Promethium is a chemical element with the symbol Pm and atomic number 61. All of its isotopes are radioactive; it is extremely rare, with only about 500–600 grams naturally occurring in Earth's crust at any given time. Promethium is one of only two radioactive elements that are followed in the periodic table by elements with stable forms, the other being technetium. Chemically, promethium is a lanthanide. Promethium shows only one stable oxidation state of +3.

In 1902 Bohuslav Brauner suggested that there was a then-unknown element with properties intermediate between those of the known elements neodymium (60) and samarium (62); this was confirmed in 1914 by Henry Moseley, who, having measured the atomic numbers of all the elements then known, found that atomic number 61 was missing. In 1926, two groups (one Italian and one American) claimed to have isolated a sample of element 61; both "discoveries" were soon proven to be false. In 1938, during a nuclear experiment conducted at Ohio State University, a few radioactive nuclides were produced that certainly were not radioisotopes of neodymium or samarium, but there was a lack of chemical proof that element 61 was produced, and the discovery was not generally recognized. Promethium was first produced and characterized at Oak Ridge National Laboratory in 1945 by the separation and analysis of the fission products of uranium fuel irradiated in a graphite reactor. The discoverers proposed the name "prometheum" (the spelling was subsequently changed), derived from Prometheus, the Titan in Greek mythology who stole fire from Mount Olympus and brought it down to humans, to symbolize "both the daring and the possible misuse of mankind's intellect". However, a sample of the metal was made only in 1963.

The two sources of natural promethium are rare alpha decays of natural europium-151 (producing promethium-147) and spontaneous fission of uranium (various isotopes). Promethium-145 is the most stable promethium isotope, but the only isotope with practical applications is promethium-147, chemical compounds of which are used in luminous paint, atomic batteries and thickness-measurement devices. Because natural promethium is exceedingly scarce, it is typically synthesized by bombarding uranium-235 (enriched uranium) with thermal neutrons to produce promethium-147 as a fission product.

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Atomic properties

Atomic mass146.915 u

Atomic radii

Radius (empirical)183 pm
Radius (calculated)205 pm
Covalent radius199 pm

Atomic shell

Electron configurationXe 4f5 6s2
Ionization energy(1st) 5.58187 eV
(2nd) 10.938 eV
(3rd) 22.44 eV
(4th) 41.17 eV
(5th) 61.7 eV
Shell model

Physical properties

Density7.2 g·cm−3 (298.1 K)
Molar volume2.01·10-5 m3·mol−1


Melting point1,315 K
Boiling point3,273 K
Liquid range1,958 K


Melting enthalpy7.7 kJ·mol-1
Enthalpy of vaporization290 kJ·mol-1
Binding energy350 kJ·mol-1

Heat and conductivity

Thermal conductivity15 W·m-1·K-1
Expansion coefficient1.1·10-5 K-1

Elastic properties

Young’s modulus46 GPa
Shear modulus18 GPa
Bulk modulus33 GPa
Poisson’s ratio0.28

Electrical properties

Electrical conductivity1.33·106 S·m-1
Resistance7.5·10-7 Ωm



Chemical properties

Basicityslightly basic
Oxidation state3
Standard potential-2.423 V (Pm3+ + 3e- → Pm)


Allred-Rochow scale1.07
Nagle scale1.04

Other properties

Natural occurrenceDecay product
Crystal structureDouble hexagonal close-packed
Goldschmidt Classificationsynthetic
Superconductorwithout transition tempperature
Radioactivitysignificantly radioactive
Price/kg460,000 USD

Natural abundances

0.005 ppb ≈ 9.94·106 Mt