Protactinium · element 91
The actinide almost nobody has seen, once purified from 60 tonnes of waste to yield a few grams
Key facts
Protactinium sits between thorium and uranium, and for most of the twentieth century it was the rarest metal a chemist could hold. It is a bright, silvery actinide that keeps its shine for a while in air before a dull oxide layer forms. Every atom of it is radioactive, and the most stable form, protactinium-231, has a half-life of roughly 32,000 years. That is long enough to build up in uranium ore, but far too short for any primordial protactinium to have survived from the birth of the Earth.
Its name is a clue to where it fits in nature. Protactinium-231 decays by giving off an alpha particle and becomes actinium-227. The name means, roughly, the parent of actinium, and that role in the uranium-235 decay chain is exactly what led to its discovery.
Where it is found
Protactinium is never mined for its own sake. It exists on Earth only because uranium-235 is constantly decaying, and protactinium-231 is one of the stepping stones in that decay chain. In a typical uranium ore such as pitchblende there is roughly one part protactinium in every few million parts of ore. A tonne of rich ore might hold a fraction of a gram.
The most famous stockpile came from the United Kingdom. In 1961 the UK Atomic Energy Authority processed around 60 tonnes of leftover material from uranium refining and recovered about 125 grams of protactinium. For decades that single batch was the main supply for laboratories around the world. Tiny amounts of protactinium-233 also appear inside nuclear reactors that use thorium fuel, where it is an intermediate on the way to uranium-233.
How it was discovered
The trail begins in 1900, when William Crookes separated a strongly radioactive substance from uranium and called it uranium X. He could not identify it as a new element. We now know he had isolated a short-lived isotope, protactinium-234m, which fades within minutes. In 1913 Kasimir Fajans and Oswald Goehring recognised that this substance belonged to a new element and called it brevium, meaning brief, because of its short life.
The long-lived isotope was found in 1917 and 1918 by two teams working independently: Lise Meitner and Otto Hahn in Berlin, and Frederick Soddy and John Cranston in Glasgow. Because this isotope lasted tens of thousands of years, its name, protoactinium, replaced brevium. IUPAC trimmed the spelling to protactinium in 1949. Aristid von Grosse produced the first metallic sample in 1934.
What we use it for
Protactinium has no commercial or industrial use. It is too scarce, too radioactive and too expensive to justify any product. Its value lies in research. Geochemists use the ratio of protactinium-231 to thorium-230 in seabed sediments to work out how quickly water moved in ancient oceans. The two isotopes are produced at a known rate from uranium dissolved in seawater but are removed at different speeds, so their ratio acts as a slow clock for ocean circulation.
The same isotope pair helps date corals and cave deposits. Protactinium-233 matters to engineers designing thorium reactors because it soaks up neutrons and must be allowed to decay to uranium-233 before it can be used as fuel. Physicists have also studied protactinium metal as a superconductor at temperatures a little above one kelvin.
Reading the numbers
Protactinium melts at about 1,570 °C and boils near 4,030 °C, so it is a robust, high-melting metal, harder to melt than steel. Its density of about 15.4 grams per cubic centimetre makes it nearly twice as dense as iron and around 15 times denser than water. The electron configuration [Rn] 5f2 6d1 7s2 shows electrons beginning to fill the inner 5f shell, which is the defining feature of the actinide row. In protactinium the 5f electrons are still fairly loosely held, so the element behaves partly like a transition metal and readily reaches the +5 oxidation state, much like tantalum and niobium. Its first ionisation energy, around 568 kilojoules per mole, is low, as expected for a large metal atom whose outer electrons sit far from the nucleus.
Safety and the environment
Protactinium-231 is an alpha emitter with a long half-life, and it concentrates in bone and kidneys if swallowed or inhaled. It is considered among the more radiotoxic natural elements, and every gram must be handled in glove boxes behind shielding. Because it is so rare there is no pollution problem from protactinium itself, but it forms a small part of the radioactivity in uranium mine tailings and in spent nuclear fuel, and it must be accounted for in long-term waste plans.
Did you know?
- The 125 gram batch purified in the UK in 1961 came from about 60 tonnes of uranium processing residue, an extraction ratio of roughly one part in half a million.
- The first protactinium isotope ever isolated, found by Crookes in 1900, lives for only a little over a minute, which is why the element was once called brevium.
- The name was shortened from protoactinium to protactinium in 1949, more than thirty years after the element was named.
- Protactinium-231 makes a good ocean clock because its half-life of about 32,000 years matches the timescales on which deep ocean water turns over.
- Protactinium metal becomes a superconductor when cooled to about 1.4 kelvin, colder than liquid helium at normal pressure.
More in the actinides
The nearest neighbours of protactinium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Frequently asked questions
Is protactinium found in nature?
Yes, in trace amounts inside uranium ores. It is formed constantly as uranium-235 decays, but it is present at only around a part per million, so it is one of the rarest natural elements.
Why is protactinium so rare?
Its most stable isotope has a half-life of around 32,000 years, so it cannot survive over geological time on its own. It exists only because uranium keeps making it, and uranium-235, its parent, is itself the scarce isotope of uranium.
What is protactinium used for?
Nothing commercial. It is used in research, especially for dating ocean sediments and corals through the ratio of protactinium-231 to thorium-230, and it is an intermediate in thorium-fuelled reactors.
Is protactinium dangerous?
Yes. It is an alpha emitter that accumulates in bone, so it is only handled in specialised laboratories. There is no route by which the public would normally encounter it.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Protactinium reference article. Spotted an error? Tell us.