A decay chain is a series of radioactive decays that occur when an unstable atomic nucleus transforms into a more stable one through a sequence of intermediate isotopes. Each step in the chain involves the emission of radiation, such as alpha or beta particles, and can result in the formation of new elements. For example, the decay chain of uranium-238 begins with its decay into thorium-234, which then decays into protactinium-234, and continues until it eventually becomes a stable isotope, lead-206. Understanding decay chains is important in fields like nuclear physics and radiometric dating.