Which element decays to produce Radon-222?

Radon-222 is a radioactive noble gas that is primarily produced through the decay of Radium-226. This decay process occurs as part of the uranium-238 decay series, where uranium-238 ultimately transforms through various radioactive elements, including radium-226, before resulting in the emission of radon-222 as a decay product.

Radium-226 itself is an alpha-emitting radionuclide that naturally occurs in uranium ore. When radium-226 undergoes alpha decay, it transforms into Radon-222, which then can accumulate in the soil, rock, and eventually in buildings. The significant aspect of this decay pathway is that it directly illustrates the connection between uranium, radium, and radon, reinforcing the importance of understanding these relationships in radon measurement and indoor air quality evaluations.

The other options, while they may be part of other decay series or have their own decay products, do not directly lead to the production of Radon-222 through their decay processes. Uranium-238 can eventually produce radon, but it first does so by decaying into radium-226, which is the immediate predecessor to radon-222. Therefore, Radium-226 is the correct answer as it directly decays to