Promethium (Pm)

Promethium (symbol Pm) is the chemical element with atomic number 61: a lanthanide (rare earth), solid at 0 °C, radioactive, with an atomic mass of 145 u. Discovered in 1945 (Jacob Marinsky, Lawrence Glendenin and Charles Coryell).

Promethium is the only lanthanide that is entirely radioactive: all its isotopes decay, the longest-lived having a half-life of 17.7 years. As a result it is almost absent from Earth and has to be made in nuclear reactors. It has been used in luminous paints and atomic micro-batteries.

Key facts

Atomic number: 61
Symbol: Pm
French name: Prométhium
Atomic mass: 145 u
Molar mass: [145] g/mol
Family: Lanthanides
Position: period 6, f-block
State at 0 °C: Solid
Melting point: 1,041.9 °C
Boiling point: 2,999.9 °C
Density: 7.26 g/cm³
Electronegativity (Pauling): 1.13
First ionization energy: 540 kJ/mol
Atomic radius: 185 pm
Electron configuration: [Xe] 4f⁵ 6s²
Indicative price: 460,000 $/kg (order of magnitude)
Radioactive: yes
Discovery: 1945, Jacob Marinsky, Lawrence Glendenin and Charles Coryell

Origin of the name

From Prometheus, who stole fire from the gods.

What does promethium look like?

A silvery metal rarely seen in any quantity. Its salts give off a blue-green glow driven by their own radioactivity.

Where is promethium found?

Only vanishing traces exist in Earth's crust, produced by the spontaneous fission of uranium in its ores: a few hundred grams at most for the whole planet. It has also been detected in the spectra of a few peculiar stars, such as Przybylski's Star.

Natural sources

Traces in uranium ores (pitchblende)

How is promethium produced?

It is recovered from the fission waste of nuclear reactors, or made by bombarding neodymium with neutrons. The isotope promethium-147 is the most widely used.

What is promethium used for?

Nuclear micro-batteries (betavoltaics) for pacemakers and space probes, in the past.
Luminous paint for watch dials, replacing radium.
Industrial gauges measuring the thickness of paper or plastic sheets.
Research in the physics and chemistry of radioactive elements.

In everyday life

Glow-in-the-dark watch dials from the 1960s–1970s

Role in living things

No biological role. If ingested, it would lodge mainly in the bones.

Hazards of promethium

Radioactive: it emits beta rays. Handling is restricted to equipped laboratories, with protection against ingestion and inhalation.

Cosmic origin

Human-made: Does not exist (or barely) in nature: synthesized in a reactor or accelerator.

History of promethium

Its empty slot in the table prompted many false discoveries (“illinium,” “florentium”). It was finally identified in 1945 by Jacob Marinsky, Lawrence Glendenin and Charles Coryell at Oak Ridge, among the fission products of uranium. Grace Mary Coryell suggested naming it after Prometheus, the Titan who stole fire from the gods.

Did you know?

It wasn't until 2024 that an Oak Ridge team characterized a promethium molecule in solution for the first time.
Its name recalls Prometheus, punished for stealing fire — a nod to the dangers of nuclear power.
Along with technetium, it is one of only two elements lighter than lead with no stable isotope.

Isotopes of promethium

Promethium has no stable isotopes at all: along with technetium, it is one of only two light elements in that situation. Only vanishing traces of it are found in uranium ores.

promethium-145: radioactive, half-life 17.7 years. The longest-lived isotope of promethium.
promethium-146: radioactive, half-life 5.53 years
promethium-147: radioactive, half-life 2.62 years. Used in luminous paints, miniature nuclear batteries and thickness gauges.
promethium-149: radioactive, half-life 2.21 days

Related elements

Same family (lanthanides): Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb), Lutetium (Lu).