Portrait of Antoine Henri Becquerel

Adapted from ⁠an image by Paul Nadar / Adam Cuerden (public domain), via Wikimedia Commons.

1852–1908

Antoine Henri Becquerel

Discovered radioactivity in 1896, and shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie.

Element

92UUranium
1896

Henri Becquerel developed photographic plates that had been kept in a dark drawer under uranium salts, and found the salts' outline on them. Uranium gave off rays of its own: radioactivity.

Explore uranium

Becquerel discovered that uranium gives off rays by itself. Watch uranium atoms decay, and how slowly.

Radioactive decay

Watching atoms decay

Uranium-238 halves so slowly that about half of what the Earth formed with is still here. Every atom that does decay sets off a relay of unstable atoms that only stops when it reaches lead.

Half of any sample of U-238 decays every 4.47 billion years, turning into Th-234.

U-238 · 100 leftTurned into Th-234 · 0
100%50%25%12.5%0123456

Half-lives passed (1 half-life = 4.47 billion years)

No time passed · 0 half-lives · 100 of 100 atoms left

Each atom decays at a random moment, so no one can say which will go next, but about half of whatever is left goes in every half-life. The dashed curve is the prediction; the solid line is this sample. The new Th-234 atoms decay too, on their own schedule, which the chain below follows.

From U-238 to stable Pb-206: 14 steps

  1. U-238Uranium4.5 billion yearsαdecays by alpha
  2. Th-234Thorium24 daysβdecays by beta
  3. Pa-234mProtactinium1.2 minutesβdecays by beta
  4. U-234Uranium250,000 yearsαdecays by alpha
  5. Th-230Thorium75,000 yearsαdecays by alpha
  6. Ra-226Radium1,600 yearsαdecays by alpha
  7. Rn-222Radon3.8 daysαdecays by alpha
  8. Po-218Polonium3.1 minutesαdecays by alpha
  9. Pb-214Lead27 minutesβdecays by beta
  10. Bi-214Bismuth20 minutesβdecays by beta
  11. Po-214Polonium160 microsecondsαdecays by alpha
  12. Pb-210Lead22 yearsβdecays by beta
  13. Bi-210Bismuth5 daysβdecays by beta
  14. Po-210Polonium140 daysαdecays by alpha
  15. Pb-206Leadstable

Arrows show how each atom decays: α gives off an alpha particle (2 protons and 2 neutrons), β turns a neutron into a proton, γ gives off a gamma ray.

Half-lives and decay modes from the IAEA Live Chart of Nuclides (evaluated ENSDF data). The chain follows each atom’s most common decay; a few steps have rare side branches.

Keep exploring

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