Promethium

Promethium is the only lanthanide with no stable isotopes, all of its atoms are radioactive and decay. It does not occur in nature in any measurable quantity on Earth. Named after Prometheus, the titan who stole fire from the gods, it was the last naturally occurring element to be discovered.

• Atomic Number6161 protons, 61 electrons
• Atomic Mass144.91276 u61× heavier than hydrogen
• State at Room TempSolidSolid
• Density7.26 g/cm³
• Melting / Boiling1041.8°C / 2999.8°C
• Discovered1945

What is Promethium?

Promethium is a radioactive lanthanide with 61 protons that produces beta radiation as it decays. The most stable isotope, promethium-145, has a half-life of just 17.7 years. Any primordial promethium formed when the Earth was born has long since decayed away entirely.

Named after Prometheus, the Titan of Greek mythology who stole fire from the gods and gave it to humans, because the element was finally produced artificially rather than found in nature. Discovered at Oak Ridge National Laboratory in 1945 by Jacob Marinsky, Lawrence Glendenin and Charles Coryell, using uranium fission products.

Fact Promethium had been predicted by Moseley's periodic table analysis. Its discovery was delayed because it has no stable isotopes. When finally found in uranium fission products in 1945, it filled the last gap in the known periodic table among the naturally occurring elements.

Where you find Promethium

On Earth

Promethium does not occur in nature. It is produced as a fission product in nuclear reactors, or by bombarding neodymium with neutrons.

How we use Promethium

• Promethium-147 was used in early atomic batteries (betavoltaic cells), the beta radiation excites a phosphor that generates electricity. Used in some satellites and pacemakers in the 1950s-60s.. Nuclear batteries
• Promethium-147 was used in luminous paint for instrument dials, replacing the radium that caused radiation poisoning in dial-painters.. Luminous dials

How it was discovered

Named after Prometheus, the Titan of Greek mythology who stole fire from the gods and gave it to humans, because the element was finally produced artificially rather than found in nature. Discovered at Oak Ridge National Laboratory in 1945 by Jacob Marinsky, Lawrence Glendenin and Charles Coryell, using uranium fission products.

Deeper dive: promethium and the lanthanide series

The lanthanides (elements 57-71) are characterised by the progressive filling of the 4f electron subshell. Because the 4f electrons are deep inside the atom and shielded by outer electrons, they have little effect on chemical bonding. All lanthanides have very similar chemical behaviour, forming +3 ions of comparable size. This similarity makes them extraordinarily difficult to separate from each other, historically requiring hundreds of fractional crystallisation steps. Ion exchange chromatography and solvent extraction methods, developed in the 1940s, finally made pure lanthanides available in quantity.

The term "rare earth" is historically misleading. Most lanthanides are as abundant as copper or nickel in the Earth's crust. The challenge is not scarcity but concentration: they are geochemically dispersed and rarely form rich mineral deposits. The name stuck from the 18th century when they were genuinely difficult to isolate.

Moving to 62 protons brings us to the next element in this remarkable family.