Krypton

83.80

Krypton gas glows with the help of a few thousand volts

Classification:	Krypton is a noble gas and a nonmetal
Atomic weight:	83.80 g/mol
Density @ 20 ^oC:	0.003708 g/cm³
Atomic volume:	38.9 cm³/mol

Krypton's spectrum.

William Ramsay pointing to the periodic table's final column containing the noble (or inert) gases. Ramsay was awarded the Nobel Prize for Chemistry in 1904 for his work in the discovery of the inert gases. Image: Vanity Fair

Discovery of Krypton

Author: Dr. Doug Stewart

William Ramsay and his assistant Morris Travers discovered krypton in 1898. Ramsay had previously discovered helium and argon. He was aware that more new elements must lie in the same group of the periodic table.

Ramsay and Travers considered the gap in the periodic table between helium and argon. They reasoned that a new element (it's now called neon) must exist to fill this gap; they were determined to find it. And find it they did, but only after their search had first revealed another new element: krypton. ⁽¹⁾

Aware of the history of chemistry, Ramsay knew that sometimes one new element can hide another. For example, John Gadolin had discovered yttrium in the mineral we now call gadolinite. Some years later Carl Gustaf Mosander discovered the new elements erbium and terbium in gadolinite; they had been there all along, but Gadolin had not found them. Ramsay wondered about the possibility of finding small amounts of the elusive new element hiding in one of his earlier discoveries, argon.

Ramsay and Travers decided they would extract a large volume of argon from air. They would liquefy the argon, and then distil it into separate fractions to see whether the lightest fractions contained the new gas. ⁽²⁾

With little experience of handling liquid gases, they obtained a liter of liquid air in order to find out how to work with it. In their practice work, they evaporated most of it, leaving just 100 ml. Ramsay knew it was highly improbable the new gas, lighter than argon, could be present in this residual sample, but the exciting idea came to him that a new, heavier, gas might just be present.

With this in mind, Travers removed oxygen and nitrogen from the gas using red-hot copper and magnesium. He placed a sample of the remaining gas in a vacuum tube and applied a high voltage to measure the gas's spectrum.

He found Argon was present, as expected, but also two new brilliant lines, one yellow and one green, which had never been seen before.

Ramsay and Travers measured the ratio of the gas's specific heat at constant pressure to its specific heat at constant volume and found it was 1.66. This value could only be obtained if the new gas consisted of single atoms, proving it could not be a compound. If it was not a compound, it had to be a new element.

Krypton had been discovered. Ramsay chose the element's name from the Greek word 'kryptos' meaning 'hidden.'

For many years after the noble gases were discovered most scientists believed that they would not form compounds. This turned out to be incorrect.

William Ramsay actually did believe compounds would be possible, writing in 1902: "I have had for a long time the idea that krypton and xenon may enter into combinations much more easily than the other gases; but how to bring it about?" ⁽³⁾

It took another 60 years before his question would be answered. (See Interesting Facts below.)

William Ramsay received the Nobel Prize for Chemistry in 1904, "in recognition of his services in the discovery of the inert gaseous elements in air, and his determination of their place in the periodic system." He was responsible for adding an entire new group to the periodic table. Radon was the only noble gas he didn't discover.

Interesting Facts: Did You Know?

Interesting Facts about Krypton: Did You Know?

Between 1960 and 1983, the scientific unit of length, the meter, was defined as 1 650 763.73 wavelengths of krypton-86's orange-red spectral line. (The meter is now defined as the distance travelled by light in a vacuum during a time interval of
1/299 792 458 of a second.)
From its discovery in 1898 until the 1960s most scientists believed it was impossible to make compounds of krypton, or any other noble gas - although see William Ramsay, above. Krypton difluoride was made in 1963. It is a white, crystalline solid, stable at temperatures below -30 ^oC. ⁽⁴⁾
Krypton is not very abundant in our planet's atmosphere: For every krypton atom, there are about 8200 argon atoms, 184 000 oxygen molecules and 685 000 nitrogen molecules.
Krypton-85 in the atmosphere can be used to detect the presence of otherwise secret nuclear weapons research and production facilities. ⁽⁵⁾
Krypton-fluorine lasers produce pulses with 500 times the power of the entire U.S. electrical grid. Not surprisingly, these pulses are of short duration: four billionths of a second. ⁽⁶⁾

States

State (s, l, g): gas
Melting point: 115.9 K (-157.3 ^oC)

Boiling point: 119.4 K (-153.2 ^oC)

Energies

Specific heat capacity: 0.248 J g^-1 K^-1
Heat of fusion: 1.638 kJ mol^-1
1^st ionization energy: 1350.7 kJ mol^-1
3^rd ionization energy: 3565.1 kJ mol^-1

Heat of atomization: 0 kJ mol^-1
Heat of vaporization: 9.029 kJ mol^-1
2^nd ionization energy: 2350.3 kJ mol^-1
Electron affinity: kJ mol^-1

Oxidation & Electrons

Shells: 2,8,18,8
Minimum oxidation number: 0
Min. common oxidation no.: 0
Electronegativity (Pauling Scale): 3

Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁶
Maximum oxidation number: 2
Max. common oxidation no.: 2
Polarizability volume: 2.5 Å³

Appearance & Characteristics

Structure: fcc: face-centered cubic
Hardness: mohs

Krypton Ion Laser

A krypton-ion 568 nm tertiary laser used in protein research. Photo: Center for Cancer Research

Krypton glowing at high voltage in an electrical discharge tube. Photo: Alchemist-hp

Color: Colorless

Harmful effects:
Krypton is considered to be non-toxic.

Characteristics:
Krypton is a colorless, odorless, inert gas.

Although it is extremely unreactive krypton can react with the very reactive gas fluorine. A few compounds of krypton have been prepared, including krypton (II) fluoride and krypton clathrates.

Solid krypton is white and crystalline.

Uses:
Krypton is used in lighting products:

An important use is in high-powered, flashing airport runway lights.

Ionized krypton gas appears whitish - see photo on left - which makes krypton-based bulbs useful as a brilliant white light source in high speed photography.

Krypton is employed alongside other gases to make luminous 'neon light' style signs that glow with a greenish-yellow light.

Krypton is used as a filling gas for energy-saving fluorescent lights and as an inert filling gas in incandescent bulbs.

The relative abundance of krypton versus hydrogen can be used by astronomers to measure how much nucleosynthesis (element formation) has taken place in any region of interstellar space. ⁽⁷⁾

Between 1960 and 1983, an international agreement defined the meter length in terms of the wavelength of light emitted from the krypton isotope, ⁸⁶Kr. (The meter is now defined as the distance traveled by light in vacuum during a time of 1/299,792,458 of a second. The time is measured using a cesium atomic clock.)

Reactions & Compounds

Reaction with air: none
Reaction with 15 M HNO₃: none
Oxide(s): none
Hydride(s): none

Reaction with 6 M HCl: none
Reaction with 6 M NaOH: none
Chloride(s): none

Radius

Atomic radius: 88 pm
Ionic radius (2+ ion): pm
Ionic radius (2- ion): pm

Ionic radius (1+ ion): pm
Ionic radius (3+ ion): pm
Ionic radius (1- ion): pm

Conductivity

Thermal conductivity: 0.01 W m^-1 K^-1

Electrical conductivity: S cm^-1

Abundance & Isotopes

Abundance earth's crust: 100 parts per trillion by weight, 30 parts per trillion by moles
Abundance solar system: parts per million by weight, parts per million by moles
Cost, pure: $33 per 100g
Cost, bulk: $ per 100g

Source: Krypton is obtained commercially by fractional distillation of liquid air.

Isotopes: Krypton has 25 isotopes whose half-lives are known, with mass numbers 71 to 95. Of these, six are stable: ⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³Kr, ⁸⁴Kr and ⁸⁶Kr. The most abundant isotope is ⁸⁴Kr at 57.03%.

References

1. William Ramsay, Nobel Prize Lecture., December 12, 1904.
2. William Ramsay, The Recently Discovered Gases and Their Relation to the Periodic Law., Science, 1898, Vol. IX, p273-280.
3. Leonello Paoloni, The noble gas compounds: the views of William Ramsay and Giuseppe Oddo in 1902., J. Chem. Educ., 1983, 60 (9), p758.
4. D. R. MacKenzie, Krypton Difluoride: Preparation and Handling., Science 20 September 1963, Vol. 141 no. 3586 p1171.
5. BBC Report, New N Korea nuclear plant suspected Sunday, 20 July, 2003.
6. Naval Research Laboratory, Krypton-fluorine lasers.
7. Stefan I. B. Cartledge et al., Interstellar Krypton Abundances: The Detection of Kiloparsec-scale Differences in Galactic Nucleosynthetic History., 2008, The Astrophysical Journal., 687, p1043.

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