The radioactive decay series that leads to radon-222 starts with which element?

The radioactive decay series that leads to radon-222 begins with uranium-238. This series, known as the uranium-238 decay series, involves a sequence of radioactive decays where uranium-238 undergoes a series of transformations through a series of alpha and beta decays, ultimately producing radon-222, among other decay products.

Initially, uranium-238 decays to thorium-234, which then decays to protactinium-234, and eventually leads to radium-226. Radium-226 further decays to radon-222. This chain of decay illustrates how uranium-238 serves as the starting point for the production of radon-222 and is pivotal in understanding radon gas in the context of exposure and health risks associated with radon in homes.

The other options, while involved in different decay chains or series, do not initiate the pathway that produces radon-222. Radium-228 starts a different decay series involving thorium, lead-210 is a product in the decay chain of uranium-238 but not a starting point, and thorium-234 is a descendant in the chain and not the origin. Thus, uranium-238 is the correct answer, illustrating its fundamental role in