Promethium Facts – Element Symbol Pm or Atomic Number 61

Promethium (symbol Pm and atomic number 61) is a rare earth element and one of only two elements with atomic numbers under 83 that have no stable isotopes (the other is technetium). It belongs to the lanthanide series and is known for its scarcity and radioactivity. Most of the element comes from synthetic processes, although it also occurs in trace amounts naturally.

History of Discovery, Naming, Synthesis, and Isolation

Prediction of Its Existence

The existence of an element between neodymium (Z = 60) and samarium (Z = 62) was predicted in the early 20th century by the gaps in the periodic table. Henry Moseley’s work in 1913 using X-ray spectra showed that no element with atomic number 61 had been identified. This missing element was provisionally called “eka-samarium” and later “illinium” or “florentium” by those claiming discovery.

False Discoveries

• 1926 – Italian Chemists: Luigi Rolla and Lorenzo Fernandes in Florence claimed discovery and proposed the name “Florentium.”
• 1926 – Illinois Team: Scientists at the University of Illinois also announced a discovery and proposed the name “Illinium.”
• These claims were later disproven due to lack of verifiable samples.

Verified Discovery

• 1945 – Oak Ridge National Laboratory: Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell chemically isolated promethium at Clinton Laboratories (now Oak Ridge National Laboratory) from fission products of uranium.
• They confirmed the element using ion-exchange chromatography but did not publish their findings until 1947 due to World War II secrecy.

Naming

The initial proposed name for the new element was “clintonium” for the laboratory. Ultimately, the element was named prometheum and finally promethium after Prometheus, the Titan in Greek mythology who stole fire from the gods and gave it to humanity. The name symbolizes the element’s radioactive energy and the potential dangers and benefits it could bring to mankind.

Location on the Periodic Table and Group

Promethium is in period 6 and the f-block of the periodic table. It is between neodymium and samarium in the lanthanide series and share many common characteristics with these elements, including its silvery metallic appearance, softness, reactivity with air and water, and a predominant +3 oxidation state. Although promethium generally follows the chemical trends of the lanthanide series, it shows a slight deviation from the smooth decrease in ionic radii known as the lanthanide contraction.

However, promethium is unique among the lanthanides in that it has no stable isotopes—all of its known isotopes are radioactive. This sets it apart from its otherwise chemically similar neighbors and gives it properties such as the ability to ionize air and induce faint luminescence in some compounds.

Compared to its homologs in group 3, such as lanthanum (above) and neptunium (below), promethium maintains the lanthanide trend of limited oxidation states and relatively straightforward chemistry. This is in contrast to the actinides like neptunium, which exhibit a wide range of oxidation states (+3 to +7) and more complex electronic structures due to the involvement of 5f orbitals.

Appearance and Allotropes

Promethium is a silvery metal with a luster similar to other rare earth elements. Its compounds faintly blue or green in the dark due to its intense radioactivity. However, the metallic element does not glow appreciably because it primarily emits beta radiation that does not ionize air significantly.

Two allotropes of promethium are the double hexagonal close packed (dhcp) alpha form that occurs under ordinary conditions and the body-centered cubic (bcc) beta form, which forms upon heating the element to 890 °C.

A solution containing Pm³⁺ ions has a red or pink color. Solid salts vary in color, including brown, white, pink, and yellow compounds.

Characteristics

In general, promethium shares characteristics with other lanthanides. The exception is its beta emission:

• Radioactive: All isotopes are unstable.
• Metallic: Soft and malleable.
• Chemically reactive: Reacts slowly with oxygen and water to form oxide layers.
• Emits beta radiation, but not significant gamma rays.
• Sparks in air when powdered or finely divided due to oxidation.

Isotopes of Promethium

Promethium has no stable isotopes. Most isotopes decay to neodymium (Z = 60) or samarium (Z = 62) by beta decay. The most stable isotope is promethium-145, which has a half-life of 17.7 years via electron capture. The element also has 18 nuclear isomers, with mass numbers ranging from 133 to 154.

Notable Isotopes:

Origin, Abundance, and Sources

Primordial Synthesis

Promethium forms during supernova nucleosynthesis or neutron star mergers, but all primordial promethium on Earth has decayed due to its short half-lives.

Natural Occurrence

However, the element does exist in nature today as part of the decay scheme of actinide elements.

• Found in trace amounts in uranium ores (e.g., pitchblende) via spontaneous fission.
• Exists in extremely small quantities in stars.
• Estimated natural Earth abundance: < 600 grams in the entire crust at any given time.

Artificial Production

For practical applications, scientists synthesize promethium rather than isolate it from ores.

• Nuclear Reactors: Produced by neutron bombardment of neodymium-146 or uranium fission.
• Example reaction:
  Nd-146 + n → Nd-147 → Pm-147 + β⁻
• Spent nuclear fuel is a major source.

Uses of Promethium

Despite its rarity, promethium has several niche but important uses:

1. Nuclear Batteries
   • Pm-147 is used in betavoltaic cells to power:
     • Pacemakers (historically)
     • Spacecraft and satellites
     • Remote sensing equipment
2. Luminous Paints
   • Combined with phosphors to produce a glow in watches, signs, and instruments (replacing radium).
3. Thickness Gauges
   • Beta radiation from Pm-147 used in measuring material thickness (paper, metals, plastics).
4. Portable X-Ray Sources
   • Beta radiation can induce X-rays in target materials.
5. Scientific Research
   • Used in studying nuclear decay processes and behavior of lanthanides.

Oxidation States

Promethium primarily shows the +3 oxidation state, which is typical of lanthanides. It also forms the +2 oxidation state, although this is much less stable.

Biological Role, Health Effects, and Toxicity

Biological Role

• No known biological function in humans or other organisms.
• Does not naturally occur in biological systems.

Health Effects

• Highly radioactive: Emits beta particles and in some cases gamma radiation that damages living tissues.
• If ingested or inhaled, it accumulates in bone marrow, posing a cancer risk.
• External beta radiation can cause skin burns.

Toxicity

• Similar to other rare earths chemically, but radioactivity makes it far more dangerous.
• Strict handling procedures required in labs.
• Limited ecological impact due to its rarity and containment.

Key Promethium Facts for Scientists

Interesting Promethium Facts

• Promethium is the only lanthanide with no stable isotopes.
• You can’t buy promethium watches anymore. They’re banned in many countries due to radioactivity.
• Promethium was the last lanthanide to be discovered.
• Some white dwarfs and red giants show promethium spectral lines, indicating recent nucleosynthesis.
• Promethium has potential use in laser devices and as a portable heat source for electronics in extreme environments.
• It takes its name for a mythical rebel, unlike most lanthanides named after places or people.

References

• Cowley, C. R.; Bidelman, W. P.; et al. (2004). “On the possible presence of promethium in the spectra of HD 101065 (Przybylski’s star) and HD 965”. Astronomy & Astrophysics. 419 (3): 1087–1093. doi:10.1051/0004-6361:20035726
• Emsley, John (2011). Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford University Press. ISBN 978-0-19-960563-7.
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Marinsky, J. A.; Glendenin, L. E.; Coryell, C. D. (1947). “The chemical identification of radioisotopes of neodymium and of element 61”. Journal of the American Chemical Society. 69 (11): 2781–5. doi:10.1021/ja01203a059
• Pallmer, P. G.; Chikalla, T. D. (1971). “The crystal structure of promethium”. Journal of the Less Common Metals. 24 (3): 233. doi:10.1016/0022-5088(71)90101-9