Promethium (Pm)
Element 61 · Lanthanide · atomic mass [145] u · f-block, period 6
Electron shells: 2, 8, 18, 23, 8, 2
Promethium is the only rare earth with no stable isotope, so it barely exists in nature. It was found in the fission products of a nuclear reactor.
There is thought to be well under a kilogram of natural promethium in the entire Earth's crust.
- Atomic number
- 61
- Atomic mass
- [145] u
- Electron configuration
- [Xe] 6s² 4f⁵
- Group · period · block
- f-block · 6 · f
- Oxidation states
- +3
- Electronegativity
- Not measured
- Melting point
- 1,042 °C
- Boiling point
- 3,000 °C
- Density
- 7.26 g/cm³
- State at 20 °C
- Solid
- Appearance
- Metallic, faintly glowing
- Discovered
- Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell, 1945
61 protons, and 61 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Shown in the strip below the table
What Promethium is used for
- Nuclear batteries for spacecraft
- Thickness-measuring gauges
- Luminous paint research
Atomic structure of Promethium
A neutral promethium atom has 61 protons and 61 electrons. Its most common isotope carries about 84 neutrons, which is where the mass number 145 comes from. Change the neutron count and you have a different isotope of promethium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 23, 8, 2. In orbital notation that is [Xe] 6s² 4f⁵. As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Promethium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Promethium sits in the periodic table
Discovery and naming
Promethium was discovered by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell in 1945. Its name comes from Prometheus, who stole fire in Greek myth, which is also why its symbol (Pm) does not match its English name.
Every isotope of promethium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Promethium?
Promethium has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 145 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Promethium have?
Promethium has 61 protons, because the atomic number is the proton count and that is what makes an atom Promethium at all. A neutral Promethium atom has the same number of electrons, 61. The neutron count varies by isotope; the most common one has about 84, since 145 − 61 = 84.
What is the electron configuration of Promethium?
The electron configuration of Promethium is [Xe] 6s² 4f⁵. Its electrons are arranged 2, 8, 18, 23, 8, 2 across 6 shells.
Is Promethium a metal?
Yes. Promethium is classified as a lanthanide, on the metallic side of the table.
What group and period is Promethium in?
Promethium sits in the f-block strip below the main table, in period 6, among the lanthanides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Promethium?
Promethium most commonly forms a 3+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound.
What is the melting point of Promethium?
Promethium melts at 1,042 °C and boils at 3,000 °C. That puts it in the solids at everyday temperatures.
What is Promethium used for?
Nuclear batteries for spacecraft, Thickness-measuring gauges, Luminous paint research. Promethium is the only rare earth with no stable isotope, so it barely exists in nature.
Who discovered Promethium, and where does its name come from?
Promethium (Pm) was discovered by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell in 1945. The name comes from Prometheus, who stole fire in Greek myth.
Is Promethium radioactive?
Yes. Every isotope of Promethium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.