Atomic number 61

Promethium

Pm
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Shells Orbitals 4f 6s

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61 electrons in 6 shells: 2, 8, 18, 23, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Lanthanide
Period: 6
Block: f
Atomic mass: 144.91276 Da (longest-lived isotope)
Electron configuration: [Xe] 4f⁵ 6s²
Phase at 25 °C: solid
Melting point: 1042 °C
Boiling point: 3000 °C
Density: 7.26 g/cm³
Discovered: 1945, Marinsky, Glendenin and Coryell

Neighbours in the table

Nd Sm

Named for Prometheus, who in Greek myth stole fire from heaven, promethium spent more than forty years as a gap in the periodic table. In 1902 the Czech chemist Bohuslav Brauner predicted an element between neodymium and samarium, Jefferson Lab records. Moseley confirmed it in 1914, Los Alamos records.

In 1941 researchers at Ohio State University aimed neutrons, deuterons and alpha particles, nuclei of heavy hydrogen and helium, at neodymium and praseodymium. Their new radioactivities were most likely element 61, in Los Alamos’s account. Wu and Segrè, and Bethe, confirmed its formation in 1942. Chemical proof was lacking, because separating the rare earths, its family of similar metals, was then so hard.

Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell found the element among uranium fission by-products at Clinton Laboratories in Oak Ridge, Tennessee, now Oak Ridge National Laboratory, according to Jefferson Lab. They had proof in 1944 but, busy with wartime defence research, waited until 1946 to claim it. Their first chemical identification, made by ion-exchange chromatography, separation by how strongly ions cling to a resin, came in 1945 by Los Alamos’s dating.

No isotope of promethium is stable, and technetium is the only other element below atomic number 83 without one, Los Alamos notes. PubChem gives the mass of promethium’s longest-lived isotope, promethium-145, as 144.913 Da.

Where it is found

Promethium appears to be absent from Earth’s crust, where every search for it has failed, Los Alamos reports. Jefferson Lab states that it does not occur naturally on Earth. In Haz-Map’s summary, quoting the Merck Index, it is present at low levels in uranium ores.

Far from Earth, promethium has been identified in the spectrum of HR465, a star in the constellation Andromeda. A half-life is the time in which half of a sample decays, and no known isotope of the element has one beyond 17.7 years. So the promethium in that star is being formed now, near its surface, in Los Alamos’s account.

Uranium fission remains a source: promethium is extracted from its by-products, Jefferson Lab reports. Neutron irradiation of neodymium-146 offers a second route. Capturing a neutron turns that isotope into neodymium-147. That nucleus, whose half-life is 11 days, beta-decays, sending out a beta particle, a fast electron from the nucleus, and becomes promethium-147. Early in 1963, ion-exchange methods yielded about 10 g of promethium from the wastes of reactor fuel processing, Los Alamos records.

Why it matters

Promethium-147 is the most generally useful promethium isotope, in Los Alamos’s judgement. Every one of its decays emits a beta particle, in the IAEA’s data. Its half-life is 2.6234 years in the IAEA’s evaluation and 2.6 years according to Los Alamos.

It serves as a beta source for thickness gauges, Los Alamos notes. In reactor fuel, it measures the graphite layer that keeps a fuel rod from touching its Zircaloy cladding, the tube holding it, according to IUPAC. In that method, beta-ray backscatter, promethium-147 laid under the graphite sends beta particles back through it. Its long half-life and steady beta emission make the method reliable and simple, IUPAC notes.

Its radiation can also make a phosphor, a material that glows under radiation, shine, and that light suits dependable signs and signals, Los Alamos reports. Photocells can turn that light into current, and a nuclear battery built this way would last about 5 years. IUPAC calls promethium-147 an ideal candidate for nuclear batteries known as beta voltaics, long-lived power for space and sea missions.

Promethium could also serve as a portable source of X-rays, and perhaps as a heat source giving auxiliary power to space probes and satellites, in Los Alamos’s view. Lasers signalling to submerged submarines are another possible use, in Jefferson Lab’s account.

Properties

Properties of promethium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	61
Atomic mass	144.91276 Da	Da	PubChem, longest-lived isotope
Melting point	1042 °C	°C	Los Alamos: 1042 °C
Boiling point	3000 °C	°C	Los Alamos: 3000 °C
Density	7.26 g/cm³	g/cm³
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f⁵
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Every isotope of promethium is radioactive, which makes ionising radiation its hazard, in Haz-Map’s radionuclide summary. Promethium is a low-energy beta emitter and gives off no gamma rays. Its beta particles can still generate X-rays when they strike elements of high atomic number. The reference sheet is PubChem’s Laboratory Chemical Safety Summary for promethium.

Questions

Does promethium glow in the dark?

Its salts do. Promethium salts give a pale blue or greenish glow in the dark, and Los Alamos traces that light to their high radioactivity. Chemists have prepared over 30 of its compounds, and most are coloured. The metal itself is little studied: few of its properties are generally known, though it exists in two allotropic forms, different arrangements of the same atoms, Los Alamos notes.

What is the most stable isotope of promethium?

Promethium-145 lasts longest, with a half-life of 17.7 years in the IAEA's evaluation. It changes into neodymium-145 by electron capture, the nucleus absorbing one of the atom's own electrons, and in the IAEA's evaluation a further 0.00000028 per cent of its decays emit an alpha particle instead. PubChem gives its mass as 144.913 Da. A half-life is the time in which half of a sample decays.

How many isotopes does promethium have?

None is stable. Haz-Map gives mass numbers from 132 to 155. According to the IAEA's Atomic Mass Data Center, promethium isotopes run from promethium-126 to promethium-165. The centre gives promethium-146 a half-life of 5.53 years; that isotope decays by electron capture 66.0 per cent of the time and by beta-minus emission 34.0 per cent.