Uranium-235 decays into thorium-231 by releasing an alpha particle (a cluster of two protons and two neutrons). But that’s only the first step. Thorium-231 is itself radioactive, and it kicks off a long chain of transformations that passes through 11 intermediate isotopes before finally ending as lead-207, a stable, non-radioactive form of lead.
The First Step: Alpha Decay to Thorium-231
When a uranium-235 atom decays, it ejects an alpha particle from its nucleus. That alpha particle carries away two protons and two neutrons, which drops the atomic number from 92 (uranium) to 90 (thorium) and the mass number from 235 to 231. The result is thorium-231.
This first step is extraordinarily slow. Uranium-235 has a half-life of about 700 million years, meaning it takes that long for half of any given sample to transform. That extreme longevity is why uranium-235 still exists on Earth billions of years after the planet formed. It makes up about 0.711% of all natural uranium, with the more common uranium-238 accounting for roughly 99.27%.
Thorium-231, by contrast, is short-lived. It has a half-life of just 25.52 hours. It decays by emitting a beta particle (a high-speed electron released when a neutron converts into a proton), turning into protactinium-231.
The Full Decay Chain
The series of isotopes that uranium-235 passes through is called the actinium series. Each step involves either alpha decay (losing two protons and two neutrons) or beta decay (converting a neutron into a proton). Here is the complete sequence, with approximate half-lives:
- Uranium-235 → alpha decay → Thorium-231 (half-life: 700 million years)
- Thorium-231 → beta decay → Protactinium-231 (half-life: 25.52 hours)
- Protactinium-231 → alpha decay → Actinium-227 (half-life: 32,760 years)
- Actinium-227 → alpha decay → Francium-223, or beta decay → Thorium-227 (half-life: 21.77 years)
- Radium-223 → alpha decay → Radon-219 (half-life: 11.43 days)
- Radon-219 → alpha decay → Polonium-215 (half-life: 3.96 seconds)
- Polonium-215 → alpha decay → Lead-211 (half-life: 1.78 milliseconds)
- Lead-211 → beta decay → Bismuth-211 (half-life: 36.1 minutes)
- Bismuth-211 → alpha decay → Thallium-207 (half-life: 2.14 minutes)
- Thallium-207 → beta decay → Lead-207, stable (half-life: 4.77 minutes)
At actinium-227, the chain branches. Most of the time actinium-227 undergoes beta decay to thorium-227, which then alpha-decays to radium-223 and continues down the chain. A small fraction takes a different alpha-decay path through francium-223, but both branches rejoin and ultimately reach the same destination: stable lead-207.
Why the Chain Ends at Lead-207
Each radioactive isotope in the series is unstable because its nucleus has too many protons, too many neutrons, or too much energy. Alpha and beta decays are the nucleus’s way of shedding that excess and moving toward a more balanced configuration. Lead-207 is the first isotope in this particular chain where the balance of protons and neutrons is stable enough that no further decay occurs. It sits there permanently.
This is actually a pattern across nature’s heaviest elements. Uranium-238 decays through its own long chain and ends at lead-206. Thorium-232 ends at lead-208. All three of the naturally occurring heavy decay series terminate at a different stable isotope of lead.
How Scientists Use This Decay Chain
The predictable, clock-like pace of this decay chain makes it a powerful dating tool. Geologists measure the ratio of uranium-235 to lead-207 in rock samples to determine how old those rocks are. Because uranium-235’s half-life is 700 million years, this method works well for dating geological formations hundreds of millions to billions of years old.
The intermediate isotopes are useful too. Protactinium-231, the third member of the chain with a half-life of about 32,760 years, builds up at a measurable rate in uranium-containing materials. Nuclear forensic scientists use the ratio of protactinium-231 to uranium-235 as a chronometer to determine when uranium was last chemically purified, a technique that can date processed nuclear materials back to the 1940s.
Natural Decay vs. Fission
It’s worth noting that the decay chain described above is what happens to uranium-235 when it’s left alone. This natural, spontaneous process releases energy gradually over hundreds of millions of years.
Fission is a completely different process. When a uranium-235 nucleus absorbs a neutron, it can split into two smaller atoms (the specific products vary, but common pairs include krypton and barium or strontium and xenon) plus two or three free neutrons. A single fission event releases about 202 MeV of energy, roughly 181 MeV of which is released instantly as kinetic energy of the fragments. The rest comes out more slowly as the radioactive fission products themselves decay.
Fission is what powers nuclear reactors and weapons. Natural radioactive decay is what happens to any uranium-235 atom sitting in a rock formation, slowly working its way down the actinium series toward lead-207 over billions of years.