86Rn

Radon

Noble gasgasblock p

A radioactive noble gas that seeps out of uranium-bearing rock into basements and is the second leading cause of lung cancer after smoking.

Atomic number86
Mass number (longest-lived isotope)222 g/mol
This element has no stable isotope, so it has no standard atomic weight. The figure above is the mass number of its longest-lived isotope — a convention, not a measurement of a natural mixture.
Group · period18 · 6
Electron configuration[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶

About Radon

Radon-222 is the gaseous daughter of radium-226 in the uranium-238 chain, and being chemically inert it diffuses freely out of granite, shale and phosphate rock instead of staying put. Its 3.8-day half-life is long enough to reach a basement and short enough to matter: the decay products are solid alpha emitters that lodge in lung tissue, and radon is now blamed for roughly 20000 lung cancer deaths a year in the United States alone, second only to smoking. Testing is cheap and mitigation is usually just a fan and a pipe under the slab. Radon is also the densest gas known at 9.7 g/L, about eight times air, and the reason it pools in cellars and mine workings rather than dispersing.