What is Radon?

Radon gas, like carbon-14 gas, is completely natural. It forms during the decay of uranium-238, an element with a fairly interesting decay sequence (to learn more about decay sequences and radioactivity in general, see How Nuclear Radiation Works):

  1. Start with a uranium-238 atom. This atom has 92 protons and 146 neutrons. It has a half-life of 4.5 billion years. When it decays it emits an alpha particle, leaving behind a thorium-234 atom.
  2. A thorium-234 atom has 90 protons and 144 neutrons. It has a half-life of 24.5 days. When it decays it emits a beta particle and a gamma ray, leaving behind a protactinium-234 atom.
  3. A protactinium-234 atom has 91 protons and 143 neutrons. It has a half-life of 269,000 years. When it decays it emits a beta particle and a gamma ray, leaving behind a thorium-230 atom.
  4. A thorium-230 atom has 90 protons and 140 neutrons. It has a half-life of 83,000 years. When it decays it emits an alpha particle and a gamma ray, leaving behind a radium-226 atom.
  5. A radium-226 atom has 88 protons and 138 neutrons. It has a half-life of 1,590 years. When it decays it emits an alpha particle and a gamma ray, leaving behind a radon-222 atom.

That radon atom is a gas atom, and it has a half-life of only 3.825 days. Accumulations of radon atoms from the natural nuclear decay of uranium-238 is where radon gas comes from. That means that radon gas concentrations are higher where uranium is plentiful in the soil. For completeness, here is the rest of the sequence:

  1. radon-222, with a half-life of 3.825 days, emits an alpha particle to become polonium-218.
  2. polonium-218, with a half-life of 3.05 minutes, emits an alpha particle to become lead-214.
  3. lead-214, with a half-life of 26.8 minutes, emits a beta particle and a gamma ray to become bismuth-214.
  4. bismuth-214, with a half-life of 19.7 minutes, emits either an alpha particle or a beta particle and a gamma ray to become either thallium-210 or polonium-214.
  5. polonium-214, with a half-life of a 150 microseconds, emits an alpha particle to become thallium-210.
  6. thallium-210, with a half-life of 1.32 minutes, emits a beta particle to become lead-210.
  7. lead-210, with a half-life of 22 years, emits a beta particle and a gamma ray to become bismuth-210.
  8. bismuth-210, with a half-life of five days, emits a beta particle to become polonium-210.
  9. polonium-210, with a half-life of 138 days, emits an alpha particle and a gamma ray to become lead-206.
  10. lead-206 is a stable isotope of lead.