Promethium (Pm)

Promethium is a chemical element with the atomic number 61 in the periodic table. Although it’s an artificially produced substance that cannot be found in Earth’s crust, promethium can also occur in trace amounts in the uranium ores. 

Element 61 is classified as a member of the lanthanides family of the periodic table. This highly radioactive and artificially produced rare-earth metal has three valence electrons in its outer shell that may participate in the formation of a chemical bond. Promethium is mainly used for scientific purposes and research. 

Chemical and Physical Properties of Promethium

Atomic number61
Atomic weight (mass)(147) g.mol-1
Group (number)Lanthanides
Period6 (f-block)
ColorSilvery-white metallic substance
Physical stateSolid at room temperature
Half-lifeFrom 0.5 seconds to 17.7 years
Electronegativity according to PaulingUnknown
Density6.475 at 20°C
Melting point1042°C, 1908°F, 1315 K
Boiling point3000°C, 5432°F, 3273 K
Van der Waals radiusUnknown
Ionic radiusUnknown
Most characteristic isotope145Pm, 147Pm
Electronic shell[Xe] 4f5 6s2
The energy of the first ionization534.6 kJ.mol-1
The energy of the second ionization1050 kJ.mol-1
Discovery dateIn 1945 by Jacob .A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell

With the periodic table symbol Pm, atomic number 61, atomic mass of X g.mol-1, and electron configuration [Xe] 4f5 6s2, promethium is a highly radioactive rare-earth metal that emits beta radiation. It reaches its boiling point at 3000°C, 5432°F, 3273 K, while the melting point is achieved at 1042°C, 1908°F, 1315 K. 

While promethium does not display ferromagnetic properties, it does respond to an external magnetic field which makes this element somewhat magnetic. Metallic promethium is silvery-white in color. On the other hand, its radioactivity makes it radiate with a pale blue or green glow in the dark.   

There are two allotropes of promethium:

  • The α-phase (a double close-packed hexagonal structure with a=3.65 Å), and
  • The β-phase (a body-centered cubic structure with a=4.10 Å).     

Until the present day, most of the physical and chemical properties of this rare-earth metal (such as the Van der Waals radius, the ionic radius, and the electronegativity) have not been studied.

How Was Promethium Discovered?

Based upon chemical calculation, the Czech chemist Bohuslav Brauner (1855 – 1935) predicted the existence of element 61 between neodymium (element 60) and samarium (element 62) in 1902. While this claim was confirmed by the British physicist Henry Moseley (1887-1915), many of his fellow chemists unsuccessfully tried to confirm his postulate. This was mainly due to the fact that the separation of promethium from other elements proved to be an especially hard task to perform at that time.

In 1926, the American chemist B. Smith Hopkins (1873-1952) analyzed rare-earth residues of the mineral monazite. The result Hopkins obtained from his experiment made him believe that he had discovered the element with atomic number 61, which he labeled as illinium. By choosing this name for the new substance, the American chemist wanted to honor the University of Illinois where he was conducting his scientific work.

The same year Hopkins revealed his scientific findings on element 61 (illinium), the Italian chemist Dr. Luigi Rolla from the Royal University in Florence published his evidence on the discovery of the new element. Rolla also conveniently labeled his discovery as florentium, as a nod to his University where he observed the new substance two years prior to Hopkins’ discovery. 

Scientists from all over the world were struggling to fill in the gap of element 61 in the periodic table for decades. Finally,  the team of American scientists which included Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell succeeded in producing the first promethium atom as a fission product of uranium. 

These scientists managed to isolate promethium-147 isotope (with a 2.62-year half-life) and promethium-149 isotope (with a 53-hour half-life) from the uranium fission products at Clinton Laboratories (now Oak Ridge National Laboratory) in Tennessee, United States. They applied the method of ion-exchange chromatography to separate promethium from the fission products of uranium fuel, which has been obtained from a nuclear reactor that was used to produce plutonium for making an atomic bomb during World War II. 

The first sample of promethium metal prepared in 1963 by reduction of the fluoride (PmF3) with lithium.

How Did Promethium Get Its Name?

This chemical element was named after the Greek God of fire – Prometheus. His name has the meaning of ‘foresight’. In Greek mythology, Prometheus was one of the Titans. He was considered to be the creator of man. 

According to the legend, Prometheus climbed Mount Olympus and stole fire from Hephaestus, the God of the forge, with a noble intention – to present humanity with the gift of fire, thus giving rise to the civilization of people. 

The name promethium for the chemical element 61 was accepted in 1949 by the International Union of Pure and Applied Chemistry (IUPAC).

Where Can You Find Promethium?

Despite being a synthetically produced element, trace quantities of promethium can be obtained as a byproduct of spontaneous fission of two uranium isotopes ( 238U and 235U) as well as by alpha decay of 151Eu isotopes. Promethium can also be produced by bombarding neodymium-146 with neutrons. In addition, astronomers have observed promethium in the spectra of stars that belong to the Andromeda galaxy.

Promethium in Everyday Life

Element 61 is not a substance that we often see integrated into products we use in our everyday lives. The reason behind the limited practical use of promethium is the rarity of this substance, as well as its high radioactivity. However, in several instances promethium is successfully applied, namely:

  • Promethium can be applied as a beta source for thickness gauges, superconductor, in atomic batteries for spacecraft, pacemakers, as well as in guided missiles;
  • Being a rare-earth metal, promethium is used in some of the world’s strongest magnets;
  • In the future, this radioactive chemical element  could  be used as a source of portable x-ray sources, as an auxiliary heat or power source for space probes and satellites;
  • Element 61 is used as a source of radioactivity in measuring instruments;
  • The beta radiation emitted by promethium is used in the light sources that use phosphors that absorb this radiation and convert it to visible light.

How Dangerous Is Promethium?

Being a highly radioactive substance, promethium can be harmful and pose great danger to the health of the scientists who study its properties or work in the industries that employ this chemical element in their production.

Exposure to any radioactive substance may lead to DNA damage by destroying the cellular mechanisms. The most frequently occurring symptoms that point out to radiation exposure are:

  • Nausea;
  • Vomiting;
  • Hair loss;
  • Diarrhea;
  • Hemorrhage;
  • Destruction of the intestinal lining;
  • Damage of the central nervous system;
  • Death.

Environmental Effects of Promethium

Due to the fact that this rare chemical element cannot be found in the environment, it cannot be considered as a hazardous substance upon the geological, biological, or aquatic systems. 

Isotopes of Promethium

There are 38 observed isotopes of promethium. All forms of this artificially produced element are radioactive and none of them are naturally occurring isotopes. Just like in all radioactive substances there aren’t any stable isotopes of promethium either. 

With a half-life of 17.7 years, the promethium-145 isotope is the most stable radioactive form of element 61. The least stable radioisotope of this chemical is promethium-128 – it has a half-life of one second.

Most of the promethium forms have a half-life of 30 seconds. The decay mode of isotopes lighter than the promethium-145 form is electron capture, while beta decay is the primary mode for the isotopes of promethium that are heavier than 145Pm.


[n 1]

ZNIsotopic mass (Da)

[n 2][n 3]


[n 4]



[n 5]



[n 6][n 7]

Spin and


[n 8][n 4]



Excitation energy[n 4]
126Pm6165125.95752(54)#0.5# s    
127Pm6166126.95163(64)#1# s  5/2+# 
128Pm6167127.94842(43)#1.0(3) sβ+128Nd6+# 
129Pm6168128.94316(43)#3# s [>200 ns]β+129Nd5/2+# 
130Pm6169129.94045(32)#2.6(2) sβ+130Nd(5+, 6+, 4+) 
β+, p (rare)129Pr
131Pm6170130.93587(21)#6.3(8) sβ+, p130Pr5/2+# 
132Pm6171131.93375(21)#6.2(6) sβ+132Nd(3+) 
β+, p (5×10−5%)131Pr
133Pm6172132.92978(5)15(3) sβ+133Nd(3/2+) 
134Pm6173133.92835(6)22(1) sβ+134Nd(5+) 
135Pm6174134.92488(6)49(3) sβ+135Nd(5/2+, 3/2+) 
136Pm6175135.92357(8)107(6) sβ+136Nd(5−) 
137Pm6176136.920479(14)2# minβ+137Nd5/2+# 
138Pm6177137.919548(30)10(2) sβ+138Nd1+# 
139Pm6178138.916804(14)4.15(5) minβ+139Nd(5/2)+ 
140Pm6179139.91604(4)9.2(2) sβ+140Nd1+ 
141Pm6180140.913555(15)20.90(5) minβ+141Nd5/2+ 
142Pm6181141.912874(27)40.5(5) sβ+142Nd1+ 
143Pm6182142.910933(4)265(7) dβ+143Nd5/2+ 
144Pm6183143.912591(3)363(14) dβ+144Nd5− 
145Pm6184144.912749(3)17.7(4) yEC145Nd5/2+ 
α (2.8×10−7%)141Pr
146Pm6185145.914696(5)5.53(5) yEC (66%)146Nd3− 
β (34%)146Sm
147Pm[n 9]6186146.9151385(26)2.6234(2) yβ147Sm7/2+Trace[n 10]
148Pm6187147.917475(7)5.368(2) dβ148Sm1− 
149Pm[n 9]6188148.918334(4)53.08(5) hβ149Sm7/2+ 
150Pm6189149.920984(22)2.68(2) hβ150Sm(1−) 
151Pm[n 9]6190150.921207(6)28.40(4) hβ151Sm5/2+ 
152Pm6191151.923497(28)4.12(8) minβ152Sm1+ 
153Pm6192152.924117(12)5.25(2) minβ153Sm5/2− 
154Pm6193153.92646(5)1.73(10) minβ154Sm(0, 1) 
155Pm6194154.92810(3)41.5(2) sβ155Sm(5/2−) 
156Pm6195155.93106(4)26.70(10) sβ156Sm4− 
157Pm6196156.93304(12)10.56(10) sβ157Sm(5/2−) 
158Pm6197157.93656(14)4.8(5) sβ158Sm  
159Pm6198158.93897(21)#1.47(15) sβ159Sm5/2−# 
160Pm6199159.94299(32)#2# sβ160Sm  
161Pm61100160.94586(54)#700# msβ161Sm5/2−# 
162Pm61101161.95029(75)#500# msβ162Sm  
163Pm61102162.95368(86)#200# msβ163Sm5/2−# 

Source: Wikipedia

List of Promethium Compounds 

The oxidation state of +3 is the only stable bond promethium forms when this chemical is a part of a compound. As a result of its radioactivity, promethium salts radiate a pale blue or green luminesce. 

The following promethium compounds are most frequently prepared:

  • Promethium(III) chloride
  • Promethium(III) fluoride
  • Promethium(III) hydroxide
  • Promethium(III) oxide

5 Interesting Facts and Explanations

  1. Promethium was discovered as the last rare-earth element in the lanthanide series of the periodic table.
  2. The work of Bohuslav Brauner was largely based on the perfection of Mendeleev’s periodic law and system of classification of the chemical elements. Employed at the University of Prague, this Czech chemist was especially interested in clarifying those positions in the periodic table that were reserved for several elements that were yet to be discovered. 
  3. Element 61 is the only rare-earth radioactive metal.
  4. Rare-earth metals are applied in the strongest magnets and superconductors that have ever been produced in the world. The group of rare-earth metals includes the elements scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium
  5. Nowadays, promethium can be successfully isolated from other rare-earth fission products by the ion-exchange method.