Xenon

Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized.
Xenon is used in flash lamps and arc lamps, and as a general anesthetic. The first excimer laser design used a xenon dimer molecule as the lasing medium, and the earliest laser designs used xenon flash lamps as pumps. Xenon is also used to search for hypothetical weakly interacting massive particles and as a propellant for ion thrusters in spacecraft.
Naturally occurring xenon consists of seven stable isotopes and two long-lived radioactive isotopes. More than 40 unstable xenon isotopes undergo radioactive decay, and the isotope ratios of xenon are an important tool for studying the early history of the Solar System. Radioactive xenon-135 is produced by beta decay from iodine-135, and is the most significant neutron absorber in nuclear reactors.

History

Xenon was discovered in England by the Scottish chemist William Ramsay and English chemist Morris Travers on July 12, 1898, shortly after their discovery of the elements krypton and neon. They found xenon in the residue left over from evaporating components of liquid air. Ramsay suggested the name xenon for this gas from the Greek word ξένον xénon, neuter singular form of ξένος xénos, meaning 'foreign', 'strange', or 'guest'. In 1902, Ramsay estimated the proportion of xenon in the Earth's atmosphere to be one part in 20 million.
During the 1930s, American engineer Harold Edgerton began exploring strobe light technology for high speed photography. This led him to the invention of the xenon flash lamp in which light is generated by passing brief electric current through a tube filled with xenon gas. In 1934, Edgerton was able to generate flashes as brief as one microsecond with this method.
In 1939, American physician Albert R. Behnke Jr. began exploring the causes of "drunkenness" in deep-sea divers. He tested the effects of varying the breathing mixtures on his subjects, and discovered that this caused the divers to perceive a change in depth. From his results, he deduced that xenon gas could serve as an anesthetic. Although Russian toxicologist Nikolay V. Lazarev apparently studied xenon anesthesia in 1941, the first published report confirming xenon anesthesia was in 1946 by American medical researcher John H. Lawrence, who experimented on mice. Xenon was first used as a surgical anesthetic in 1951 by American anesthesiologist Stuart C. Cullen, who successfully used it with two patients.
Xenon and the other noble gases were for a long time considered to be completely chemically inert and not able to form compounds. However, while teaching at the University of British Columbia, Neil Bartlett discovered that the gas platinum hexafluoride was a powerful oxidizing agent that could oxidize oxygen gas to form dioxygenyl hexafluoroplatinate. Since O₂ and xenon have almost the same first ionization potential, Bartlett realized that platinum hexafluoride might also be able to oxidize xenon. On March 23, 1962, he mixed the two gases and produced the first known compound of a noble gas, xenon hexafluoroplatinate.
Bartlett thought its composition to be Xe⁺⁻, but later work revealed that it was probably a mixture of various xenon-containing salts. Since then, many other xenon compounds have been discovered, in addition to some compounds of the noble gases argon, krypton, and radon, including argon fluorohydride, krypton difluoride, and radon fluoride. By 1971, more than 80 xenon compounds were known.
In November 1989, IBM scientists demonstrated a technology capable of manipulating individual atoms. The program, called IBM in atoms, used a scanning tunneling microscope to arrange 35 individual xenon atoms on a substrate of chilled crystal of nickel to spell out the three-letter company initialism. It was the first-time atoms had been precisely positioned on a flat surface.

Characteristics

Xenon has atomic number 54; that is, its nucleus contains 54 protons. At standard temperature and pressure, pure xenon gas has a density of 5.894 kg/m³, about 4.5 times the density of the Earth's atmosphere at sea level, 1.217 kg/m³. As a liquid, xenon has a density of up to 3.100 g/mL, with the density maximum occurring at the triple point. Liquid xenon has a high polarizability due to its large atomic volume, and thus is an excellent solvent. It can dissolve hydrocarbons, biological molecules, and even water. Under the same conditions, the density of solid xenon, 3.640 g/cm³, is greater than the average density of granite, 2.75 g/cm³. Under gigapascals of pressure, xenon forms a metallic phase.
Solid xenon changes from Face-centered cubic to hexagonal close packed crystal phase under pressure and begins to turn metallic at about 140 GPa, with no noticeable volume change in the hcp phase. It is completely metallic at 155 GPa. When metallized, xenon appears sky blue because it absorbs red light and transmits other visible frequencies. Such behavior is unusual for a metal and is explained by the relatively small width of the electron bands in that state.
Liquid or solid xenon nanoparticles can be formed at room temperature by implanting Xe⁺ ions into a solid matrix. Many solids have lattice constants smaller than solid Xe. This results in compression of the implanted Xe to pressures that may be sufficient for its liquefaction or solidification.
Xenon is a member of the zero-valence elements that are called noble or inert gases. It is inert to most common chemical reactions because the outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.
In a gas-filled tube, xenon emits a blue or lavenderish glow when excited by electrical discharge. Xenon emits a band of emission lines that span the visual spectrum, but the most intense lines occur in the region of blue light, producing the coloration.

Occurrence and production

Xenon is a trace gas in Earth's atmosphere, occurring at a volume fraction of , or approximately 1 part per 11.5 million. It is also found as a component of gases emitted from some mineral springs. Given a total mass of the atmosphere of, the atmosphere contains on the order of of xenon in total when taking the average molar mass of the atmosphere as 28.96 g/mol which is equivalent to some 394-mass ppb.

The missing Xe problem

The concentration of Xe in the atmosphere is much lower than Ar and Kr, a geological mystery known as "the missing Xe problem". Numerous proposals have been made to explain the mystery, including formation of Xe–Fe oxides in the Earth's lower mantle, formation of xenon dioxide in silica, and reactions between Xe and Fe/Ni in the Earth's core.

Commercial

Xenon is obtained commercially as a by-product of the separation of air into oxygen and nitrogen. After this separation, generally performed by fractional distillation in a double-column plant, the liquid oxygen produced will contain small quantities of krypton and xenon. By additional fractional distillation, the liquid oxygen may be enriched to contain 0.1–0.2% of a krypton/xenon mixture, which is extracted either by adsorption onto silica gel or by distillation. Finally, the krypton/xenon mixture may be separated into krypton and xenon by further distillation.
Worldwide production of xenon in 1998 was estimated at. At a density of this is equivalent to roughly. Because of its scarcity, xenon is much more expensive than the lighter noble gases—approximate prices for the purchase of small quantities in Europe in 1999 were 10 €/L for xenon, 1 €/L for krypton, and 0.20 €/L for neon, while the much more plentiful argon, which makes up over 1% by volume of earth's atmosphere, costs less than a cent per liter.

Solar System

Within the Solar System, the nucleon fraction of xenon is, for an abundance of approximately one part in 630 thousand of the total mass. Xenon is relatively rare in the Sun's atmosphere, on Earth, and in asteroids and comets. The abundance of xenon in the atmosphere of planet Jupiter is unusually high, about 2.6 times that of the Sun. This abundance remains unexplained, but may have been caused by an early and rapid buildup of planetesimals—small, sub-planetary bodies—before the heating of the presolar disk; otherwise, xenon would not have been trapped in the planetesimal ices. The problem of the low terrestrial xenon may be explained by covalent bonding of xenon to oxygen within quartz, reducing the outgassing of xenon into the atmosphere.

Stellar

Unlike the lower-mass noble gases, the normal stellar nucleosynthesis process inside a star does not form xenon. Nucleosynthesis consumes energy to produce nuclides more massive than iron-56, and thus the synthesis of xenon represents no energy gain for a star. Instead, xenon is formed during supernova explosions during the r-process, by the slow neutron-capture process in red giant stars that have exhausted their core hydrogen and entered the asymptotic giant branch, and from radioactive decay, for example by beta decay of extinct iodine-129 and spontaneous fission of thorium, uranium, and plutonium.

Nuclear fission

is a notable neutron poison with a high fission product yield. As it is relatively short lived, it decays at the same rate it is produced during steady operation of a nuclear reactor. However, if power is reduced or the reactor is scrammed, less xenon is destroyed than is produced from the beta decay of its parent nuclides. This phenomenon called xenon poisoning can cause significant problems in restarting a reactor after a scram or increasing power after it had been reduced and it was one of several contributing factors in the Chernobyl nuclear accident.
Stable or extremely long lived isotopes of xenon are also produced in appreciable quantities in nuclear fission. Xenon-136 is produced both as a fission product and when xenon-135 undergoes neutron capture before it can decay. The ratio of xenon-136 to xenon-135 can give hints as to the power history of a given reactor or identify a nuclear explosion, as xenon-135 is mostly produced by successive beta decays of more neutron-rich fission products. These short-lived nuclides do not share its neutron-absorbing prowess, and so absorb fewer neutrons during the brief moment of a nuclear explosion, lowering the ratio of mass-136 to mass-135 products.
The stable isotope xenon-132 has a fission product yield of over 4% in the thermal neutron fission of which means that stable or nearly stable xenon isotopes have a higher mass fraction in spent nuclear fuel than it does in air. However, there is as of 2022 no commercial effort to extract xenon from spent fuel during nuclear reprocessing.