Promethium · element 61
The only rare earth with no stable atoms, made in reactors and named after the thief of fire
Key facts
Promethium is the gap in the lanthanide row. Every other rare earth metal has at least one stable isotope that has survived since the Earth formed. Promethium has none. All of its known isotopes are radioactive, and the longest lived, promethium-145, has a half-life of only about 17 to 18 years. Any promethium present when the planet formed decayed away billions of years ago.
That makes promethium one of only two elements lighter than bismuth with no stable form, the other being technetium. Almost everything we know about it comes from material made in nuclear reactors, where it forms when uranium atoms split. Chemically it behaves like a typical lanthanide, forming pink or salmon coloured salts in the +3 oxidation state, but handling it always means handling a radioactive source.
Where it is found
For practical purposes promethium does not occur in nature. Tiny traces do exist. Uranium in ores such as uraninite very occasionally splits spontaneously, and a small share of those fission events produce promethium atoms. Estimates suggest the whole of the Earth's crust holds only a few hundred grams at any moment, constantly being made and constantly decaying.
Promethium has been spotted far from Earth. Astronomers have identified its spectral lines in the light of a peculiar star known as Przybylski's Star, which shows signatures of several short lived elements.
All usable promethium comes from spent nuclear fuel or from bombarding neodymium with neutrons in a reactor. Chemists then separate it from the other fission products using ion exchange columns.
How it was discovered
Moseley's X-ray work had shown that an element belonged between neodymium and samarium. Several teams in the 1920s claimed to have found it in minerals and proposed names such as illinium and florentium. None of the claims held up, because there was almost nothing there to find.
The real discovery came out of the wartime atomic programme. In 1945 Jacob Marinsky, Lawrence Glendenin and Charles Coryell, working at Oak Ridge in Tennessee, separated element 61 from the fission products of uranium using ion exchange chromatography. They announced the result in 1947. The name was suggested by Grace Mary Coryell, Charles Coryell's wife. It honours Prometheus, who in Greek myth stole fire from the gods, a pointed reference to both the promise and the danger of nuclear energy.
What we use it for
Promethium-147, with a half-life of about 2.6 years, emits beta particles but very little penetrating gamma radiation. That combination makes it useful and fairly easy to shield. Its earliest job was in luminous paint. Mixed with a phosphor, it made glowing dials for watches and aircraft instruments, replacing the far more hazardous radium used before it.
The same isotope powers small nuclear batteries. In a betavoltaic cell the beta particles strike a semiconductor and generate a trickle of electricity for years without maintenance. They have powered remote sensors and early pacemakers.
Promethium also works as a beta source in thickness gauges. A sheet of paper, plastic or metal passes between the source and a detector, and the amount of radiation getting through reveals how thick the sheet is.
Reading the numbers
Promethium melts at about 1042 °C and boils near 3000 °C, so it stays solid well beyond the heat of a kitchen oven or a wood fire. Its density of 7.26 grams per cubic centimetre is a little below that of iron. The atomic mass is listed as a round 145 rather than a decimal because there is no natural mix of isotopes to average; the table simply quotes the mass number of the longest lived isotope.
The electron configuration [Xe] 4f5 6s2 places it in the f-block. The two outer 6s electrons and one 4f electron are given up readily, so promethium forms +3 ions like almost every lanthanide. Its first ionisation energy of roughly 540 kJ/mol is typical for the row. Position 61 sits between neodymium at 60 and samarium at 62, exactly where the missing element was predicted to be.
Safety and the environment
Every promethium isotope is radioactive, so the element is treated as a radiological hazard rather than a chemical poison. Beta particles from promethium-147 travel only a few millimetres in tissue and are stopped by a thin sheet of metal or plastic, so sealed sources present a low risk. The danger lies in inhaling or swallowing loose material, because lanthanides tend to lodge in bone, where the radiation is then delivered directly.
Because it is produced in such small quantities and used in sealed devices, promethium is not a meaningful environmental pollutant.
Did you know?
- Promethium is the only lanthanide with no stable isotope, and one of just two elements lighter than bismuth without one (technetium is the other).
- Its salts glow faintly in the dark with a pale blue or green light, caused by their own radioactivity exciting the surrounding material.
- The table gives its atomic mass as a whole number, 145, because there is no natural isotope mixture from which to calculate a standard atomic weight.
- Promethium has been detected in the spectrum of Przybylski's Star, even though its longest lived isotope survives for only a couple of decades.
- Promethium-147 paint replaced radium in glowing watch and instrument dials in the mid twentieth century.
- The name was proposed by Grace Mary Coryell, the wife of one of the discoverers, after the Greek figure who stole fire from the gods.
More in the lanthanides
The nearest neighbours of promethium in the family, by atomic number.
See all 15 lanthanides on one page, or browse all 118 elements.
Frequently asked questions
Is promethium radioactive?
Yes, every known isotope of promethium is radioactive. The most useful one, promethium-147, has a half-life of about 2.6 years and gives off beta particles that are easily blocked by thin shielding. No stable form of the element exists.
Where does promethium come from if it does not occur naturally?
Almost all promethium is made in nuclear reactors. It appears among the fission products when uranium splits, and it can also be produced by irradiating neodymium with neutrons. Chemists then separate it from the other elements using ion exchange columns.
Why is promethium named after Prometheus?
The discoverers at Oak Ridge chose the name in the late 1940s at the suggestion of Grace Mary Coryell. Prometheus stole fire from the gods in Greek myth, and the name was meant to reflect both the power and the peril of the nuclear energy that produced the element.
What is promethium used for?
Its main uses have been in luminous paint, in small long lasting nuclear batteries for remote devices, and as a beta radiation source in gauges that measure the thickness of sheet materials. All of these rely on promethium-147 and use it in sealed form.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Promethium reference article. Spotted an error? Tell us.