Promethium, 61Pm
Pronunciationm/ (MEE-thee-əm)
Mass number[145]
Promethium in the periodic table
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon


Atomic number (Z)61
Groupgroup n/a
Periodperiod 6
Element category  Lanthanide
Electron configuration[Xe] 4f5 6s2
Electrons per shell2, 8, 18, 23, 8, 2
Physical properties
Phase at STPsolid
Melting point1315 K ​(1042 °C, ​1908 °F)
Boiling point3273 K ​(3000 °C, ​5432 °F)
Density (near r.t.)7.26 g/cm3
Heat of fusion7.13 kJ/mol
Heat of vaporization289 kJ/mol
Atomic properties
Oxidation states+2, +3 (a mildly basic oxide)
ElectronegativityPauling scale: 1.13 (?)
Ionization energies
  • 1st: 540 kJ/mol
  • 2nd: 1050 kJ/mol
  • 3rd: 2150 kJ/mol
Atomic radiusempirical: 183 pm
Covalent radius199 pm
Color lines in a spectral range
Spectral lines of promethium
Other properties
Natural occurrencefrom decay
Crystal structuredouble hexagonal close-packed (dhcp)
Double hexagonal close packed crystal structure for promethium
Thermal expansion9.0 µm/(m·K)[1] (at r.t.)
Thermal conductivity17.9 W/(m·K)
Electrical resistivityest. 0.75 µΩ·m (at r.t.)
Magnetic orderingparamagnetic[2]
Young's modulusα form: est. 46 GPa
Shear modulusα form: est. 18 GPa
Bulk modulusα form: est. 33 GPa
Poisson ratioα form: est. 0.28
CAS Number7440-12-2
DiscoveryChien Shiung Wu, Emilio Segrè, Hans Bethe (1942)
First isolationCharles D. Coryell, Jacob A. Marinsky, Lawrence E. Glendenin (1945)
Named byGrace Mary Coryell (1945)
Main isotopes of promethium
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
145Pmtrace17.7 yε145Nd
146Pmsyn5.53 yε146Nd
147Pmtrace2.6234 yβ147Sm
| references

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.

There are two possible sources for natural promethium: rare decays of natural europium-151 (producing promethium-147) and uranium (various isotopes). Practical applications exist only for chemical compounds of promethium-147, which are used in luminous paint, atomic batteries and thickness-measurement devices, even though promethium-145 is the most stable promethium isotope. 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.


Physical properties

A promethium atom has 61 electrons, arranged in the configuration [Xe]4f56s2.[3] In forming compounds, the atom loses its two outermost electrons and one of the 4f-electrons, which belongs to an open subshell. The element's atomic radius is the second largest among all the lanthanides but is only slightly greater than those of the neighboring elements.[3] It is the most notable exception to the general trend of the contraction of lanthanide atoms with the increase of their atomic numbers (see lanthanide contraction[4]). Many properties of promethium rely on its position among lanthanides and are intermediate between those of neodymium and samarium. For example, the melting point, the first three ionization energies, and the hydration energy are greater than those of neodymium and lower than those of samarium;[3] similarly, the estimate for the boiling point, ionic (Pm3+) radius, and standard heat of formation of monatomic gas are greater than those of samarium and less than those of neodymium.[3]

Promethium has a double hexagonal close packed (dhcp) structure and a hardness of 63 kg/mm2.[5] This low-temperature alpha form converts into a beta, body-centered cubic (bcc) phase upon heating to 890 °C.[6]

Chemical properties and compounds

Promethium belongs to the cerium group of lanthanides and is chemically very similar to the neighboring elements.[7] Because of its instability, chemical studies of promethium are incomplete. Even though a few compounds have been synthesized, they are not fully studied; in general, they tend to be pink or red in color.[8][9] Treatment of acidic solutions containing Pm3+ ions with ammonia results in a gelatinous light-brown sediment of hydroxide, Pm(OH)3, which is insoluble in water.[10] When dissolved in hydrochloric acid, a water-soluble yellow salt, PmCl3, is produced;[10] similarly, when dissolved in nitric acid, a nitrate results, Pm(NO3)3. The latter is also well-soluble; when dried, it forms pink crystals, similar to Nd(NO3)3.[10] The electron configuration for Pm3+ is [Xe] 4f4, and the color of the ion is pink. The ground state term symbol is 5I4.[11] The sulfate is slightly soluble, like the other cerium group sulfates. Cell parameters have been calculated for its octahydrate; they lead to conclusion that the density of Pm2(SO4)3·8 H2O is 2.86 g/cm3.[12] The oxalate, Pm2(C2O4)3·10 H2O, has the lowest solubility of all lanthanide oxalates.[13]

Unlike the nitrate, the oxide is similar to the corresponding samarium salt and not the neodymium salt. As-synthesized, e.g. by heating the oxalate, it is a white or lavender-colored powder with disordered structure.[10] This powder crystallizes in a cubic lattice upon heating to 600 °C. Further annealing at 800 °C and then at 1750 °C irreversibly transforms it to a monoclinic and hexagonal phases, respectively, and the last two phases can be interconverted by adjusting the annealing time and temperature.[14]

Formula symmetry space group No Pearson symbol a (pm) b (pm) c (pm) Z density,
α-Pm dhcp[5][6] P63/mmc 194 hP4 365 365 1165 4 7.26
β-Pm bcc[6] Fm3m 225 cF4 410 410 410 4 6.99
Pm2O3 cubic[14] Ia3 206 cI80 1099 1099 1099 16 6.77
Pm2O3 monoclinic[14] C2/m 12 mS30 1422 365 891 6 7.40
Pm2O3 hexagonal[14] P3m1 164 hP5 380.2 380.2 595.4 1 7.53

Promethium forms only one stable oxidation state, +3, in the form of ions; this is in line with other lanthanides. According to its position in the periodic table, the element cannot be expected to form stable +4 or +2 oxidation states; treating chemical compounds containing Pm3+ ions with strong oxidizing or reducing agents showed that the ion is not easily oxidized or reduced.[7]

Promethium halides[15]
Formula color coordination
symmetry space group No Pearson symbol m.p. (°C)
PmF3 Purple-pink 11 hexagonal P3c1 165 hP24 1338
PmCl3 Lavender 9 hexagonal P63/mc 176 hP8 655
PmBr3 Red 8 orthorhombic Cmcm 63 oS16 624
α-PmI3 Red 8 orthorhombic Cmcm 63 oS16 α→β
β-PmI3 Red 6 rhombohedral R3 148 hR24 695


Promethium is the only lanthanide and one of only two elements among the first 83 that has no stable or long-lived (primordial) isotopes. This is a result of a rarely occurring effect of the liquid drop model and stabilities of neighbor element isotopes; it is also the least stable element of the first 84.[16] The primary decay products are neodymium and samarium isotopes (promethium-146 decays to both, the lighter isotopes generally to neodymium via positron decay and electron capture, and the heavier isotopes to samarium via beta decay). Promethium nuclear isomers may decay to other promethium isotopes and one isotope (145Pm) has a very rare alpha decay mode to stable praseodymium-141.[16]

The most stable isotope of the element is promethium-145, which has a specific activity of 940 Ci/g (35 TBq/g) and a half-life of 17.7 years via electron capture.[16][17] Because it has 84 neutrons (two more than 82, which is a magic number which corresponds to a stable neutron configuration), it may emit an alpha particle (which has 2 neutrons) to form praseodymium-141 with 82 neutrons. Thus it is the only promethium isotope with an experimentally observed alpha decay.[18] Its partial half-life for alpha decay is about 6.3×109 years, and the relative probability for a 145Pm nucleus to decay in this way is 2.8×107 %. Several other promethium isotopes such as 144Pm, 146Pm, and 147Pm also have a positive energy release for alpha decay; their alpha decays are predicted to occur but have not been observed.

The element also has 18 nuclear isomers, with mass numbers of 133 to 142, 144, 148, 149, 152, and 154 (some mass numbers have more than one isomer). The most stable of them is promethium-148m, with a half-life of 43.1 days; this is longer than the half-lives of the ground states of all promethium isotopes, except for promethium-143 to 147. In fact, promethium-148m has a longer half-life than its ground state, promethium-148.[16]