36 Kr Krypton 83.798
Noble gas p-block Period 4 Group 18

Krypton

Kr · Element 36 · Noble gas

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Named "hidden", used to define the metre, and briefly the fictional weakness of a superhero.

State at 20°C Gas
Atomic mass 83.798 u
Electron configuration[Ar] 3d¹⁰ 4s² 4p⁶
35Br All 118 37Rb

Structure

The krypton atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where krypton sits among all 118 elements for that property.

Physical

Density 0.004 g/cm³ 7%
Melting point 115.8 K 6%
Boiling point 119.9 K 7%
Specific heat 0.248 J/g·K
Thermal conductivity 0.00943 W/m·K 3%

Atomic

Atomic radius 116 pm 12%
Covalent radius 116 pm
Van der Waals radius 202 pm

Electronic

Electronegativity 3 96%
Ionisation energy 1350.8 kJ/mol 96%
Electron affinity —

Occurrence

Abundance in crust 1.00e-4 mg/kg 30%

Identity

SymbolKr
Atomic number36
Atomic mass83.798 u
CategoryNoble gas
Blockp
Crystal structureface-centered cubic
Oxidation states0, +2
Discovered1898
Discovered byRamsay & Travers

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Same element, different neutrons

Isotopes of krypton

All krypton atoms have the same number of protons, 36, and differ only in their number of neutrons: those variants are its isotopes. The most common, Kr-84, has 48 neutrons and makes up 56.987% of natural krypton.

Isotopes of krypton found in nature, with their share of the natural element, and well-known radioactive ones
ISOTOPE Neutrons Natural Abundance Half-life Decay Mode
Kr-78 42 0.355% Stable —
Kr-80 44 2.286% Stable —
Kr-82 46 11.593% Stable —
Kr-83 47 11.500% Stable —
Kr-84 most common 48 56.987% Stable —
Kr-85 49 — 10.739 yrs β⁻
Kr-86 50 17.279% Stable —

Here the classroom shortcut, rounded atomic mass (83.798) minus atomic number, gives the same answer: 48.

Decay modes

β⁻
beta-minus decay

Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.

Size, to scale

How big is a krypton atom?

Radius 116 pm — that is 0.116 nm, so about 4.31 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Krypton is liquid over a 4 K window, from 116 K to 120 K.

Where it sits

Position in the table

Krypton sits in period 4, group 18. Everything in group 18 shares the same outer-electron count, which is why they behave so similarly.

Other noble gases

2He 10Ne 18Ar 54Xe 86Rn 118Og
All noble gases

The story

What krypton is, and how we found it

Krypton (Kr) is element 36, a colorless, odorless noble gas in group 18, between argon and xenon. It makes up about one part per million of the air. Krypton condenses to a liquid at −153.22 °C (119.93 K) and freezes at −157.36 °C (115.79 K). As a gas its density is 3.733 g/L, nearly three times that of air.

Although it is a noble gas, krypton does form a few compounds: krypton difluoride, KrF₂, first made in 1963, is a powerful fluorinating agent.

Atomic structure and isotopes

Krypton has 36 electrons in four shells, arranged 2, 8, 18, 8, with its outer 4s and 4p subshells full (configuration [Ar] 3d¹⁰ 4s² 4p⁶).

Natural krypton is a mixture of six isotopes, the most abundant being krypton-84, at about 57%. Radioactive krypton-85 (half-life 10.7 years) is released by nuclear fuel reprocessing, and krypton-81, produced by cosmic rays, is used to date groundwater and polar ice hundreds of thousands of years old.

The discovery of krypton

Searching the air (1898)

After argon was found in 1894 and helium in 1895, William Ramsay suspected that more unknown gases were hidden in the air. On 30 May 1898, he and his assistant Morris Travers, at University College London, slowly evaporated a sample of liquefied air and examined the last gas that remained.

A hidden element

Its spectrum showed bright yellow and green lines that belonged to no known element. They named it krypton, from the Greek kryptos, meaning hidden. Within weeks they found neon and xenon in the same way.

Krypton and the meter

In 1960 the General Conference on Weights and Measures redefined the meter using light from krypton-86, replacing the platinum-iridium prototype bar. In 1983 the krypton definition was replaced by one based on the speed of light.

Applications

What krypton is used for

Lighting

  • Lamps: krypton is used in some fluorescent tubes and in premium incandescent and halogen bulbs, where its heavy atoms slow the evaporation of the tungsten filament.
  • Flash lamps: krypton gives an intense white light in photographic flashes and in lamps that pump some lasers.

Insulating windows

Krypton fills the gap between the panes of some high-performance double and triple glazing. It conducts heat less than argon, so a thinner gap gives the same insulation.

Lasers, space and science

  • Excimer lasers: krypton fluoride lasers emit ultraviolet light at 248 nm, used in chip-making lithography and in fusion research.
  • Electric propulsion: some satellite thrusters, including those of SpaceX's first-generation Starlink satellites, use krypton as a cheaper propellant than xenon.
  • Length standard: from 1960 to 1983 the meter was defined as 1,650,763.73 wavelengths of an orange-red spectral line of krypton-86.

In everyday life

You are most likely to meet krypton in energy-efficient windows filled with krypton gas, in some halogen and specialty light bulbs, and in camera flashes. The name is also familiar from kryptonite, the fictional mineral of the Superman stories, which has nothing to do with the real element.

Where it comes from

Natural occurrence

1.00e-4 mg/kg of Earth's crust · more abundant than 30% of elements

In the air

Krypton is one of the rarer gases of the atmosphere, at about 1.1 parts per million by volume. It is not found in useful amounts in rocks or minerals, so air is its only commercial source.

Production

Krypton is a byproduct of air separation plants that liquefy air to make oxygen and nitrogen. A krypton and xenon mixture is collected from the liquid oxygen and then separated by distillation. Because only large plants recover it, supply is limited and prices can change sharply.

In space

Krypton is made in stars, mostly by neutron capture. It is present in the atmosphere of Mars and in meteorites, where its isotope ratios record the history of the solar system.

Handling

Safety

Health hazards

Krypton is not toxic, but in an enclosed space it can displace oxygen and cause suffocation without warning, because it has no smell. At high pressures it has an anesthetic, narcotic effect.

Cold and pressure

Liquid krypton causes cold burns. Gas cylinders are under high pressure and must be secured and kept away from heat.

Radioactive krypton

Krypton-85, present in traces from nuclear activities and used in some leak detectors and lamp starters, is a weak radiation source that is controlled under radiation safety rules.

Keep exploring

Use krypton in the tools

Quick answers

Krypton: common questions

What is krypton?

Krypton (symbol Kr) is element 36 on the periodic table, a noble gas in period 4, group 18. Named "hidden", used to define the metre, and briefly the fictional weakness of a superhero. At room temperature it is a gas.

What is the electron configuration of krypton?

Krypton's ground-state electron configuration is [Ar] 3d¹⁰ 4s² 4p⁶, giving 4 occupied shells with the distribution 2, 8, 18, 8. Its outer shell holds 8 electrons, which is what sets its bonding behaviour.

What are the melting and boiling points of krypton?

Krypton melts at 115.8 K (-157.4 °C) and boils at 119.9 K (-153.2 °C).

What is the atomic mass of krypton?

The standard atomic weight of krypton is 83.798 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

How many neutrons does krypton have?

The most common krypton atom, Kr-84, has 48 neutrons: its mass number, 84, minus its atomic number, 36. It makes up 56.987% of natural krypton. The classroom shortcut, rounded atomic mass (83.798) minus atomic number, gives the same 48.

How dense is krypton?

Krypton is a gas at room temperature; at 0 °C and 1 atm its density is about 3.733 g/L, so it is heavier than air.

What is the electronegativity of krypton?

Krypton has a Pauling electronegativity of 3. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this high means it tends to pull electrons away from almost anything it bonds with.

Who discovered krypton, and when?

Krypton was discovered in 1898 by Ramsay & Travers. Origin of the name: Greek kryptos, "hidden".

How common is krypton on Earth?

Krypton makes up about 1.00e-4 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.

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