Rare gases

The so-called "rare" gases Neon (Ne), Krypton (Kr) and Xenon (Xe), are present in air in very low concentrations. Like the other "noble" or "inert" gases, helium (He), argon (Ar) and radon (Rn), Neon, Krypton and Xenon remain in the air because they do not combine with other materials to form solid or liquid compounds. All of these gases are monatomic.

Neon, Krypton and Xenon are valued for their light emitting properties when electrically charged. Krypton and Xenon are also valued for their total inertness coupled with high molecular weight (83.80 and 131.30, respectively). Krypton and Xenon are about two to three times as heavy as argon (molecular weight 39.95) and approximately three to four times as heavy as nitrogen (molecular weight 28.0) which is used as in inert gas in many applications, but is not a true inert gas. These properties are put to good use in multi-pane windows to reduce heat loss due to convection between the panes; and in light bulbs, where their high molecular weight slows evaporation of the hot tungsten filament, leading to longer useful operating life. Krypton and Xenon have also been considered for a more exotic application - as the propulsion gas for deep space exploration using ion engines.

Neon, Krypton and Xenon can be economically recovered by adding additional purification steps in large air separation plants or ammonia production plants (which use large amounts of air as a raw material). The boiling point of Neon is significantly lower than nitrogen (lower than all the gases except helium and hydrogen). It can be used as a very low temperature working fluid in refrigeration cycles. Neon can be recovered from large nitrogen plants as well as multi-product air separation units. Krypton and Xenon have higher boiling points than oxygen, from which they can be separated by distillation in air separation plants. When these products are recovered from ammonia plant purge gas, the neon must be separated from hydrogen and nitrogen, and the krypton and xenon from methane.

All of the naturally inert or "noble" gases are members of "Group 18" of the Periodic Table. Group 18 materials have a complete outermost electron shell; the "valence" shell that is highly involved in the formation of compounds. Moving down the Periodic Table from Helium, to Neon, Argon, Krypton, Xenon and Radon, the valence shells are located further from the nucleus, above the previous element's valence shell. Helium has two valence electrons, the other noble gases have eight.

Properties

Rare Gases Applications and Uses for Neon (Ne), Krypton (Kr) and Xenon (Xe):

Lighting:

• Neon is commonly recognized as the gas that produces the glow in "neon" lights (which often contain other gases as well). Neon's natural red color can be turned into a wide range of effective decorative lighting colors by mixing neon with other gases, by using colored glass tubes or by depositing fluorescent powder coatings inside the glass tubes. Neon is also used to produce a red glow in indicator lamps and lasers.

• Krypton is used in halogen sealed beam headlights to increase light output by allowing thinner filaments to be used with acceptable useful lifetimes. Krypton is also used in in lasers, in particular mixed with fluorine to create an "excimer" mixture that is a precursor to a molecule which exists in the excited state but not in the ground state. In excimer lasers, the gas mixture is pulsed to form short-lived excited molecules which release energy by light emission as the constituents return to the ground state. Krypton-fluorine excimer lasers produce high-power ultraviolet light used in eye surgery. Other applications are sterilization of fluids and lithographic fabrication of semiconductors.

• Xenon has a light spectrum that is much wider than neon or krypton and Xenon, with an overall bluish hue that is perceived as being similar to "daylight". It is used in high-intensity aviation approach lights, in high-efficiency incandescent bulbs for automotive and stage lighting uses, in plasma display panels, in operating room and internal examination lighting, and in ultraviolet lasers.

Construction:

Argon and Krypton are used as a premium filler gases for high-efficiency dual-pane (and triple pane) windows. Argon is about one-third heavier than nitrogen or dry air, and Krypton is twice as heavy as Argon. They may be used individually or in a mixture. These heavier filler gases minimize heat transmission by convective movement of the filler gas between the panes of glass. The insulating value of the window (measured by R value) is roughly proportional to the molecular weight of the filler gas, holding other possible construction differences such as the impact of high efficiency (Low E) glass coatings and triple versus dual-pane construction constant. Noise transmission through windows is also reduced as the molecular weight of the filler gas increases.

Argon is about 5 times as expensive as dry nitrogen, but so little is used in a window that the benefits of using it are easily justified. Argon has become the preferred gas to use in most multi-paned windows. Krypton costs much more than argon, often about 100 times as much for the same volume. This price disparity is mainly due to the much lower concentration of Krypton than Argon in air. Only a small number of air separation plants process enough air to make production of Krypton economically attractive.

Low Temperature Refrigeration:

Neon, with a boiling point lower than all the gases except helium and hydrogen, can be used as a very low temperature refrigerant. On a volume basis, Neon has 3 times the refrigerating capacity of liquid hydrogen and over 40 times the refrigerating capacity of liquid helium.