4.5 Billion Years
in 14 Steps
The radioactive decay chain of Uranium-238
Every atom of uranium in the Earth is a clock, counting down. Follow the journey from unstable uranium to stable lead — through 8 alpha decays and 6 beta decays, across time scales from microseconds to billions of years.
What is Nuclear Decay?
The periodic table tells us what elements exist. The nuclide chart tells us what they do. Some atomic nuclei are stable — they will exist unchanged for eternity. Others are unstable, and will eventually transform into something else.
Alpha decay (α): The nucleus ejects a helium-4 particle (2 protons + 2 neutrons). The atom loses 4 units of mass and becomes a completely different element — two steps down the periodic table.
Beta-minus decay (β⁻): A neutron transforms into a proton, releasing an electron and an antineutrino. The atom's mass stays the same, but it gains a proton — becoming the next element up.
The U-238 decay chain uses both. Eight alpha decays strip away mass. Six beta decays adjust the proton-neutron balance. The result: uranium becomes lead, via 14 transformations.
21 Orders of Magnitude
The longest step in the chain — U-238's first decay — takes 4.468 billion years. Almost exactly the age of the Earth. Half of all the uranium that existed when our planet formed has already decayed.
The fastest step — Po-214 to Pb-210 — takes 164 microseconds. In that time, light travels just 49 kilometres.
The ratio between these two? 8.6 × 10²⁰. That's 860 billion trillion times faster. If the first step were one second, the fastest step would be 0.00000000000000000001 seconds.
This extraordinary range is explained by quantum tunneling. Alpha particles must tunnel through an energy barrier to escape the nucleus. A tiny increase in energy makes tunneling exponentially more likely — transforming billion-year half-lives into microsecond flashes.
Why This Matters
Uranium-Lead Dating
The ratio of U-238 to Pb-206 in a rock tells us when it formed. The oldest minerals on Earth — Jack Hills zircons from Western Australia — are 4.4 billion years old. Every Pb-206 atom in that zircon was once U-238.
Radon in Buildings
Rn-222 is the only gas in the chain. It seeps through cracks in floors and foundations into basements. Radon is the second leading cause of lung cancer after smoking — estimated 21,000 deaths per year in the US alone. The UK mandates radon testing in high-risk areas.
Polonium: The Assassin's Element
Po-210 — the final unstable isotope in the chain — was used to assassinate Alexander Litvinenko in London in 2006. Intensely radioactive, it delivers devastating alpha radiation to biological tissue at close range. It exists naturally in tiny amounts in all uranium-bearing rock.
Further Exploration
Related Explainers
- The Nuclide Chart — The full landscape of nuclear stability
- Stellar Evolution — Where heavy elements are made
Key Figures
- Marie Curie — Discovered radium (Ra-226) and polonium (Po-210)
- Ernest Rutherford — Identified alpha and beta radiation
- Clair Cameron Patterson — Used U-Pb dating to determine the age of Earth (1956)
- George Gamow — Explained alpha decay via quantum tunneling (1928)