Archive for the ‘KRYPTON’ Tag

KRYPTON   Leave a comment

Krypton (from Greek: κρυπτός kryptos “the hidden one”) is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 (noble gases) elements. A colourless, odourless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other rare gases in fluorescent lamps. Krypton is inert for most practical purposes.

Krypton, like the other noble gases, can be used in lighting and photography. Krypton light has a large number of spectral lines, and krypton’s high light output in plasmas allows it to play an important role in many high-powered gas lasers (krypton ion and excimer lasers), which pick out one of the many spectral lines to amplify. There is also a specific krypton fluoride laser. The high power and relative ease of operation of krypton discharge tubes caused (from 1960 to 1983) the official length of a meter to be defined in terms of the 605 nm (red-orange) spectral line of krypton-86.

 

History

Krypton was discovered in Britain in 1898 by Sir 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, an international agreement defined the meter in terms of wavelength of light emitted by the krypton-86 isotope (wavelength of 605.78 nanometres). This agreement replaced the long-standing standard meter located in Paris, which was a metal bar made of a platinum-iridium alloy (the bar was originally estimated to be one ten-millionth of a quadrant of the Earth’s polar circumference), and was itself replaced by a definition based on the speed of light — a fundamental physical constant. However, in 1927, the International Conference on Weights and Measures had redefined the meter in terms of a red cadmium spectral line (1 m = 1,553,164.13 wavelengths). In October 1983, the same bureau defined the meter as the distance that light travels in a vacuum during 1/299,792,458 s.

 

Characteristics

Krypton is characterized by several sharp emission lines (spectral signatures) the strongest being green and yellow. It is one of the products of uranium fission. Solidified krypton is white and crystalline with a face-centred cubic crystal structure, which is a common property of all noble gases (except helium, with a hexagonal close-packed crystal structure).

 

Isotopes

Naturally occurring krypton is made of six stable isotopes. In addition, about thirty unstable isotopes and isomers are known. 81Kr, the product of atmospheric reactions, is produced with the other naturally occurring isotopes of krypton. Being radioactive, it has a half-life of 230,000 years. Krypton is highly volatile when it is near surface waters 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 non-reactive. However, following the first successful synthesis of xenon compounds in 1962, synthesis of krypton di-fluoride (KrF2) was reported in 1963. In the same year, KrF4 was reported by Grosse, et al., but was subsequently shown to be a mistaken identification. 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+.

 

Compounds with krypton bonded to atoms other than fluorine have also been discovered. The reaction of KrF2 with B(OTeF5)3 produces an unstable compound, Kr(OTeF5)2, that contains a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation [HC≡N–Kr–F]+, produced by the reaction of KrF2 with [HC≡NH]+[AsF−6] below −50 °C. HKrCN and HKrC≡CH (krypton hydride-cyanide and hydro-krypto-acetylene) were reported to be stable up to 40 K.

 

Natural occurrence

The Earth has retained all of the noble gases that were present at its formation except for 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, as the amount is derived from the meteoric activity and that from solar winds. The first measurements suggest an over-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 brilliant white light source. Krypton is thus used in some types of photographic flashes used in high speed photography. Krypton gas is also combined with other gases to make luminous signs that glow with a bright greenish-yellow light.

 

Krypton mixes with argon as the fill gas of energy saving fluorescent lamps. This reduces their power consumption. Unfortunately this also reduces their light output and raises their 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 to be used for the filament. A brighter light results which contains more blue than conventional lamps.

 

Krypton’s white discharge is often used to good effect in coloured gas discharge tubes, which are then simply painted or stained in other ways to allow the desired colour (for example, “neon” type advertising signs where the letters appear in differing colours are often entirely krypton-based). Krypton is also capable of much higher light power density 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 which select out the red spectral line for laser amplification and emission, rather than the more familiar helium-neon variety, which could never practically achieve the multi-watt red laser light outputs needed for this application.

 

Krypton has an important role in production and usage of the krypton fluoride laser. The laser has been important in the nuclear fusion energy research community in confinement experiments. The laser has high beam uniformity, short wavelength, and the ability to modify the spot size 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 the cheaper liquid argon is typically used. The advantage of krypton over argon is a small Molière radius of 4.7 cm, which allows for excellent spatial resolution and low degree of overlapping. The other parameters relevant for calorimetry application are: radiation length of X0=4.7 cm, density of 2.4 g/cm3.

 

The sealed spark gap assemblies contained in ignition exciters used in some older jet engines contain a very small amount of Krypton-85 to obtain consistent ionization levels and uniform operation.

 

Krypton-83 has application in magnetic resonance imaging (MRI) for imaging airways. In particular, it may be used 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 anaesthetic properties limit its fraction in the breathing gas to 35%. The use of a breathing mixture containing 30% xenon and 30% krypton is comparable in effectiveness for CT to a 40% xenon fraction, while avoiding the unwanted effects of a high fraction xenon gas.

 

Precautions

Krypton is considered to be a non-toxic asphyxiant. Krypton has a narcotic potency seven times greater than air, so breathing a gas containing 50% krypton and 50% air would cause narcosis similar to breathing air at four times atmospheric pressure. This would be comparable to scuba diving at a depth of 30 m (100 ft) and potentially could affect anyone breathing it. Nevertheless, that mixture would contain only 10% oxygen and hypoxia would be a greater concern.

 

Synopsis: https://en.wikipedia.org/wiki/Krypton

 

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Posted 2018/03/27 by Stelios in Education

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NOBLE GAS   Leave a comment

The noble gases are the elements in group 18 (also sometimes Group 0 IUPAC Style, or Group 8) of the periodic table. The group is also called the helium family or neon family. Chemically, the noble gases are very stable due to having the maximum number of valence electrons their outer shell can hold. Noble gases rarely react with other elements since they are already stable. Under normal conditions, they occur as odourless, colourless, monatomic gases. Each of them has its melting and boiling point close together, so that only a small temperature range exists for each noble gas in which it is a liquid. Noble gases have numerous important applications in lighting, welding and space technology.

The seven noble gases are: helium, neon, argon, krypton, xenon, radon, and ununoctium.

Etymology

“Noble gas” is the translation of the German Edelgas, which was in use as early as 1898. This refers to the extremely low level of reactivity under normal conditions. The noble gases have also been referred to as inert gases, but these terms are not strictly accurate because several of them do take part in chemical reactions. Another old term is rare gases, although argon forms a fairly considerable part (0.93% by volume, 1.29% by mass) of the Earth’s atmosphere.

History

The existence of noble gases was not known until after the advent of the periodic table. In the late nineteenth century, Lord Rayleigh discovered that some samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Along with scientist William Ramsay, Lord Rayleigh theorized that the nitrogen extracted from air was associated with another gas, argon. With this discovery, they realized that a whole class of gases was missing from the periodic table. Eventually, all the known noble gases except for helium were discovered in the air, with argon being much more common than the others, and the table was completed. Helium was detected spectrographically in the Sun in 1868. The isolation of helium on Earth had to wait until 1895. Under standard conditions, the noble gases all occur as monatomic gases.

Chemical make-up

Noble gases have full valence electron shells. Valence electrons are the outermost electrons of an atom and are normally the only electrons which can participate in chemical bonding. According to atomic theory derived from quantum mechanics and experimental trends, atoms with full valence electron shells are extraordinarily stable and therefore do not form chemical bonds.

All of them exhibit an extremely low chemical reactivity and very few noble gas compounds have been prepared. No conventional compounds of helium or neon have yet been prepared, while xenon and krypton are known to show some reactivity in the laboratory. Recently argon compounds have also been successfully characterised. The noble gases’ lack of reactivity can be explained in terms of them having a “complete valence shell”. They have little tendency to gain or lose electrons. The noble gases have high ionization energies and negligible electro negativities The noble gases have very weak inter-atomic forces of attraction, and consequently very low melting points and boiling points. This is why they are all monatomic gases under normal conditions, even those with larger atomic masses than many normally solid elements.

Applications

One of the most commonly encountered uses of the noble gases in everyday life is in lighting. Argon is often used as a suitable safe and inert atmosphere for the inside of filament light bulbs, and is also used as an inert atmosphere in the synthesis of air and moisture sensitive compounds (as an alternative for nitrogen). Some of the noble gases glow distinctive colours when used inside lighting tubes ( neon lights). Helium, due to its non reactivity (compared with flammable hydrogen) and lightness, is often used in blimps and balloons. Helium and argon are commonly used to shield a welding arc, and the surrounding base metal from the atmosphere during welding. Krypton is also used in lasers, which are used by doctors for eye surgery. Xenon is used in xenon arc lamps, and it has anaesthetic properties.

Noble gas notation

The noble gases can be used in conjunction with the electron configuration notation to make what is called the Noble Gas Notation. For example: while the electron notation of the element carbon is 1s²2s² 2p², the Noble Gas notation would be [He] 2s²2p².

This notation makes the identification of elements faster, and is shorter and easier than writing out the full notation of orbitals.

Synopsis from: https://en.wikipedia.org/wiki/Noble_gas

Posted 2018/02/01 by Stelios in Education

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