Three stable lead nuclides are the end products of radioactive decay in the three natural decay series: uranium (decays to lead-206), thorium (decays to lead-208), and actinium (decays to lead-207).

What does uranium break down into?

The half-life of uranium-238 is 4.5 billion years. It decays into radium-226, which in turn decays into radon-222. Radon-222 becomes polonium-210, which finally decays into a stable nuclide, lead.

Why does uranium decay stop at lead?

A nucleus of uranium 238 decays by alpha emission to form a daughter nucleus, thorium 234. ... Any such decay chain is only stopped by the formation of a stable nucleus. This occurs at the fourteenth generation of the uranium 238 family, when lead 206 is finally produced.