======================================================================
= Krypton =
======================================================================
Introduction
======================================================================
Krypton (from 'the hidden one') is a chemical element; it has symbol
Kr and atomic number 36. It is a colorless, odorless noble gas that
occurs in trace amounts in the atmosphere and is often used with other
rare gases in fluorescent lamps. Krypton is chemically inert.
Krypton, like the other noble gases, is used in lighting and
photography. Krypton light has many spectral lines, and krypton plasma
is useful in bright, high-powered gas lasers (krypton ion and excimer
lasers), each of which resonates and amplifies a single spectral line.
Krypton fluoride also makes a useful laser medium. From 1960 to 1983,
the official definition of the metre was based on the wavelength of
one spectral line of krypton-86, because of the high power and
relative ease of operation of krypton discharge tubes.
History
======================================================================
Krypton was discovered in Britain in 1898 by William Ramsay, a
Scottish chemist, and Morris Travers, an English chemist, in residue
left from evaporating nearly all components of liquid air. Neon was
discovered by a similar procedure by the same workers just a few weeks
later. William Ramsay was awarded the 1904 Nobel Prize in Chemistry
for discovery of a series of noble gases, including krypton.
In 1960, the International Bureau of Weights and Measures defined the
meter as 1,650,763.73 wavelengths of light emitted in the vacuum
corresponding to the transition between the 2p10 and 5d5 levels in the
isotope krypton-86. This agreement replaced the 1889 international
prototype meter, which was a metal bar located in Sèvres. This also
made obsolete the 1927 definition of the ångström based on the red
cadmium spectral line, replacing it with 1 Å = 10−10 m. The krypton-86
definition lasted until the October 1983 conference, which redefined
the meter as the distance that light travels in vacuum during
1/299,792,458 s.
Characteristics
======================================================================
Krypton is characterized by several sharp emission lines (spectral
signatures) the strongest being green and yellow. Krypton is one of
the products of uranium fission. Solid krypton is white and has a
face-centered cubic crystal structure, which is a common property of
all noble gases (except helium, which has a hexagonal close-packed
crystal structure).
Isotopes
==========
Naturally occurring krypton in Earth's atmosphere is composed of five
stable isotopes, plus one isotope (78Kr) with such a long half-life
(9.2×1021 years) that it can be considered stable. (This isotope has
the third-longest known half-life among all isotopes for which decay
has been observed; it undergoes double electron capture to 78Se). In
addition, about thirty unstable isotopes and isomers are known. Traces
of 81Kr, a cosmogenic nuclide produced by the cosmic ray irradiation
of 80Kr, also occur in nature: this isotope is radioactive with a
half-life of 230,000 years. Krypton is highly volatile and does not
stay in solution in near-surface water, but 81Kr has been used for
dating old (50,000-800,000 years) groundwater.
85Kr is an inert radioactive noble gas with a half-life of 10.76
years. It is produced by the fission of uranium and plutonium, such as
in nuclear bomb testing and nuclear reactors. 85Kr is released during
the reprocessing of fuel rods from nuclear reactors. Concentrations at
the North Pole are 30% higher than at the South Pole due to convective
mixing.
Chemistry
===========
Like the other noble gases, krypton is chemically highly unreactive.
The rather restricted chemistry of krypton in the +2 oxidation state
parallels that of the neighboring element bromine in the +1 oxidation
state; due to the scandide contraction it is difficult to oxidize the
4p elements to their group oxidation states. Until the 1960s no noble
gas compounds had been synthesized.
Following the first successful synthesis of xenon compounds in 1962,
synthesis of krypton difluoride () was reported in 1963. In the same
year, was reported by Grosse, 'et al.', but was subsequently shown to
be a mistaken identification. Under extreme conditions, krypton reacts
with fluorine to form KrF2 according to the following equation:
:Kr + F2 -> KrF2
Krypton gas in a krypton fluoride laser absorbs energy from a source,
causing the krypton to react with fluorine gas, producing the exciplex
krypton fluoride, a temporary complex in an excited energy state:
:2Kr + F2 -> 2KrF
The complex can undergo spontaneous or stimulated emission, reducing
its energy state to a metastable, but highly repulsive ground state.
The ground state complex quickly dissociates into unbound atoms:
:2KrF -> 2Kr + F2
The result is an exciplex laser which radiates energy at 248 nm, near
the ultraviolet portion of the spectrum, corresponding with the energy
difference between the ground state and the excited state of the
complex.
Compounds with krypton bonded to atoms other than fluorine have also
been discovered. There are also unverified reports of a barium salt of
a krypton oxoacid. ArKr+ and KrH+ polyatomic ions have been
investigated and there is evidence for KrXe or KrXe+.
The reaction of with produces an unstable compound, , that contains
a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation
[HC≡N-Kr-F], produced by the reaction of with [HC≡NH][AsF] below −50
°C. HKrCN and HKrC≡CH (krypton hydride-cyanide and
hydrokryptoacetylene) were reported to be stable up to 40 K.
Krypton hydride (Kr(H2)4) crystals can be grown at pressures above 5
GPa. They have a face-centered cubic structure where krypton octahedra
are surrounded by randomly oriented hydrogen molecules.
Natural occurrence
====================
Earth has retained all of the noble gases that were present at its
formation except helium. Krypton's concentration in the atmosphere is
about 1 ppm. It can be extracted from liquid air by fractional
distillation. The amount of krypton in space is uncertain, because
measurement is derived from meteoric activity and solar winds. The
first measurements suggest an abundance of krypton in space.
Applications
======================================================================
Krypton's multiple emission lines make ionized krypton gas discharges
appear whitish, which in turn makes krypton-based bulbs useful in
photography as a white light source. Krypton is used in some
photographic flashes for high speed photography. Krypton gas is also
combined with mercury to make luminous signs that glow with a bright
greenish-blue light.
Krypton is mixed with argon in energy efficient fluorescent lamps,
reducing the power consumption, but also reducing the light output and
raising the cost. Krypton costs about 100 times as much as argon.
Krypton (along with xenon) is also used to fill incandescent lamps to
reduce filament evaporation and allow higher operating temperatures.
Krypton's white discharge is sometimes used as an artistic effect in
gas discharge "neon" tubes. Krypton produces much higher light power
than neon in the red spectral line region, and for this reason, red
lasers for high-power laser light-shows are often krypton lasers with
mirrors that select the red spectral line for laser amplification and
emission, rather than the more familiar helium-neon variety, which
could not achieve the same multi-watt outputs.
The krypton fluoride laser is important in nuclear fusion energy
research in confinement experiments. The laser has high beam
uniformity, short wavelength, and the spot size can be varied to track
an imploding pellet.
In experimental particle physics, liquid krypton is used to construct
quasi-homogeneous electromagnetic calorimeters. A notable example is
the calorimeter of the NA48 experiment at CERN containing about 27
tonnes of liquid krypton. This usage is rare, since liquid argon is
less expensive. The advantage of krypton is a smaller Molière radius
of 4.7 cm, which provides excellent spatial resolution with little
overlapping. The other parameters relevant for calorimetry are:
radiation length of X0=4.7 cm, and density of 2.4 g/cm3.
Krypton-83 has application in magnetic resonance imaging (MRI) for
imaging airways. In particular, it enables the radiologist to
distinguish between hydrophobic and hydrophilic surfaces containing an
airway.
Although xenon has potential for use in computed tomography (CT) to
assess regional ventilation, its anesthetic properties limit its
fraction in the breathing gas to 35%. A breathing mixture of 30% xenon
and 30% krypton is comparable in effectiveness for CT to a 40% xenon
fraction, while avoiding the unwanted effects of a high partial
pressure of xenon gas. The metastable isotope krypton-81m is used in
nuclear medicine for lung ventilation/perfusion scans, where it is
inhaled and imaged with a gamma camera. Krypton-85 in the atmosphere
has been used to detect clandestine nuclear fuel reprocessing
facilities in North Korea and Pakistan. Those facilities were detected
in the early 2000s and were believed to be producing weapons-grade
plutonium. Krypton-85 is a medium lived fission product and thus
escapes from spent fuel when the cladding is removed.
Krypton is used occasionally as an insulating gas between window
panes. SpaceX Starlink uses krypton as a propellant for their electric
propulsion system.
Precautions
======================================================================
Krypton is considered to be a non-toxic asphyxiant. Being lipophilic,
krypton has a significant anaesthetic effect (although the mechanism
of this phenomenon is still not fully clear, there is good evidence
that the two properties are mechanistically related), with narcotic
potency seven times greater than air, and breathing an atmosphere of
50% krypton and 50% natural air (as might happen in the locality of a
leak) causes narcosis in humans similar to breathing air at four times
atmospheric pressure. This is comparable to scuba diving at a depth of
30 m and could affect anyone breathing it.
Further reading
======================================================================
* William P. Kirk
[
https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9100FW8L.txt "Krypton 85:
a Review of the Literature and an Analysis of Radiation Hazards"],
Environmental Protection Agency, Office of Research and Monitoring,
Washington (1972)
External links
======================================================================
* [
http://www.periodicvideos.com/videos/036.htm Krypton] at 'The
Periodic Table of Videos' (University of Nottingham)
*
[
https://web.archive.org/web/20070203205901/http://other.nrl.navy.mil/LaserFusionEnergy/lasercreation.htm
Krypton Fluoride Lasers], Plasma Physics Division Naval Research
Laboratory
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Krypton
