Xenon: Encyclopedia - Xenon

Xenon

Xenon is a chemical element in the periodic table that has the symbol Xe and atomic number 54. A colorless, very heavy, odorless noble gas, xenon occurs in the earth's atmosphere in trace amounts and was part of the first noble gas compound synthesized.

Xenon - Notable characteristics

Xenon is a member of the zero-valence elements that are called noble or inert gases. The word "inert" is no longer used to describe this chemical series since some zero valence elements do form compounds. In a gas filled tube, xenon emits a blue glow when the gas is excited by electrical discharge. Using tens of gigapascals of pressure, metallic xenon has been made. Xenon can also form clathrates with water when atoms of it are trapped in a lattice of the water molecules.

Xenon - Applications

This gas is most widely and most famously used in light-emitting devices called Xenon flash lamps, which are used in photographic flashes, stroboscopic lamps, to excite the active medium in lasers which then generate coherent light, in bactericidal lamps (rarely), and in certain dermatological uses. Continuous, short-arc, high pressure Xenon arc lamps have a color temperature closely approximating noon sunlight and are used in solar simulators, some projection systems, and other specialized uses. They are an excellent source of short wavelength ultraviolet light and they have intense emissions in the near infrared, which are used in some night vision systems. Other uses of Xenon:

• Has been used as a general anaesthetic, though the cost is prohibitive.
• In nuclear energy applications it is used in bubble chambers, probes, and in other areas where a high molecular weight and inert nature is a desirable quality.
• Perxenates are used as oxidizing agents in analytical chemistry.
• The isotope Xe-133 is useful as a radioisotope.
• Hyperpolarized MRI of the lungs and other tissues using ¹²⁹Xe.[1]
• Preferred fuel for Ion propulsion because of high molecular weight, ease of ionization, store as a liquid at near room temperature (but at high pressure) yet easily converts back into a gas to fuel the engine, inert nature makes it environmentally friendly and less corrosive to ion engine then other fuels such a mercury or cesium. However, there is an emerging controversy over its possible future widespread use in the aerospace industry, as it is permanently lost to space, further decreasing the limited supply in the Earth's atmosphere.
• Is commonly used in protein crystallography. Applied at high pressure (~600 psi) to a protein crystal, xenon atoms bind in predominantly hydrophobic cavities, often creating a high quality, isomorphous, heavy-atom derivative.

Xenon - History

Xenon (Greek xenon meaning "stranger") was discovered in England by William Ramsay and Morris Travers in 1898 in the residue left over from evaporating components of liquid air.

Xenon - Occurrence

Xenon is a trace gas in Earth's atmosphere, occurring in one part in twenty million. The element is obtained commercially through extraction from the residues of liquefied air. This noble gas is naturally found in gases emitted from some mineral springs. Xe-133 and Xe-135 are synthesized by neutron irradiation within air-cooled nuclear reactors.

Xenon - Compounds

Before 1962, xenon and the other noble gases were generally considered to be chemically inert and not able to form compounds. Evidence since this time has been mounting that xenon, along with other noble gases, do in fact form compounds. Some of the xenon compounds are xenon difluoride, tetrafluoride, hexafluoride, hydrate, and deuterate, as well as sodium perxenate. The highly explosive compound xenon trioxide has also been made. There are at least 80 xenon compounds in which fluorine or oxygen is bonded to xenon. Some compounds of xenon are colored but most are colorless.

Xenon - Isotopes

Naturally occurring xenon is made of seven stable and two slightly radioactive isotopes. Beyond these stable forms, there are 20 unstable isotopes that have been studied. Xe-129 is produced by beta decay of I-129 (half-life: 16 million years); Xe-131m, Xe-133, Xe-133m, and Xe-135 are some of the fission products of both U-235 and Pu-239, and therefore used as indicators of nuclear explosions.

The artificial isotope Xe-135 is of considerable significance in the operation of nuclear fission reactors. Xe-135 has a huge cross section for thermal neutrons, 2.65x10⁶ barns, so it acts as a neutron absorber or "poison" that can slow or stop the chain reaction after a period of operation. This was a significant problem in the earliest nuclear reactors built by the American Manhattan Project for plutonium production. The problem was eventually overcome by redesigning the reactor to increase its reactivity (the number of neutrons per fission that go on to fission other atoms of nuclear fuel).

Relatively high concentrations of radioactive xenon isotopes are also found emanating from nuclear reactors due to the release of this fission gas from cracked fuel rods or fissioning of uranium in cooling water. The concentrations of these isotopes are still usually low compared to naturally occurring radioactive noble gases such as Rn-222.

Because xenon is a tracer for two parent isotopes, Xe isotope ratios in meteorites are a powerful tool for studying the formation of the solar system. The I-Xe method of dating gives the time elapsed between nucleosynthesis and the condensation of a solid object from the solar nebula. Xenon isotopes are also a powerful tool for understanding terrestrial differentiation. Excess Xe-129 found in carbon dioxide well gases from New Mexico was believed to be from the decay of mantle-derived gases soon after Earth's formation.

Xenon - Precautions

The gas can be safely kept in normal sealed glass containers at standard temperature and pressure. Xenon is non-toxic, but many of its compounds are toxic due to their strong oxidative properties.

Because xenon is heavier than air, the speed of sound in xenon is slower than that in air, and when inhaled, lowers the resonant frequencies of the vocal tract. This produces a characteristic lowered voice pitch, analogous to the high-pitched voice caused by inhalation of helium. For several reasons, the inhalation of xenon is more dangerous than the inhalation of helium. First, xenon has poorly understood anaesthetic effects, similar to those of nitrous oxide. Inhalation can cause mild-to-moderate, short-lasting effects, including a pounding sensation associated with light and sound.

A myth exists that xenon is too heavy for the lungs to expel unassisted, and that after inhaling xenon, it is necessary to bend over completely at the waist to allow the excess gas to "spill" out of the body. In fact, the lungs mix gases very effectively and rapidly, so that a heavy gas would be purged from the lungs within a breath or two (in fact, carbon dioxide, which the lungs are designed to exhaust from the body, is quite heavy). There is a danger associated with any heavy gas in large quantities: it may sit invisibly in a container, and if a person enters a container filled with an odorless, colorless gas, they may find themselves breathing it unknowingly. Xenon is rarely used in large enough quantities for this to be a concern.

Because of the dangers associated with xenon inhalation, many universities no longer allow the practice as a general chemistry demonstration. Xenon is in any case quite expensive. The gas sulfur hexafluoride is generally used as a replacement.

Adapted from the Wikipedia article "Xenon", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki