusters. The thrusters ionize and expel the xenon at up to hundreds of kilometers per second providing in-space propulsion many times more efficient than conventional chemical propulsion. Xenon electric propulsion has been used for orbit-raising and station keeping of satellites and has been used to send space probes to Mercury and the asteroid belt. Xenon’s physical and chemical properties make it useful in lighting applications. It is used in incandescent, halogen, fluorescent, and high intensity discharge lamps. It is used for production of semiconductors in annealing, etching and lithography applications. Krypton acts both as surface disruption agent and a plasma modulator, influencing the composition of plasmas through secondary ionization. Historically secondary ionization and surface disruption was achieved using argon due to cost and availability, but krypton provides the ability to select for ions present in plasma in additional ways. Xenon mixes are used in detectors including those used to detect radiation and infrared light. Xenon is even used in the very large detectors used to find dark matter. Xenon even has some medical applications, including use to enhance MRI imaging or as anesthesia.