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What is Radon

Radon (pronunciation: RAY-don) is an odorless, radioactive element that belongs to the family of noble gases and is represented by the chemical symbol Rn [1, 2]. It is colorless at room temperatures, but becomes phosphorescent when cooled below its freezing temperature and gradually turns into orange-red when the temperature is further lowered to the boiling point of liquid air [4].

Radon Symbol


There are more than 35 radioactive isotopes of radon, ranging from 195Rn-229Rn, out of which 222Rn is the most stable with a half-life period of 3.823 days and forms 218Po through α-decay [5]. It has four naturally occurring isotopes, including 218Rn, 219Rn, 220Rn, and 222Rn [5].

Where is Radon Found

Radon is always found in nature because it is produced from the radioactive decomposition of radium-226 that is present in phosphate rocks, uranium ores, metamorphic rocks, and common rocks like limestone [1, 3]. A small amount of radon also occurs in the atmosphere [1].

Radon Test Kit


Origin of its Name: It is named for the radioactive metallic element radium because radon was first identified during the radioactive decay of radium [1].

Who discovered it: The German physicist Friedrich Ernst Dorn is known for its discovery [1].

When and How was it Discovered

In 1899, Robert B. Owens and Ernest Rutherford found a radioactive gas emanating from thorium [1]. In that same year, Marie and Pierre Curie detected a gas being released by radium [1]. In 1900, Ernst Dorn discovered a gas while studying the decay chain of radium at the German city of Halle [1].

We now know that the radioactive gas observed by the Curies and Ernst Dorn was Radon-222 while the gas detected by Rutherford was Radon-220 [1]. In 1908, Robert Whytlaw-Gray and William Ramsay collected enough radon and determined the properties of radon at the University College, London [1].

Epa Radon Gas Map

Radon Identification

Atomic number 86 [1]
CAS number 10043-92-2 [1]
Position in the periodic table Group Period Block
  18 [1] 6 [1] p [1]

Location of Radon on the Periodic Table

Properties and Characteristics of Radon

General Properties

Relative atomic mass [222] [1]
Atomic mass [222] amu [1]
Molar mass 210.9906010 ± 0.0000070 g/mol [6]

Physical Properties

Color Colorless [1, 7]
Melting point/freezing point -71 °C, -96 °F [1]
Boiling point -61.7 °C, -79.1 °F [1]
Density 0.009074 g cm-3 [1]
State of matter at room temperature (solid/liquid/gas) Gas [1, 7]
Charge  Unknown [8]
Thermal (heat) conductivity 0.00361 W/(m K) [9]
Flammability Not flammable [10]
Specific heat 94 J kg-1 K-1 [1]
Bulk modulus Unknown [1]
Shear modulus Unknown [1]
Young’s modulus Unknown [1]
Vapor pressure
– Temperature (K) 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
– Pressure (Pa)

Chemical Properties

Oxidation state/Oxidation number +2, (+6)[1]
Isotopes Isotope Mass Abundance (%) Half-life Mode of decay
  211Rn 210.991 14.6 h β+, EC
  220Rn 220.011 55.6 s α
  222Rn 128.905 3.823 d α

Atomic Data of Radon (Element 86)

Valence electrons 8 [11]
Quantum numbers
– n 6 [11]
– ℓ 1 [11]
– m 1 [11]
– ms -1/2 [11]
Electron configuration (noble gas configuration) [Xe] 4f145d106s26p6 [1]
Atomic structure
– Number of electrons 86 [7]
– Number of neutrons 136 [7]
– Number of protons 86 [7]
Radius of Atom
– Atomic radius 2.20 Å [1]
– Covalent radius 1.46 Å [1]
Electronegativity (Pauling-scale) Unknown [1]
Electron affinity Not stable [1]
Ionization energy (kJ mol-1) 1st 2nd 3rd 4th 5th 6th 7th 8th

Radon Electron Configuration (Bohr Model)

What is it Used for

  • Radioactive decomposition of radon is useful for obtaining polonium [1].
  • Radon emitted from a radium source can be used in cancer therapy [1]. It was commonly used in hospitals for the treatment of tumors through implantation of minute tubes in which the gas was sealed [1, 7]. Today, it has been replaced by safer treatment options [1, 7].
  • Radon testing kits are used for analyzing indoor radon levels in places where large concentrations of radon gas can accumulate indoors [1]. If the test result shows the level is 4 pCi/L or more, mitigation systems are used to reduce radon concentrations [12].
  • Researchers use radon soil-concentration to map subsurface geological faults since concentrations generally increase over the faults [13]. Groundwater radon concentrations are also analyzed for earthquake prediction [14].

Radon Toxicity and Health Effects

The decay products of radon have been considered as being carcinogenic [7]. Since it can be inhaled, people exposed to high levels of radon are at risk of developing lung cancer [15]. The chances of getting affected by lung cancer are much higher in smokers who are exposed to radon [15]. It is thought to have had played a role in evolution and could be responsible for the background radiation of the Earth which can cause genetic modifications [1].

Interesting Facts

  • It is the heaviest known chemically inactive gas that can form compounds with other substances only under extreme conditions [3, 4].
  • It is graphically represented by an image of the radiation hazard symbol with background images of houses indicating that the gas can accumulate in houses [1].

Radon Mitigation System

Radon Cost

The price of radon gas is about $4 per m [16].



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