What is the approximate half-life of radium-226 in years?

The approximate half-life of radium-226 is 1,600 years.

Radium-226 is the longest-lived naturally occurring isotope of radium. Its half-life means that after 1,600 years, half of any original sample will have decayed into other products. After another 1,600 years, half of what remains will decay, leaving one-quarter of the starting amount. Radioactive decay is statistical, so the half-life describes a large population of atoms rather than a predictable lifetime for one atom.

Radium-226 undergoes alpha decay, producing radon-222. It is part of the uranium-238 decay series, a chain of transformations that eventually ends in stable lead-206. Because uranium occurs naturally in rocks and minerals, small quantities of radium-226 can also be found in soil and groundwater.

The other choices represent very different timescales. About 87.7 years is associated with radium-228, while 12.3 years is the half-life of tritium. The billions-of-years figure is closer to the half-life of uranium-238, the parent isotope in radium-226’s decay chain.