How many neutrons does radon have?

An isotope is a form of an element that has the same number of protons but a different number of neutrons in the nucleus, giving it a different atomic mass. Thirty-nine radon isotopes are known, so radon has no single neutron count. Radon-222 is the most common and its most stable, which is why 136 is the usual answer.

Radon-220 and radon-222 both have practical uses: scientists use them to study how gases move underground and in the atmosphere, and radon's behavior in streams and rivers makes it a tracer in groundwater studies.

Radon-222 sits in the decay chain that begins with uranium-238 and ends at lead-206, the final, stable atom; every decay product before it is radioactive. In alpha decay a nucleus loses two protons and two neutrons, so each alpha step takes two from each count.

Radon-222 forms when radium-226 decays through alpha decay. Radium-226, radium's most stable isotope, has a half-life of about 1,600 years and 138 neutrons; losing one alpha particle takes it to radon-222, with 136. Radon is formed as part of three radioactive decay chains that begin with uranium or thorium.

The counts balance: 86 protons plus 2 is 88, and 136 neutrons plus 2 is 138. The same bookkeeping applies one step later, when radon-222 gives off an alpha particle and becomes polonium-218, with 84 protons and 134 neutrons. The rule works for any isotope of any element: the atomic number fixes the protons, and the rest of the mass number is neutrons.

Radon-222 is unstable: it has a half-life of 3.823 days and decays into polonium-218 through alpha decay. The alpha particle it throws off is two neutrons and two protons. Alpha particles pose no direct external threat, but they can pose a serious health threat if inhaled, and since radon is a gas, it is easily inhaled.