Uranium decays into lead. Specifically, uranium-238 (the most common form) ends as stable lead-206, and uranium-235 ends as stable lead-207. But neither isotope makes that jump in a single step. Each one passes through a long chain of intermediate elements, transforming more than a dozen times before finally settling into a form that no longer emits radiation.
The Uranium-238 Decay Chain
Uranium-238 makes up about 99.3% of all natural uranium. It has a half-life of 4.47 billion years, roughly the age of Earth itself, which means a chunk of U-238 loses half its atoms over that span. When a U-238 atom decays, it ejects an alpha particle (a cluster of two protons and two neutrons) and becomes thorium-234. That’s just the first of 14 separate decay steps before the chain reaches stable lead-206.
The full sequence looks like this:
- Uranium-238 (4.47 billion years) → alpha decay → Thorium-234
- Thorium-234 (24.1 days) → beta decay → Protactinium-234m
- Protactinium-234m (1.17 minutes) → beta decay → Uranium-234
- Uranium-234 (245,000 years) → alpha decay → Thorium-230
- Thorium-230 (77,000 years) → alpha decay → Radium-226
- Radium-226 (1,600 years) → alpha decay → Radon-222
- Radon-222 (3.8 days) → alpha decay → Polonium-218
- Polonium-218 (3.11 minutes) → alpha decay → Lead-214
- Lead-214 (26.8 minutes) → beta decay → Bismuth-214
- Bismuth-214 (19.9 minutes) → beta decay → Polonium-214
- Polonium-214 (163.7 microseconds) → alpha decay → Lead-210
- Lead-210 (22.3 years) → beta decay → Bismuth-210
- Bismuth-210 (5 days) → beta decay → Polonium-210
- Polonium-210 (138.4 days) → alpha decay → Lead-206 (stable)
Across this entire chain, eight alpha decays and six beta decays occur. Each alpha decay sheds 4 units of mass and drops the atomic number by 2. Each beta decay converts a neutron into a proton, raising the atomic number by 1 without changing mass. The net result: the atom loses 32 units of mass (from 238 down to 206) and drops 10 places on the periodic table (from element 92 to element 82).
The Uranium-235 Decay Chain
Uranium-235 is far rarer, making up only about 0.72% of natural uranium, but it’s the isotope used in nuclear reactors and weapons because it can sustain a fission chain reaction. Its half-life is 708 million years, much shorter than U-238’s. When U-235 decays naturally (rather than being split by a neutron), it follows its own chain, sometimes called the actinium series, passing through isotopes of thorium, protactinium, actinium, francium, radium, radon, polonium, and bismuth before ending as stable lead-207. The chain involves seven alpha decays and four beta decays across 11 steps.
Alpha vs. Beta Decay
Two types of radioactive decay drive these chains. In alpha decay, the atom ejects a small package containing two protons and two neutrons. This is the heavier form of radiation. It can’t pass through a sheet of paper, but if alpha-emitting material gets inside the body (through breathing, swallowing, or an open wound), the particles deliver concentrated energy to nearby cells.
In beta decay, a neutron inside the nucleus converts into a proton and releases a fast-moving electron. Beta particles penetrate farther than alpha particles but carry less energy per impact. Both types occur multiple times throughout the uranium decay chains, which is why uranium ore produces a mix of radiation types.
Why Radon Matters
One of the most significant stops along the U-238 decay chain is radon-222. Every other isotope in the chain is a solid metal, but radon is a gas. When radium-226, sitting in soil or rock, decays into radon-222, the newly formed gas can seep upward through cracks and accumulate in basements and enclosed spaces. With a half-life of just 3.8 days, radon itself decays quickly into polonium-218, then continues down the chain through several short-lived isotopes that cling to dust particles. When inhaled, these particles lodge in lung tissue and deliver alpha radiation directly to cells, which is why radon exposure is the second leading cause of lung cancer after smoking.
Why Polonium-210 Is Dangerous
Near the very end of the U-238 chain sits polonium-210, with a half-life of about 138 days. It emits alpha particles that carry enough energy to damage or destroy genetic material inside cells. Polonium-210 poses no threat from outside the body, as neither the element nor its radiation can penetrate unbroken skin. The danger comes from ingestion, inhalation, or contamination of a wound. Once inside the body, some of it passes through the digestive system, but the portion that reaches the bloodstream concentrates in organs like the spleen, kidneys, and liver. High doses over a short period can be fatal. Lower, sustained exposure raises long-term cancer risk.
Polonium-210 also forms in small amounts in tobacco leaves, which is one reason cigarette smoke delivers alpha radiation directly to lung tissue.
How Scientists Use the Decay Chain
The predictable, clock-like nature of uranium’s decay makes it one of the most powerful tools in geology. Because U-238 decays into lead-206 at a known rate, measuring the ratio of uranium to lead in a mineral sample reveals how long ago that mineral formed. This technique, called uranium-lead dating, is the gold standard for determining the age of ancient rocks. It’s how scientists pinned Earth’s age at roughly 4.5 billion years, a number that lines up almost exactly with uranium-238’s half-life.
The decay chain also explains why lead is so abundant in Earth’s crust relative to other heavy elements. A significant fraction of the lead on the planet wasn’t there when Earth formed. It accumulated over billions of years as the endpoint of uranium and thorium decay.