Krypton (Kr): Properties & Uses

Feature

In 1898,  a Scottish Scientist Sir William Ramsay and an English Scientist Sir Morris William Travers discovered few gases in the residue of a completely evaporated sample of liquid air. Krypton, along with Xenon and Neon, was found in that process. Krypton is a chemical element with atomic no. 36 and, atomic mass 83.03 u, positioned in Group 18 of the periodic table, right below argon.

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Its name comes from the Greek word “Kryptos,”  which means hidden. As a noble gas, krypton is generally inert and forms very few chemical compounds. In 1960, the General Conference on Weight and Measurement defined the meter as the length equal to 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the p-d transitions between the levels of 86-Krypton atom. It is also known for having a comic counterpart in a famous comic book series, Superman. However, that resemblance is fictional and has nothing to do with the facts.

Natural Occurrence

Earth has managed to retain almost all of the noble gases, except for Helium. Krypton is naturally present in meteorites and minerals in trace quantities. Krypton concentration in the earth’s atmosphere is around 1 part per million by volume. In other words, for every 100 litres of air, there would be 1/10th of the millilitre of Krypton. An insignificant amount of Krypton could also appear in the earth’s crust due to the decay of Uranium and other radioactive elements. The abundance of Krypton in space is still uncertain. However, measurements show that all noble gases are abundant in the universe.

Isotopes

Krypton exists in five stable isotopes and one radioactive isotope (_{ }^{ 78 }{ Kr }). However, _{ }^{ 78 }{ Kr } has a very long half-life period of 9200 trillion years, which makes it relatively stable. Krypton-84 is the most prevalent, comprising about 57% of natural krypton. The other five stable isotopes and their relative abundances are krypton-78 (0.4%), krypton-80 (2.3%), krypton-82 (12%), krypton-83 (11%), and krypton-86 (17%).  There are thirty other radioactive isotopes and isomers of Krypton, out of which, only Krypton-81 and Krypton-85 have a half-life long enough to be of any concerns. Other isotopes have a half-life of a maximum of two days.

Krypton-81 has a half-life of 230,000 years and is mainly a cosmogenic nuclide, i.e., it is produced by the irradiation of krypton-80 due to interaction with cosmic rays. Whereas, Krypton-85 is produced by fissioning of Uranium and Plutonium and released during chopping and dissolution of spent fuel rods in the nuclear-reprocessing facilities. The half-life of Krypton-85 is 11 years, which gives it a great spot-light in several Departments Of Energy around the globe. The low specific activity of krypton-81 limits its radioactive hazard.

Properties of Krypton

Krypton is characterised by its brilliant green and orange spectral lines when it is in a gaseous state, whereas solid krypton is white in colour. One should not confuse Krypton with Kryptonite, the Superman repellant. Kryptonite is purely a fictional concept and has nothing to do with Krypton, and NO, there is no real planet by the name Krypton.

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Physical Properties

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Krypton is a colourless (except for solid kryptonite), odourless, and a reasonably expensive element. Like all other inert gases melting point and boiling point of krypton also have a narrow range. The boiling point of Krypton is –153.4 ºC, and the melting point is –157.4 ºC. The solubility of Krypton in water is about 3 x { 10 }^{ -7 } mg/L. Krypton is 2.8 times as dense as air at Normal Temperature and Pressure conditions with a density of 0.003425  gram per cubic centimetre.

Chemical Properties

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Krypton has an electronic configuration of [Ar] 3{ d }^{ 10 }4{ s }^{ 2 }4{ p }^{ 6 } Like other noble gases, Krypton was also considered chemically inert initially. However, after the successful attempt of synthesizing compounds of Xenon, scientists were able to isolate a compound Krypton Difluoride KrF2. Under extreme conditions, Krypton reacts with fluorine to form Krypton Difluoride, and since fluorine has an oxidation state of -1, to isolate a neutral compound oxidation state of Krypton is found to be 2.

KrF2

Lewis Dot structure of KrF2

In Krypton Fluorine Laser, Krypton absorbers energy from a source and reacts with the fluorine gas, producing exciplex Krypton Fluoride, an unstable complex in an excited energy state.

2 Kr + F2 → 2 KrF

This complex then goes under simulated or spontaneous emission of laser light.

2 KrF → 2 Kr + F2

 

 

Krypton Hydride (Kr(H2)4) crystals can form under an intense high Pressure of 5 Gpa.

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Structure of Kr(H2)4. Krypton octahedra (green) are surrounded by randomly oriented hydrogen molecules.

Uses Of Krypton

Commercial Applications

Neonlites

Krypton is used to manufacture white lighting bulbs for cinematographic purposes. It has been used in the manufacture of camera flashes for high-speed photography. Mixed with Mercury, krypton emits a brilliant light that has used to make the luminous sign for directing aeroplanes on the runway during foggy weather. Krypton is also used to make energy-efficient fluorescent lamps. Most of the neon-hoardings are krypton based since Krypton produces a much brighter red light, and for the same reason, red light used in laser shows is also made up of krypton based amplification devices.

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A young girl holding Krypton Gas Lightsabres.

Many small satellites of SpaceX Starlink, uses Krypton as a propellent for Hall thrusters in their electronic propulsion system.

 

Research Applications

Liquid Krypton is used to construct quasi-homogeneous electromagnetic calorimeters. Krypton has a radioactive isotope, 85-Kr, which is a beta emitter with a maximum energy of 670 KeV and a half-life of 11 years. Although Krypton is expensive than argon, it provides excellent spatial resolution with little overlapping.

NA48 Setup

NA48 Setup used at CERN for electromagnetic calorimetry.

Krypton-81 has a half-life of 230,000 years. This radioactive property enables archaeologists and Scientists in Antarctica to use this isotope for dating. The Krypton dating process is much like carbon-14 dating, but unlike carbon, krypton is chemically inert and much more stable than carbon.

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Top: satellite imagery of Taylor Glacier. Kr-81 sampling locations are indicated as blue dots. Bottom: location of Taylor Glacier on the map of Antarctica.

The presence of Krypton-85 in the atmosphere helps to detect clandestine nuclear fuel reprocessing facilities all around the globe. With the help of this technology in the early 2000s, it was detected that such facilities were producing weapon-grade plutonium in North Korea and Pakistan.

Medical Applications

Hyper-polarised MRI scanners based on another isotope of Krypton, 83-Kr, are used across the globe for the study of the spatial distribution of tobacco smoke condensate in the human lungs, which can cause major pulmonary diseases in humans such as lung cancer. Also, Krypton-81, a radioactive isotope of Krypton, can be used as a nuclear medicine to treat severe pulmonary embolism.

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Hp-83Kr MRI scan representation for a rat.

Although Xenon is an inert gas and has the potential to be used in computed tomography (CT-Scan), it is moderately soluble in blood and tissue and has anaesthetic properties. Krypton, on the other hand, is completely inert and used as a supplement with xenon to bring the total radiodensity of the inhaled contrast agent in computed tomography.

Kr-Xe CT

Miscellaneous Applications

A mixture of Krypton and Xenon is used in Filament bulbs to provide an atmosphere for the filament to withstand higher operating temperatures. KrF lasers have outstanding beam spatial uniformity. They have an inherent short wavelength (248 nm) that increases the rocket efficiency and raises the threshold for harmful laser-plasma instabilities.

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Xe-Kr Filled Bulb

Krypton is occasionally used as a filler gas in glass window panes to reduce thermal conductivity.

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Health Aspects

Warning

Although Krypton is a non-toxic inert gas, it can act as an asphyxiant by displacing the oxygen in a closed and contained environment below the required point to support life. Being an odourless, colourless, tasteless, and non-irritating gas, it does not have any warning indicators, and humans do not possess any senses that can detect a Krypton leak in the air. Inhalation of argon in excessive amounts can cause dizziness, nausea, vomiting, loss of consciousness, and death. Death may result from errors in judgment, confusion, or loss of consciousness that prevents self-rescue. Other than that, there are no evident harmful effects of Krypton on humans or “Superhumans.”

Health Impact

Current Research

In July 2014, U.S. naval research laboratory set a world record in the Guinness Book of World Records for accelerating plastic foils to the speed of 1,000 kilometres per second (more than 2.2 million mph) in less than a millimetre of distance, using a powerful Krypton-Fluorine laser. Many researchers are trying to advance this application around the globe.

Award

Researchers at the U.S. Naval Research Laboratory (NRL), Nike krypton fluoride laser facility, are the recipients of the Guinness World Records certificate for Highest Projectile Velocity.

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