Xenon

For other uses, see Xenon (disambiguation).

54 iodine ← xenon → caesium Kr ↑ Xe ↓ Rn periodic table
General
Name, Symbol, Number	xenon, Xe, 54
Chemical series	noble gases
Group, Period, Block	18, 5, p
Appearance	colorless
Atomic mass	131.293(6) g/mol
Electron configuration	[Kr] 4d10 5s2 5p6
Electrons per shell	2, 8, 18, 18, 8
Physical properties
Phase	gas
Density	(0 °C, 101.325 kPa) 5.894 g/L
Melting point	161.4 K (-111.7 °C, -169.1 °F)
Boiling point	165.03 K (-108.12 °C, -162.62 °F)
Critical point	289.77 K, 5.841 MPa
Heat of fusion	2.27 kJ/mol
Heat of vaporization	12.64 kJ/mol
Heat capacity	(25 °C) 20.786 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 83 92 103 117 137 165
Atomic properties
Crystal structure	cubic face centered
Oxidation states	0, +1, +2, +4, +6, +8 (rarely more than 0) (weakly acidic oxide)
Electronegativity	2.6 (Pauling scale)
Ionization energies	1st: 1170.4 kJ/mol
	2nd: 2046.4 kJ/mol
	3rd: 3099.4 kJ/mol
Atomic radius (calc.)	108 pm
Covalent radius	130 pm
Van der Waals radius	216 pm
Miscellaneous
Magnetic ordering	nonmagnetic
Thermal conductivity	(300 K) 5.65 mW/(m·K)
Speed of sound	(liquid) 1090 m/s
CAS registry number	7440-63-3
Notable isotopes
Main article: Isotopes of xenon iso NA half-life DM DE (MeV) DP 124Xe 0.1% 1.1×1017y ε ε no data 124Te 125Xe syn 16.9 h ε 1.652 125I 126Xe 0.09% Xe is stable with 72 neutrons 127Xe syn 36.4 d ε 0.662 127I 128Xe 1.91% Xe is stable with 74 neutrons 129Xe 26.4% Xe is stable with 75 neutrons 130Xe 4.1% Xe is stable with 76 neutrons 131Xe 21.29% Xe is stable with 77 neutrons 132Xe 26.9% Xe is stable with 78 neutrons 133Xe syn 5.243 d Beta- 0.427 133Cs 134Xe 10.4% Xe is stable with 80 neutrons 135Xe syn 9.10 h Beta- 1.16 135Cs 136Xe 8.9% 2.36×1021y Beta- no data 136Ba

Xenon (chemical symbol Xe, atomic number 54) is a colorless, odorless, heavy noble gas. It occurs in the Earth's atmosphere in trace amounts and was part of the first noble gas compound synthesized.

Occurrence and extraction

Xenon is a trace gas in the Earth's atmosphere, occurring in one part in twenty million. In addition, it is found in gases emitted from some mineral springs.

This element can be extracted by fractional distillation of liquid air or by selective adsorption (surface binding) on activated carbon. The isotopes Xe-133 and Xe-135 are synthesized by neutron irradiation within air-cooled nuclear reactors.

History

Xenon (from the Greek word ξένος, meaning "strange") was discovered in England by William Ramsay and Morris Travers on July 12, 1898, shortly after they had discovered the elements krypton and neon. They found it in the residue left over from evaporating components of liquid air.

Notable characteristics

Xenon is a member of the noble gas series in the periodic table. It is situated between krypton and radon in group 18 (former group 8A), and is placed after iodine in period 5.

As the noble gases are chemically very inert, they are said to have a chemical valence of zero. Nonetheless, the term "inert" is not an entirely accurate description of this group of elements, because some of them—including xenon—have been shown to 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, xenon has been forced into a metallic phase. Xenon can also form "clathrates" (cage-like molecules) with water, when xenon atoms are trapped in a lattice of water molecules.

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 discovered in the earliest nuclear reactors built by the American Manhattan Project for plutonium production. Fortunately the designers had made provisions in the design to increase the reactor's 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.

Compounds

Xenon tetrafluoride

Xenon and the other noble gases had long been considered completely chemically inert and unable to form compounds. In 1962, however, at the University of British Columbia, the first xenon compound—xenon hexafluoroplatinate—was synthesized. Now, many compounds of xenon are known, including xenon difluoride, xenon tetrafluoride, xenon hexafluoride, xenon tetroxide, xenon hydrate, xenon deuterate, and sodium perxenate. A 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.

Crystals of xenon tetrafluoride.

Recently, researchers (M. Räsänen at al.) at the University of Helsinki in Finland made xenon dihydride (HXeH), xenon hydride-hydroxide (HXeOH), and hydroxenoacetylene (HXeCCH). These compounds are stable up to 40K.

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, automotive HID headlights 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.

Xenon in shaped Geissler tubes.

Other uses of Xenon:

• It 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.
• 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 an ion engine than other fuels such as mercury or cesium. Europe's SMART-1 spacecraft utilized Xenon in its engines.
• 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.

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 denser 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, opposite the high-pitched voice caused by inhalation of helium. Like helium, xenon does not satisfy the body's need for oxygen and is a simple asphyxiant; consequently, many universities no longer allow the voice stunt as a general chemistry demonstration. As xenon is expensive, the gas sulfur hexafluoride, which is similar to xenon in molecular weight (146 vs 131), is generally used in this stunt, although it too is an asphyxiant.

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, such that xenon would be purged from the lungs within a breath or two. 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, though the potential for danger exists any time a tank or container of xenon is kept in an unventilated space.

External links

• WebElements.com – Xenon
• Xenon as an anaesthetic
• USGS Periodic Table - Xenon

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