What is the atomic mass of radon?

For most purposes, including homework, the answer is 222. In periodic table data, radon's row reads 86, Rn, Radon, 222.01758.

Three terms get mixed up. The mass number counts the protons and neutrons in a nucleus: radon-222 has 86 protons and 136 neutrons, which make 222. The atomic mass is the measured mass of the atom, 222.017576017 for radon-222, very close to its mass number. The atomic weight is the value listed for the element, written [222] for radon.

Thirty-nine radon isotopes are known, and each has its own mass, because isotopes share a proton count but differ in neutrons. Radon-222 is the most common, so the listed mass is its mass. It is also radon's most stable isotope, and typically the most stable form of an element is the most common in nature.

See how many neutrons radon has.

Mass numbers are how scientists tell radon's isotopes apart. Radon-222, with a half-life of about 3.8 days, and radon-220, with a half-life of 56 seconds, are both used to study how gases move underground and in the atmosphere.

Radon is the heaviest of the noble gases with measured values, and by a wide margin: from xenon to radon the atomic mass rises by about 91, more than the whole atomic mass of krypton.

Radon-222 forms in the decay chain that begins with uranium-238 and ends at lead-206, the final, stable atom. Each alpha decay removes two protons and two neutrons, so the mass number drops by four at each alpha step. From uranium-238 to radon-222, the mass number falls by 16, four alpha decays' worth.

A heavy atom makes a dense gas. Radon's density is 0.00973 grams per cubic centimeter, against 0.0012506 for nitrogen and 0.001429 for oxygen, about 7.8 and 6.8 times as dense, and in 1908 Ramsay and Gray found it to be the heaviest known gas. Weight does not keep radon out of a home, though: a home traps radon inside, where it can build up.