radon

[rey-don]
noun Chemistry.
a chemically inert, radioactive gaseous element produced by the decay of radium: emissions produced by outgassing of rock, brick, etc. are a health hazard. Symbol: Rn; atomic number: 86; atomic weight: 222.

Origin:
1915–20; rad(ium) + -on2

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World English Dictionary
radon (ˈreɪdɒn)
 
n
a colourless radioactive element of the rare gas group, the most stable isotope of which, radon-222, is a decay product of radium. It is used as an alpha particle source in radiotherapy. Symbol: Rn; atomic no: 86; half-life of 222Rn: 3.82 days; valency: 0; density: 9.73 kg/m³; melting pt: --71°C; boiling pt: --61.7°C
 
[C20: from radium + -on]

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Dictionary.com's 21st Century Lexicon
Main Entry:  radon
Part of Speech:  n
Definition:  See radon gas
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Etymonline
Word Origin & History

radon
heaviest gaseous element, 1918, from Ger. Radon (C. Schmidt, 1918), from radium (q.v.) + -on suffix of inert gases (neon, argon, etc.). The element is formed by radioactive decay of radium.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

radon ra·don (rā'dŏn)
n.
Symbol Rn
A radioactive, largely inert gaseous element formed by the radioactive decay of radium and used as a radiation source in radiotherapy and research; its most stable isotope is Rn 222 with a half-life of 3.82 days. Atomic number 86; melting point -71°C; boiling point -61.7°C; specific gravity (solid) 4.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
radon   (rā'dŏn)  Pronunciation Key 
Symbol Rn
A colorless, odorless, radioactive element in the noble gas group. It is produced by the radioactive decay of radium and occurs in minute amounts in soil, rocks, and the air near the ground. Radon is used as a source of radiation for the treatment of cancer and other diseases. Its most stable isotope is Rn 222 with a half-life of 3.82 days. Atomic number 86; melting point -71°C; boiling point -61.8°C; specific gravity (solid) 4. See Periodic Table.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary
radon [(ray-don)]

A colorless, odorless, radioactive gas that is produced by the decay of radium in the soil.

Note: Radon seeping through the ground and into buildings is a major source of indoor air pollution and may represent a significant risk for lung cancer.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Exposure to radon is the second leading cause of lung cancer after smoking.
Prolonged exposure to elevated levels of radon can be harmful when found in
  homes.
Radon gas is a naturally occurring radioactive gas that is invisible and
  odorless.
Radon is a radioactive gas that comes from the natural decay of uranium that is
  found in nearly all soils.
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