KRYPTON: HISTORY, PROPERTIES, STRUCTURE, OBTAINING, RISKS, USES - CHEMISTRY - 2024

The krypton is a noble gas which is represented by the symbol Kr and is located in group 18 of the periodic table. It is the gas that follows argon, and its abundance is so low that it was considered hidden; that's where its name comes from. It is not found almost in mineral stones, but in masses of natural gases and hardly dissolved in the seas and oceans.

His name alone evokes the image of Superman, his planet Krypton and the famous kryptonite, a stone that weakens the superhero and deprives him of his super powers. You can also think of cryptocurrencies or crypto when you hear about it, as well as other terms that are far from this gas in their essence.

Vial with krypton excited by an electrical discharge and glowing with white light. Source: Hi-Res Images of Chemical Elements

However, this noble gas is less extravagant and "hidden" compared to those figures mentioned above; although its lack of reactivity does not take away all the potential interest that can arouse in research focused on different fields, especially the physical one.

Unlike the other noble gases, the light emitted by krypton when excited in an electric field is white (top image). Because of this, it is used for various uses in the lighting industry. It can replace practically any neon light and emit its own, which is distinguished by being yellowish green.

It occurs in nature as a mixture of six stable isotopes, not to mention some radioisotopes intended for nuclear medicine. To obtain this gas, the air we breathe must be liquefied, and its resulting liquid is subjected to a fractional distillation, where the krypton is then purified and separated into its constituent isotopes.

Thanks to krypton, it has been possible to advance in studies of nuclear fusion, as well as in the applications of lasers for surgical purposes.

History

- Discovery of the hidden element

In 1785 the English chemist and physicist Henry Cavendish discovered that air contained a small proportion of a substance even less active than nitrogen.

A century later the English physicist Lord Rayleigh, isolated from the air a gas that he thought was pure nitrogen; but then he discovered that it was heavier.

In 1894 the Scottish chemist, Sir William Ramsey, collaborated to isolate this gas, which turned out to be a new element: argon. A year later, he isolated the helium gas by heating the mineral cleveite.

Sir William Ramsey himself, together with his assistant, the English chemist Morris Travers, discovered krypton on May 30, 1898, in London.

Ramsey and Travers believed that there was a space in the periodic table between the elements argon and helium, and a new element had to fill this space. Ramsey, a month after the discovery of krypton, June 1898, discovered neon; element that filled the space between helium and argon.

Methodology

Ramsey suspected the existence of a new element hidden within his previous discovery, that of argon. Ramsey and Travers, to test their idea, decided to get a large volume of argon from the air. For this they had to produce the liquefaction of the air.

Then, they distilled the liquid air to separate it into fractions and explore in the lighter fractions for the presence of the desired gaseous element. But they made a mistake, apparently they overheated the liquefied air and evaporated a lot of the sample.

In the end they only had 100 mL of the sample and Ramsey was convinced that the presence of the element lighter than argon in that volume was unlikely; but he decided to explore the possibility of an element heavier than argon in the residual sample volume.

Following his thought, he removed oxygen and nitrogen from the gas using red-hot copper and magnesium. He then placed a sample of the remaining gas in a vacuum tube, applying a high voltage to it to obtain the spectrum of the gas.

As expected, argon was present, but they noticed the appearance of two new bright lines in the spectrum; one yellow and the other green, both of which had never been observed.

- Emergence of the name

Ramsey and Travers calculated the relationship between the specific heat of gas at constant pressure and its specific heat at constant volume, finding a value of 1.66 for that relationship. This value corresponded to a gas formed by individual atoms, showing that it was not a compound.

Therefore, they were in the presence of a new gas and krypton had been discovered. Ramsey decided to call it Krypton, a word derived from the Greek word "krypto" which means "hidden." William Ramsey received the Nobel Prize in Chemistry in 1904 for the discovery of these noble gases.

Physical and chemical properties

Appearance

It is a colorless gas that exhibits an incandescent white color in an electric field.

Standard atomic weight

83,798 u

Atomic number (Z)

36

Melting point

-157.37 ºC

Boiling point

153,415 ºC

Density

Under standard conditions: 3,949 g / L

Liquid state (boiling point): 2.413 g / cm ³

Relative gas density

2.9 in relation to air with value = 1. That is, krypton is three times as dense as air.

Water solubility

59.4 cm ³ / 1,000 g at 20 ºC

Triple point

115.775 K and 73.53 kPa

Critical point

209.48 K and 5.525 MPa

Heat of fusion

1.64 kJ / mol

Heat of vaporization

9.08 kJ / mol

Molar caloric capacity

20.95 J / (mol K)

Vapor pressure

At a temperature of 84 K it has a pressure of 1 kPa.

Electronegativity

3.0 on the Pauling scale

Ionization energy

First: 1,350.8 kJ / mol.

Second: 2,350.4 kJ / mol.

Third: 3,565 kJ / mol.

Speed ​​of sound

Gas (23 ºC): 220 m / s

Liquid: 1,120 m / s

Thermal conductivity

9.43 · 10 ^-3 W / (m · K)

Order

Diamagnetic

Oxidation number

Krypton, being a noble gas, is not very reactive and does not lose or gain electrons. If it manages to form a solid of defined composition, as it happens with the clathrate Kr ₈ (H ₂ O) ₄₆ or its hydride Kr (H ₂) ₄, then it is said that it participates with a number or oxidation state of 0 (Kr ⁰); that is, its neutral atoms interact with a matrix of molecules.

However, krypton can formally lose electrons if it forms bonds with the most electronegative element of all: fluorine. In KrF ₂ its oxidation number is +2, so the existence of the divalent cation Kr ²⁺ (Kr ²⁺ F ₂ ^-) is assumed.

Reactivity

In 1962 the synthesis of krypton difluoride (KrF ₂) was reported. This compound is a highly volatile, colorless, crystalline solid and decomposes slowly at room temperature; but it is stable at -30 ºC. Krypton Fluoride is a powerful oxidizing and fluorinating agent.

Krypton reacts with fluorine when combined in an electric discharge tube at -183 ° C, forming KrF ₂. The reaction also occurs when krypton and fluorine are irradiated with ultraviolet light at -196 ° C.

KrF ⁺ and Kr ₂ F ₃ ⁺ are compounds formed by the reaction of KrF ₂ with strong fluoride acceptors. Krypton is part of an unstable compound: K (OTeF ₅) ₂, which has a bond between krypton and an oxygen (Kr-O).

A krypton-nitrogen bond is found in the HCΞN-Kr-F cation. Krypton hydrides, KrH ₂, can be grown at pressures greater than 5 GPa.

At the beginning of the 20th century, all these compounds were considered impossible given the zero reactivity that was conceived of this noble gas.

Structure and electronic configuration

Krypton atom

Krypton, being a noble gas, has its entire valence octet; that is, its s and p orbitals are completely filled with electrons, which can be verified in their electronic configuration:

3d ¹⁰ 4s ² 4p ⁶

It is a monatomic gas regardless (to date) of the pressure or temperature conditions that operate on it. Therefore, its three states are defined by the interatomic interactions of its Kr atoms, which can be imagined as marbles.

These Kr atoms, like their congeners (He, Ne, Ar, etc.), are not easy to polarize, since they are relatively small and also have a high electron density; that is, the surface of these marbles are not appreciably deformed to generate an instantaneous dipole that induces another in a neighboring marble.

Interaction interactions

It is for this reason that the only force that holds the Kr atoms together are the London scattering force; but they are very weak in the case of krypton, so low temperatures are required for its atoms to define a liquid or crystal.

However, these temperatures (boiling and melting points, respectively) are higher compared to argon, neon, and helium. This is due to the greater atomic mass of krypton, equivalent to a larger atomic radius and therefore more polarizable.

For example, the boiling point of krypton is around -153 ºC, while those of the noble gases argon (-186 ºC), neon (-246 ºC) and helium (-269 ºC) are lower; that is, its gases need colder temperatures (closer to -273.15 ºC or 0 K) to be able to condense into the liquid phase.

Here we see how the size of their atomic radii is directly related to their interactions. The same happens with their respective melting points, the temperature at which krypton finally crystallizes at -157 ºC.

Krypton crystal

When the temperature drops to -157 ° C, the Kr atoms approach slowly enough to coalesce further and define a white crystal with a face-centered cubic structure (fcc). Thus, there is now a structural order governed by its dispersal forces.

Although there is not much information about it, krypton fcc crystal can undergo crystalline transitions to denser phases if it is subjected to enormous pressures; like the compact hexagonal (hcp), in which the Kr atoms will be more grouped.

Also, without leaving this point aside, Kr atoms can be trapped in ice cages called clathrates. If the temperature is low enough, perhaps there may be mixed krypton-water crystals, with the Kr atoms arranged and surrounded by water molecules.

Where to find and obtaining

Atmosphere

Krypton is diffused throughout the atmosphere, unable to escape the gravitational field of the Earth unlike helium. In the air we breathe, its concentration is around 1 ppm, although this may vary depending on the gaseous emissions; be it volcanic eruptions, geysers, hot springs, or perhaps natural gas deposits.

Because it is poorly soluble in water, its concentration in the hydrosphere is likely to be negligible. The same happens with minerals; few krypton atoms can be trapped within them. Therefore, the only source of this noble gas is air.

Liquefaction and fractional distillation

To obtain it, the air has to go through a liquefaction process, so that all its component gases condense and form a liquid. This liquid is then heated by applying fractional distillation at low temperatures.

Once the oxygen, argon and nitrogen have been distilled, the krypton and xenon remain in the remaining liquid, which is adsorbed on activated carbon or silica gel. This liquid is heated to -153 ºC in order to distill the krypton.

Finally, the collected krypton is purified by passing through hot metallic titanium, which removes gaseous impurities.

If the separation of its isotopes is desired, the gas is made to rise through a glass column where it undergoes thermal diffusion; the lighter isotopes will rise towards the top, while the heavier ones will tend to stay at the bottom. Thus, the ⁸⁴ Kr and ⁸⁶ Kr isotope, for example, collect separately at the bottom.

Krypton can be stored in Pyrex glass bulbs at ambient pressure, or in airtight steel tanks. Before packaging it is subjected to a quality control by spectroscopy, to certify that its spectrum is unique and does not contain lines of other elements.

Nuclear fision

Another method to obtain krypton lies in the nuclear fission of uranium and plutonium, from which a mixture of their radioactive isotopes is also produced.

Isotopes

Krypton occurs in nature as six stable isotopes. These, with their corresponding abundances on Earth, are: ⁷⁸ Kr (0.36%), ⁸⁰ Kr (2.29%), ⁸² Kr (11.59%), ⁸³ Kr (11.50%), ⁸⁴ Kr (56.99%) and ⁸⁶ Kr (17.28%). The ⁷⁸ Kr is a radioactive isotope; but its half-life (t _1/2) is so long (9.2 · 10 ²¹ years) that it is practically considered stable.

This is why its standard atomic mass (atomic weight) is 83.798 u, closer to the 84 u of the ⁸⁴ Kr isotope.

The radioisotope ⁸¹ Kr (t _1/2 = 2.3 · 10 ⁵) is also found in trace amounts, which is produced when ⁸⁰ Kr receives cosmic rays. In addition to the isotopes already mentioned, there are two synthetic radioisotopes: ⁷⁹ Kr (t _1/2 = 35 hours) and ⁸⁵ Kr (t _1/2 = 11 years); the latter is what is produced as a product of the nuclear fission of uranium and plutonium.

Risks

Krypton is a non-toxic element, as it does not react under normal conditions, nor does it represent a fire risk when mixed with strong oxidizing agents. A leak of this gas does not pose any danger; unless you breathe directly, displacing oxygen and causing suffocation.

Kr atoms enter and are expelled from the body without participating in any metabolic reaction. However, they can displace the oxygen that should reach the lungs and be transported through the blood, so the individual may suffer from narcosis or hypoxia, as well as other conditions.

Otherwise, we constantly breathe krypton in every breath of air. Now, with respect to its compounds, the story is different. For example, KrF ₂ is a powerful fluorinating agent; and therefore, it will “give” anions F ^- to any molecule of the biological matrix it encounters, being potentially dangerous.

Possibly a krypton clathrate (trapped in an ice cage) is not considerably dangerous, unless there are certain impurities that do add toxicity.

Applications

The flashes from high-speed cameras are due in part to the excitation of krypton. Source: Mhoistion

Krypton is present in various applications around artifacts or devices designed for lighting. For example, it is part of the "neon lights" of yellowish green colors. Krypton's "legal" lights are white, as their emission spectrum encompasses all colors in the visible spectrum.

The white light of krypton has actually been used for photographs, as they are very intense and fast, being perfect for high-speed camera flashes, or for instantaneous flashes on airport runways.

Likewise, the electric discharge tubes that emanate this white light can be covered with colored papers, giving the effect of displaying lights of many colors without the need to excite using other gases.

It is added to tungsten filament bulbs to increase their useful life, and to argon fluorescent lamps for the same purpose, also reducing their intensity and increasing their costs (as it is more expensive than argon).

When krypton makes up the gaseous fill in incandescent bulbs, it increases its brightness and makes it more bluish.

Lasers

The red lasers seen in light shows are based on the spectral lines of krypton rather than the helium-neon mixture.

On the other hand, powerful ultraviolet radiation lasers can be made with krypton: those of krypton fluoride (KrF). This laser is used for photolithography, medical surgeries, research in the field of nuclear fusion, and micro-machining of solid materials and compounds (modifying their surface through the action of the laser).

Definition of the meter

Between 1960 and 1983, the wavelength of the red-orange spectral line of the ⁸⁶ Kr isotope (multiplied by 1,650,763.73) was used, in order to define the exact length of one meter.

Detection of nuclear weapons

Because the radioisotope ⁸⁵ Kr is one of the products of nuclear activity, where it is detected is an indication that there was the detonation of a nuclear weapon, or that illegal or clandestine activities of said energy are being carried out.

Medicine

Krypton has been used in medicine as an anesthetic, an X-ray absorber, a detector for heart abnormalities, and to cut the retina of the eyes with its lasers in a precise and controlled manner.

Its radioisotopes also have applications in nuclear medicine, to study and scan the flow of air and blood within the lungs, and to obtain nuclear magnetic resonance images of the patient's airways.

References

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