Promethium

At a glance

Symbol

Pm

Atomic number

61

Mass number of its longest-lived isotope

[145]

Group

none; it sits in the lanthanide row below the table

Period

6

Family

lanthanide

Electron configuration

[Xe] 4f⁵ 6s²

Electronegativity

not measured

Common ions

Pm³⁺

Isotopes

none are stable; ¹⁴⁵Pm lasts longest, with a half-life of about 18 years, and ¹⁴⁷Pm is the most used

On the table

Period 6, in the lanthanide row set below the main table. The other lanthanides are tinted, and every cell opens its own article.

Electrons and bonding

Like the other lanthanides, it gives up three electrons to form the Pm³⁺ ion in most of its compounds.

Properties

State at 25 °C

solid

What it looks like

silvery, radioactive metal

Melting point

1042 °C

Boiling point

3000 °C

Density

7.26 g/cm³

Where it is found

Almost none exists in nature: only about half a kilogram is thought to exist in the Earth's crust at any moment, made by the rare spontaneous splitting of uranium atoms. Nearly all of it is made in nuclear reactors.

Discovery and name

Its place in the table sat empty for decades while several claimed discoveries fell apart. Jacob Marinsky, Lawrence Glendenin and Charles Coryell identified it in 1945 at Oak Ridge, among the products of uranium fission. Coryell's wife, Grace Mary, suggested naming it after Prometheus, who stole fire from the gods.