Question

Radium-226 is a common isotope on earth, but it has a half-life of about 1620 years. given that earth is some 5 billion years old, why is there any radium at all?

Answer 1

Radium-226 is a daughter isotope and the result of the uranium radioactive decay.

At each of their respective half-life cycles, Radium-226 and Astatine-218 are converted back and forth by transmutation.