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Reference.org
Radon-222
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<p>Radon-222 (222Rn, Rn-222, historically radium emanation or radon) is the most stable <a href="/facts/Isotope/KD9B1y6o">isotope</a> of <a href="/facts/Radon/0V9arWKl">radon</a>, with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of approximately 3.8215(2) days. It is transient in the <a href="/facts/Decay_chain/JvTwPNBn">decay chain</a> of <a href="/facts/Primordial_radionuclide/lFAUqHDv">primordial</a> <a href="/facts/Uranium-238/QhcHm6q8">uranium-238</a> and is the immediate decay product of <a href="/facts/Radium-226/T0SlGtRH">radium-226</a>. Radon-222 was first observed in 1899, and was identified as an isotope of a new <a href="/facts/Chemical_element/0jducgWD">element</a> several years later. In 1957, the name <i>radon</i>, formerly the name of only radon-222, became the name of the element. Owing to its gaseous nature and high radioactivity, radon-222 is one of the leading causes of <a href="/facts/Lung_cancer/LHLdR454">lung cancer</a>. </p>