In by Mark Goodman

A naturally-occurring radioactive gas found in soil that is heavier than air. Radon gas exposure in abnormally high levels is associated with lung cancer. Mitigation measures may involve crawlspace and basement venting and installation of various forms of vapor barriers and fans.

Merriam-Webster Online Dictionary
radon (noun)
a heavy radioactive gaseous element formed by the decay of radium - see element table
radon (Wikipedia)
Radon,  86Rn
General properties
Pronunciation /ˈrdɒn/ (RAY-don)
Appearance colorless gas
Mass number 222 (most stable isotope)
Radon in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Atomic number (Z) 86
Group group 18 (noble gases)
Period period 6
Element category   noble gas
Block p-block
Electron configuration [Xe] 4f14 5d10 6s2 6p6
Electrons per shell
2, 8, 18, 32, 18, 8
Physical properties
Phase at STP gas
Melting point 202 K ​(−71 °C, ​−96 °F)
Boiling point 211.5 K ​(−61.7 °C, ​−79.1 °F)
Density (at STP) 9.73 g/L
when liquid (at b.p.) 4.4 g/cm3
Critical point 377 K, 6.28 MPa
Heat of fusion 3.247 kJ/mol
Heat of vaporization 18.10 kJ/mol
Molar heat capacity 5R/2 = 20.786 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 110 121 134 152 176 211
Atomic properties
Oxidation states 6, 2, 0
Electronegativity Pauling scale: 2.2
Ionization energies
  • 1st: 1037 kJ/mol
Covalent radius 150 pm
Van der Waals radius 220 pm
Color lines in a spectral range
Crystal structureface-centered cubic (fcc)
Face-centered cubic crystal structure for radon
Thermal conductivity 3.61×103  W/(m·K)
Magnetic ordering non-magnetic
CAS Number 10043-92-2
Discovery Ernest Rutherford and Robert B. Owens (1899)
First isolation William Ramsay and Robert Whytlaw-Gray (1910)
Main isotopes of radon
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
210Rn syn 2.4 h α 206Po
211Rn syn 14.6 h ε 211At
α 207Po
222Rn trace 3.8235 d α 218Po
224Rn syn 1.8 h β 224Fr
| references | in Wikidata

Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tastelessnoble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through which thorium and uranium slowly decay into lead and various other short-lived radioactive elements; radon itself is the immediate decay product of radium. Its most stable isotope, 222Rn, has a half-life of only 3.8 days, making radon one of the rarest elements since it decays away so quickly. However, since thorium and uranium are two of the most common radioactive elements on Earth, and they have three isotopes with very long half-lives, on the order of several billions of years, radon will be present on Earth long into the future in spite of its short half-life as it is continually being generated. The decay of radon produces many other short-lived nuclides known as radon daughters, ending at stable isotopes of lead.

Unlike all the other intermediate elements in the aforementioned decay chains, radon is, under normal conditions, gaseous and easily inhaled. Radon gas is considered a health hazard. It is often the single largest contributor to an individual's background radiation dose, but due to local differences in geology, the level of the radon-gas hazard differs from location to location. Despite its short lifetime, radon gas from natural sources, such as uranium-containing minerals, can accumulate in buildings, especially, due to its high density, in low areas such as basements and crawl spaces. Radon can also occur in ground water – for example, in some spring waters and hot springs.

Epidemiological studies have shown a clear link between breathing high concentrations of radon and incidence of lung cancer. Radon is a contaminant that affects indoor air quality worldwide. According to the United States Environmental Protection Agency, radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the United States. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA estimates. As radon itself decays, it produces decay products, which are other radioactive elements called radon daughters (also known as radon progeny). Unlike the gaseous radon itself, radon daughters are solids and stick to surfaces, such as dust particles in the air. If such contaminated dust is inhaled, these particles can also cause lung cancer.

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