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Is radon a noble gas?

By Radon411 Editorial TeamUpdated How we research

Short answer

Yes: radon is in group 18, the noble gases, with helium, neon, argon, krypton and xenon, and it is the heaviest and densest of them. It is a naturally occurring, radioactive noble gas that is odorless, colorless and tasteless, and it can build up in homes.

Next step: a test is the only way to know your home's level. How to test your home

Radon is in group 18, the noble gases, with helium, neon, argon, krypton and xenon. It is essentially inert, and it is a naturally occurring, radioactive, noble gas with no color, odor or taste.

Its atomic mass is 222.01758, above xenon's 131.29 and krypton's 83.80. Its electron configuration, [Xe] 6s2 4f14 5d10 6p6, ends in a filled 6s2 6p6 outer shell, the same pattern of eight outer electrons that krypton (4s2 4p6) and xenon (5s2 5p6) show.

Noble does not mean it never reacts. Fluorine has been reported to react with radon, forming a fluoride, and radon fluoride is the only confirmed compound of radon.

Radon is also radioactive. It comes from the natural decay of uranium and radium found in nearly all rocks and soils, its half-life is 3.8 days, and exposure to it is the second leading cause of lung cancer after smoking.

What to do next: read the radon element guide for its chemistry and decay chain, then check your area on the radon map and test your home.

What makes radon a noble gas

Radon is classified as a non-metal and sits in group 18, the noble gases, in period 6. Like the rest of the group it is a colorless gas at ordinary temperatures, it is essentially inert, and its electron configuration ends in a filled outer shell. Its atomic number is 86 and its symbol Rn.

Radon's noble gas credentials
TraitRadon
Group18, the noble gases
State at room temperatureGas
Electron configuration[Xe] 6s2 4f14 5d10 6p6
ReactivityEssentially inert
Color, odor, tasteNone

Radon beside the other noble gases

Down the group, each noble gas is heavier and boils at a higher temperature than the one before it. Radon, the last of them with measured values, is the heaviest and densest.

The noble gases compared
ElementAtomic numberAtomic massBoiling point (K)Density (g/cm3)Discovered
Helium24.002604.220.00017851868
Neon1020.18027.070.00089991898
Argon1839.987.30.00178371894
Krypton3683.80119.930.0037331898
Xenon54131.29165.030.0058871898
Radon86222.01758211.450.009731900

Oganesson, element 118, is also listed in the noble gas block, expected to be a gas, with no measured density or boiling point. Radon was discovered in 1900 by Dorn, and in 1908 Ramsay and Gray found it to be the heaviest known gas.

A narrow band as a liquid

Every noble gas turns liquid only in a narrow band of temperatures just above its melting point, and all of them are gases at room temperature. Radon has the widest band of the group, about 9 kelvin, and the highest melting and boiling points: in Celsius, it melts at minus 71 degrees and boils at minus 61.7 degrees.

Melting and boiling points of the noble gases (liquid range is our arithmetic)
ElementMelting point (K)Boiling point (K)Liquid range (K)
Helium0.954.223.27
Neon24.5627.072.51
Argon83.887.33.5
Krypton115.79119.934.14
Xenon161.36165.033.67
Radon202211.459.45

What sets radon apart: radioactivity

Radon is a naturally occurring, radioactive noble gas. It comes from the natural decay of uranium and radium in nearly all rocks and soils, as part of decay chains that begin with uranium or thorium. Its most stable isotope, radon-222, has a half-life of about 3.8 days and decays into polonium-218.

Decay chain from uranium-238 through uranium-234, thorium-230 and radium-226 to radon-222, radon's decay products and stable lead-206, with radium-226's half-life of 1600 years and radon-222's of 3.823 days.
Read this figure as text

  1. Uranium-238

    Solid

  2. Uranium-234

    Solid

  3. Thorium-230

    Solid

  4. Radium-226

    Solid

    Half-life 1600 years

  5. Radon-222

    Gas

    Half-life 3.823 days

  6. Radon's decay products

  7. Lead-206

    Solid

  • Uranium-238. More than 99 percent of the uranium found in the environment is in the form of U-238. Uranium decays by alpha particles.
  • Uranium-234. Less than one percent of natural uranium, but much more radioactive.
  • Thorium-230. An intermediate EPA names in the uranium-238 chain.
  • Radium-226. Radium decays to produce radon gas.
  • Radon-222. The most common radon isotope, and an alpha emitter. As a gas it can move up through the ground and into a building.
  • Radon's decay products. Radon gas decays into radioactive particles that can get trapped in your lungs when you breathe.
  • Lead-206. Where the chain ends. Only the final, stable atom in the chain is not radioactive.

EPA names uranium-234, thorium-230, radium-226 and radon-222 as intermediates "such as" these, so the chain has more steps than are drawn. Half-lives are shown only where a cited source states one.

Radon in the uranium-238 decay chain, between radium and polonium.

That radioactivity is what makes radon a health matter: exposure to radon is the second leading cause of lung cancer after smoking. See is radon radioactive.

Noble, but not completely unreactive

Noble does not mean radon never reacts. Fluorine has been reported to react with radon, forming a fluoride, and radon fluoride is the only confirmed compound of radon. Otherwise it is essentially inert.

Why being a noble gas matters in a home

Radon forms in the ground, and as a noble gas it does not stay bound there. Radon typically moves up through the ground to the air above and into a home through cracks and other holes in the foundation, entering through openings in floors or walls in contact with the ground. Your home then traps it, and it can build up.

Cross-section drawing of a house with a basement and a crawl space, numbered at the seven ways EPA lists for soil gas to get in: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, the water supply.
Read this figure as text
House in cross-section showing where soil gas entersGround levelSoil and rockBasementSolid floorCrawl spaceSuspended floor1234567
  1. Cracks in solid floors
  2. Construction joints
  3. Cracks in walls
  4. Gaps in suspended floors
  5. Gaps around service pipes
  6. Cavities inside walls
  7. The water supply
The routes radon takes from the soil into a house.

Questions about radon as a noble gas

Is radon a noble gas?

Yes. It is in group 18, the noble gases, and it is a naturally occurring, radioactive noble gas.

Is radon the heaviest noble gas?

Of the noble gases with measured values, yes: its atomic mass is 222.01758 and its density 0.00973 grams per cubic centimeter, both above xenon's.

Is radon a gas at room temperature?

Yes. It is a colorless gas at ordinary temperatures, and it boils at 211.45 K, far below room temperature.

Does radon react with anything?

Rarely. Fluorine has been reported to react with it, and radon fluoride is its only confirmed compound.

Is radon an inert gas?

Essentially, yes. It is described as essentially inert and occupies the last place in the zero group of gases.

Where does radon come from?

From the natural decay of uranium and radium found in nearly all rocks and soils.

When was radon discovered?

In 1900, by Dorn, who called it radium emanation. See when was radon discovered.

Learn

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