Promethium (Pm)

Promethium is the only lanthanide with no stable isotope, and there is thought to be well under a kilogram of it in the entire Earth's crust at any moment, produced by rare spontaneous fission in uranium ore. It was the last of the naturally occurring rare earths to be identified, and only after nuclear reactors made enough of it to work with — it was pulled out of fission products at Oak Ridge in 1945. Named after Prometheus, who stole fire, which the discoverers meant as a comment on nuclear energy as much as on the element.

Properties

Atomic number

61

Symbol

Pm

Mass number of most stable isotope

[145]

Category

Lanthanide

Period

6

Block

f-block

Electron configuration

[Xe] 6s² 4f⁵

Electrons per shell

2, 8, 18, 23, 8, 2

Melting point

1,315 K — 1041.8 °C — 1907.3 °F

Boiling point

3,273 K — 2999.8 °C — 5431.7 °F

Density

7.26 g/cm³

Oxidation states

+3

Discovered

1945 by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f⁵

Noble gas shorthand

[Xe] 6s² 4f⁵

Shells

K: 2 · L: 8 · M: 18 · N: 23 · O: 8 · P: 2

What it is used for

• Nuclear batteries for pacemakers and remote instruments, historically
• Beta-emitting thickness gauges in paper and sheet metal mills
• Luminous paint on aircraft instrument dials, replacing radium

Hazards

• All isotopes are radioactive; promethium-147 is a beta emitter with a 2.6-year half-life

No stable isotope; the mass given is the mass number of promethium-145, the longest-lived at about 17.7 years. No Pauling electronegativity is listed in the standard tables. The melting point and density are measured, but on samples of a few milligrams.

Neighbours in the table