Krypton (Kr)

Krypton (symbol Kr) is the chemical element with atomic number 36: a noble gas, with an atomic mass of 83.798 u. Discovered in 1898 (William Ramsay and Morris Travers).

Krypton is a noble gas, colorless and odorless, making up about one part per million of the air. It insulates our double glazing, lights some camera flashes and now propels thousands of satellites. From 1960 to 1983, the meter was defined by the light it emits.

Key facts

Atomic number: 36
Symbol: Kr
French name: Krypton
Atomic mass: 83.798 u
Molar mass: 83.798 g/mol
Family: Noble gases
Position: period 4, group 18, p-block
State at 0 °C: Gas
Melting point: -157.4 °C
Boiling point: -153.2 °C
Density: 0.00375 g/cm³
Electronegativity (Pauling): 3
First ionization energy: 1,351 kJ/mol
Atomic radius: 88 pm
Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁶
Abundance in Earth's crust: 0.0001 ppm (82nd most abundant element)
Indicative price: 290 $/kg (order of magnitude)
Radioactive: no
Discovery: 1898, William Ramsay and Morris Travers

Origin of the name

From the Greek kryptos, hidden.

What does krypton look like?

A colorless, odorless gas. Excited by an electric discharge, it glows a violet-white; frozen at around -157 °C, it is a white solid.

Where is krypton found?

The atmosphere contains about 1.1 ppm (one part per million by volume). Radioactive krypton-85 is also released by the reprocessing of nuclear fuel.

Natural sources

Air (traces)

How is krypton produced?

It is recovered by distilling liquid air, as a by-product of the big plants that make oxygen and nitrogen. Huge amounts of air must be processed to get a little, which makes it expensive.

What is krypton used for?

Insulating fill for double and triple glazing.
High-performance lamps and photographic flashes.
Krypton fluoride excimer lasers for etching chips.
Ion propulsion for satellites (such as Starlink's).

In everyday life

Triple-glazed window
Professional camera flash
Some high-end headlamps and flashlights

Role in living things

No biological role. Like the other noble gases, it is chemically inert in the body.

Hazards of krypton

Non-toxic, but it can cause asphyxiation by displacing oxygen in a confined space. At high pressure it has a narcotic effect, like xenon.

Cosmic origin

Supernovae: Forged in the cores of massive stars, then scattered when they explode.
Dying stars: Made slowly in red giants (stars like the Sun at the end of their lives).

History of krypton

In 1898, British chemists William Ramsay and Morris Travers distilled liquefied air and found, in the residue, an unknown gas with a yellow and green spectrum. They named it krypton, from the Greek kryptos, “hidden.” They would find neon and xenon in the following weeks.

Did you know?

Between 1960 and 1983, the meter was officially defined as 1,650,763.73 wavelengths of an orange line of krypton-86.
Contrary to popular belief, krypton does form compounds: krypton difluoride (KrF₂) was made in 1963.
Superman's home planet is named after it, but kryptonite exists only in comic books.
Krypton-85 in the atmosphere can reveal clandestine reprocessing of nuclear fuel.

Isotopes of krypton

Five stable isotopes, plus krypton-78, with a vanishingly faint radioactivity. Krypton-85, released by nuclear fuel reprocessing, now turns up everywhere in the atmosphere.

krypton-78: 0.355% of natural krypton, radioactive, half-life 9.2 × 10²¹ years
krypton-80: 2.286% of natural krypton, stable
krypton-81: radioactive, half-life 229,000 years. Made by cosmic rays, it dates very old groundwater aquifers.
krypton-82: 11.593% of natural krypton, stable
krypton-83: 11.5% of natural krypton, stable
krypton-84: 56.987% of natural krypton, stable
krypton-85: radioactive, half-life 10.7 years. Its presence in the air gives away nuclear fuel reprocessing.
krypton-86: 17.279% of natural krypton, stable

Related elements

Same family (noble gases): Helium (He), Neon (Ne), Argon (Ar), Xenon (Xe), Radon (Rn).

Same column (group 18): Helium (He), Neon (Ne), Argon (Ar), Xenon (Xe), Radon (Rn), Oganesson (Og).