Atomic number 36

Krypton

Kr
+ phase
− phase

Shells Orbitals 3d 4s 4p

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36 electrons in 4 shells: 2, 8, 18, 8.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Noble gas
Period: 4
Group: 18
Block: p
Standard atomic weight: 83.798(2) Da
Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁶
Phase at 25 °C: gas
Melting point: −157.37 °C
Boiling point: −153.22 °C
Density: 3.733 g/L
Electronegativity (Pauling): 3.0
Discovered: 30 May 1898, William Ramsay and Morris Travers, in liquefied air

Neighbours in the table

Br Rb

Gram quantities of krypton difluoride, KrF₂, have been made by several methods, Los Alamos notes. Jefferson Lab, quoted in PubChem’s element record, calls it the easiest krypton compound to make. OpenStax adds that it is unstable to heat even at room temperature. Los Alamos also reports a second, higher fluoride and a salt derived from a krypton oxyacid.

Compounds like these were once thought impossible. The group 18 gases were called noble because chemists took them to be unreactive. In 1962 Neil Bartlett proved that assumption false, OpenStax records, and Jefferson Lab now describes krypton as forming a few compounds.

Over four shells, krypton’s electrons are spaced 2, 8, 18 and 8, written [Ar] 3d¹⁰ 4s² 4p⁶. OpenStax counts a completely filled d subshell as core rather than valence, leaving the eight 4s and 4p electrons as the outer set. An added electron would have to enter a higher shell, a harder step according to OpenStax. PubChem’s record gives two values for the energy released when a krypton atom gains an electron, 0 eV and −0.42 eV. Only weak London dispersion forces hold noble-gas atoms together, OpenStax explains, so krypton melts at −157.37 °C and boils at −153.22 °C.

Where it is found

Los Alamos puts krypton at about 1 part per million of the atmosphere, and Jefferson Lab at 0.0001 per cent. Los Alamos also gives 0.3 parts per million for the atmosphere of Mars. Supply comes from the fractional distillation of liquid air, which separates a liquid mixture by the boiling points of its parts.

The element was first found in the same raw material. On 30 May 1898, Jefferson Lab records, William Ramsay and Morris Travers were studying liquefied air when a little liquid krypton stayed behind after the more volatile parts had evaporated. Neon followed shortly afterwards, then xenon on 12 July.

Jefferson Lab counts five stable isotopes in natural krypton. Krypton-84 makes up about 57 per cent of the atoms. IAEA nuclear data in PubChem’s record list krypton-78 as stable, with a half-life above 110 exayears (1.1 × 10²⁰ years) and a possible double beta decay. Radioactive krypton-85 has two origins, IUPAC notes: fission in nuclear reactors, and cosmic-ray neutrons striking krypton-84 in the air.

Why it matters

For 23 years the metre was a krypton measurement. In 1960, Los Alamos records, nations agreed to define the unit of length by one orange-red line in the spectrum of krypton-86. That replaced the standard metre kept in Paris, a bar of platinum-iridium alloy. In October 1983 the International Bureau of Weights and Measures redefined the metre as the path light covers in a vacuum in 1/299,792,458 of a second.

Los Alamos characterises krypton by the brilliant green lines in its spectrum. OpenStax describes krypton-xenon flash tubes for high-speed photographs, whose flash is intense and lasts 1/50,000 of a second. Some fluorescent bulbs contain a krypton-argon mixture, and signs using krypton blended with other gases glow greenish yellow, Jefferson Lab notes. The cost of separating it from air has kept such uses limited.

Its radioactive isotopes date water. Krypton-81 is inert and lingers in the atmosphere, so IUPAC expects its supply from the air to stay steady, and it has dated groundwater more than a million years old. IAEA data give it a half-life of 229,000 years. Krypton-85 has a half-life of 10.728 years in the IAEA data. It enters water by equilibrating with air, so its activity shows how many decades have passed since the water was sealed off.

Properties

Properties of krypton Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	36
Standard atomic weight	83.798(2) Da	Da	IUPAC/CIAAW 2024
Melting point	−157.37 °C	°C	Los Alamos: −157.37 °C
Boiling point	−153.22 °C	°C	PubChem: 119.93 K
Density	3.733 g/L	g/L
Electronegativity (Pauling)	3.0
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶
Phase at 25 °C	gas

Safety

In everyday life krypton poses essentially no hazard; CAMEO, a US chemical-hazards database quoted in PubChem, lists the noble gases as nontoxic and nonflammable. The GHS (Globally Harmonized System) classification, signal word Warning, concerns only the container: gas under pressure may explode if heated, and the refrigerated liquid can cause cold burns. The gas is heavier than air, so a large release can gather in low or confined spaces and leave too little oxygen there. The PubChem Laboratory Chemical Safety Summary for krypton is the sheet for anyone storing or moving it.

Questions

What does the name krypton mean?

The name comes from the Greek kryptos, meaning hidden or concealed, as Los Alamos and CIAAW both explain. Krypton is scarce: Los Alamos puts it at about one part per million of air. William Ramsay and Morris Travers found it in 1898 in what remained once liquid air had almost all boiled off. It is element 36, a noble gas in period 4.

What is krypton-85 used for?

Krypton-85 is a radioactive isotope with several jobs, according to IUPAC's notes in PubChem's record. Sealed inside a container, it reveals leaks through the radioactivity that escapes; because the gas is inert, it reacts with nothing inside. Mixed with phosphors it makes materials glow in the dark, since its radiation makes the phosphor give off light. Inhaled by a patient, it lets doctors study blood flow.

Is krypton radioactive?

Most natural krypton is not: Jefferson Lab counts five stable isotopes. Two radioactive isotopes, krypton-81 and krypton-85, form in the air through cosmic rays, and nuclear reactors release krypton-85 as well. OpenStax notes that krypton and xenon escaping from reactors spread readily through the atmosphere, and as noble gases they are not taken up by plants or animals in the food chain.

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