What is the half-life of Uranium-235?

The story behind the answer

The half-life of Uranium-235 is about 703.8 million years, meaning half of a sample decays to thorium-231 in that time. It is not stable, and the much longer multi-billion-year figure belongs to Uranium-238, not U-235. Because U-235 decays faster, it’s rarer in nature (about 0.72% of natural uranium).

U-235 is fissile—its nuclei can split with slow neutrons—making it crucial for nuclear reactors and weapons. A handy memory cue: 235 is “smaller” than 238, so its half-life is shorter—roughly 0.7 billion years. Another: “Two-three-five stays alive for three-quarters of a billion.”