When was radon discovered?

Radon has had three names, each telling part of its story. Dorn's name tied the gas to its source, Ramsay and Gray's name meant shining, and the modern name points back to radium.

Radon was discovered by studying radium because radium makes it. Radium's most stable isotope, radium-226, has a half-life of about 1,600 years and decays into radon-222 through alpha decay; one gram of radium produces about 0.0001 milliliter of radon gas per day. Today, radon is still primarily obtained through the decay of radium. The link runs further back: uranium eventually decays to radium, and radium decays to release a radioactive gas called radon, which is why radon comes from the natural decay of uranium and radium found in nearly all rocks and soils.

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1. 238U

   Uranium-238

   Solid

2. 234U

   Uranium-234

   Solid

3. 230Th

   Thorium-230

   Solid

4. 226Ra

   Radium-226

   Solid

   Half-life 1600 years

5. 222Rn

   Radon-222

   Gas

   Half-life 3.823 days

6. ···

   Radon's decay products

7. 206Pb

   Lead-206

   Solid

• Uranium-238. More than 99 percent of the uranium found in the environment is in the form of U-238. Uranium decays by alpha particles.
• Uranium-234. Less than one percent of natural uranium, but much more radioactive.
• Thorium-230. An intermediate EPA names in the uranium-238 chain.
• Radium-226. Radium decays to produce radon gas.
• Radon-222. The most common radon isotope, and an alpha emitter. As a gas it can move up through the ground and into a building.
• Radon's decay products. Radon gas decays into radioactive particles that can get trapped in your lungs when you breathe.
• Lead-206. Where the chain ends. Only the final, stable atom in the chain is not radioactive.

EPA names uranium-234, thorium-230, radium-226 and radon-222 as intermediates "such as" these, so the chain has more steps than are drawn. Half-lives are shown only where a cited source states one.

When Ramsay and Gray isolated radon in 1908, they determined its density and found it to be the heaviest known gas. Today's figures bear that out: radon's density is 0.00973 grams per cubic centimeter, against xenon's 0.005887, and it turns liquid only between 202 K and 211.45 K.

Soon after their discovery, radium and radon were put to work in medicine. Radon was purged from radium and sealed in minute tubes used in the treatment of cancer and other diseases. That use faded: other radioisotopes, such as cobalt-60, are now used in place of radium, and hospitals can get radon seeds directly from suppliers.

Radon was the last of these noble gases to be discovered, after a burst of discoveries in the 1890s.

For a time radon was a medical tool: it was pumped from a radium source and sealed in tiny tubes, called seeds or needles, for treating patients, a practice since largely discontinued. Its hazards came into view later. An increased rate of lung cancer was first seen in uranium miners exposed to very high concentrations, and radon build-up in homes has become a concern.