Promethium (Pm), Element 61: Facts and Uses

What is promethium?

Promethium is a radioactive metal. All its forms are unstable, so it is almost absent from the Earth's crust and is made in reactors.

What is promethium used for?

Small specialised uses such as thickness gauges and research. In the past it was used in glow-in-the-dark paint.

Where the name comes from

From Prometheus of Greek myth, who stole fire from the gods.

Key facts

Symbol: Pm
Atomic number: 61
Atomic mass: 144.91276 u
Family: Lanthanide
Period: 6
Group: none (f-block)
Block: f
State at room temperature: Solid
Melting point: 1,042 °C (1,315 K, 1,907 °F)
Boiling point: 3,000 °C (3,273 K, 5,432 °F)
Density: 7.26 g/cm³
Electrons per shell: 2, 8, 18, 23, 8, 2
Electron configuration: [Xe]6s2 4f5
Common oxidation states: +3
Electronegativity (Pauling): no value
First ionisation energy: 5.55 eV
Electron affinity: no value
Atomic radius (van der Waals): 236 pm
Discovered: 1945

Place in the periodic table

Promethium is element number 61. It sits in period 6 (row 6) of the periodic table, in the separate row of lanthanides drawn below the main table, and belongs to the f-block. Just before it comes neodymium (60), and just after it comes samarium (62).

Family: Lanthanides.

Physical properties

At room temperature promethium is a solid. It melts at 1,042 °C and boils at 3,000 °C, so it stays liquid over a range of 1,958 degrees. Its density is 7.26 grams per cubic centimetre, so a one-litre block would weigh about 7.26 kilograms. By density it ranks 46th among the 96 elements with a measured value.

Inside the atom

An atom of promethium has 61 protons in its nucleus and, when neutral, 61 electrons around it. The electrons are arranged in 6 shells: 2, 8, 18, 23, 8, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Xe]6s2 4f5.

Removing one electron from an atom takes 5.55 electronvolts (about 535 kilojoules per mole). One mole of promethium weighs 144.91276 grams, so a single gram contains about 4.16 × 10²¹ atoms.

The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.

Next steps

Work out the weight of a compound that contains promethium with the molar mass calculator.
Compare it with every other element in the sortable list.
Test yourself with the symbol quiz.

Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.