Krypton

Krypton is a noble gas whose main claim to fame among the general public is being Superman's home planet, a fictional invention. The real krypton is a colourless, odourless gas that makes up just 1 part per million of the air, glows brilliant white in discharge tubes, and was once used to define the international standard for the metre.

  • Atomic Number3636 protons, 36 electrons
  • Atomic Mass83.80 uAbout 84× heavier than hydrogen
  • State at Room TempGascolourless, odourless gas
  • Density0.003733 g/cm³About 3× heavier than air
  • Melting / Boiling-157.4°C / -153.2°CLiquefies at −153°C
  • Discovered1898Ramsay & Travers, 1898

Krypton in context: comparing the noble gases by mass.

Noble gases increase steadily in mass down Group 18. Krypton sits between argon and xenon.

Atomic Mass Comparison
Argon40 u
Krypton84 u
Xenon131 u
Iron55.8 u
Radon222 u

Krypton (84 u) is more than twice the mass of argon (40 u) and about two thirds the mass of xenon (131 u). Despite being denser than argon, krypton, like all noble gases, is completely unreactive under normal conditions.

What is krypton?

Krypton is a noble gas in Group 18 of the periodic table. It has 36 protons and a complete outer electron shell of eight electrons. Like all noble gases, krypton is essentially non-reactive under normal conditions. It is denser than air and produces a striking white or blue-white glow when an electric current passes through it. Krypton has a small number of known compounds, krypton difluoride (KrF₂) can be made at very low temperatures, but these are exotic and unstable.

Krypton gets its name from the ancient Greek word kryptos, meaning hidden, a reference to how difficult it was to find and isolate. The name was proposed by William Ramsay and Morris Travers, who discovered it in 1898 by carefully evaporating liquefied argon and studying what remained. Comic book writer Jerry Siegel chose Krypton as Superman's home planet in 1938, inspired by the element's name meaning "hidden", appropriate for a planet no one knew existed.

Fact From 1960 to 1983, the international definition of the metre was based on krypton. The metre was defined as 1,650,763.73 wavelengths of the orange-red light emitted by the krypton-86 isotope. This gave an extremely precise and reproducible standard that could be reproduced in any laboratory worldwide, far more reliable than the old platinum-iridium bar kept in Paris.

Where you find krypton

In space and on Earth

Krypton is produced in large stars and is found throughout the universe. On Earth, krypton makes up only approx. 1 part per million of the atmosphere, much rarer than argon (9,300 ppm) or neon (18 ppm). It is entirely obtained by industrial fractional distillation of liquid air.

  • Atmospheric krypton. Air is approx. 0.000114% krypton by volume. A cubic metre of air contains approx. 1.14 millilitres of krypton.
  • Industrial production. Krypton is separated from other atmospheric gases during the industrial production of liquid oxygen and liquid nitrogen in air separation plants.

How we use krypton

  • Photography flashes. High-speed photography uses krypton flash tubes, they can produce a very brief, brilliant flash of light far faster than conventional flash bulbs, freezing rapid motion.
  • Lighting. Krypton-filled light bulbs run hotter and more efficiently than argon-filled ones, giving a slightly brighter light for the same power. Krypton is used in certain fluorescent lamps and projection lights.
  • Laser systems. Krypton fluoride (KrF) excimer lasers produce ultraviolet light used in microchip manufacture and eye surgery (LASIK). The 248 nm ultraviolet beam can engrave features 100 times thinner than a human hair.
  • Scientific measurement. Until 1983, krypton-86 defined the international standard for the metre. The element is still used in precision spectroscopy and atomic physics research.
Did you know? Krypton-85, a radioactive isotope of krypton, leaks from nuclear fuel reprocessing plants. Because it is a noble gas, it spreads globally through the atmosphere. Scientists have used atmospheric krypton-85 levels as a covert way of tracking which countries are reprocessing nuclear fuel, since the krypton spreads globally but its concentration reveals the location of the source.

How it was discovered

Krypton was discovered in 1898 by William Ramsay and Morris Travers in London. After discovering argon (1894) and neon (1898), Ramsay predicted there should be other noble gases. He and Travers evaporated liquid argon carefully and collected the tiny residue that did not evaporate easily. In a discharge tube this residue glowed with a brilliant white-yellow light unlike anything they had seen. They identified it as a new element and named it krypton: the hidden one. Its discovery was announced just six weeks after neon.

Deeper dive: noble gas chemistry and krypton compounds

For most of the 20th century, "noble gas chemistry" was considered an oxymoron, a contradiction in terms. The noble gases were believed to be completely inert. This changed dramatically in 1962 when Neil Bartlett reacted xenon with platinum hexafluoride, producing the first noble gas compound. This inspired attempts with other noble gases. Krypton difluoride (KrF₂) was synthesised in 1963 and remains the only krypton compound stable at room temperature, though it decomposes slowly even then. It is a powerful fluorinating agent but so reactive that it will oxidise even substances that do not normally react with fluorine.

The KrF excimer laser is far more practically important. "Excimer" stands for excited dimer, KrF molecules that exist only in an excited electronic state produced by electrical discharge. When these molecules fall back to their ground state, they emit ultraviolet light at 248 nm. This UV light has a wavelength short enough to engrave microscopic features on silicon chips and to reshape the cornea in LASIK eye surgery, where it removes precisely controlled amounts of corneal tissue to correct vision. The procedure has been performed on more than 40 million people worldwide.

Krypton is the noble gas hiding in the air, invisible but useful. Moving to 37 protons brings us to rubidium, an alkali metal so reactive it spontaneously ignites in air.