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Promethium

Element 61 · Lanthanide

Specimen

Atomic number
61
Atomic mass
[145]
State at room temperature
Solid
Melting point
1,042 °C
Boiling point
3,000 °C
Density
7.26 g/cm³
Discovered
1945

Mass number of the longest-lived isotope, from IUPAC.1 Other physical data from PubChem.2

Can I eat it?

In food and water
Radioactive. All promethium is radioactive. Its longest-lived isotope has a half-life of 18 years, so it does not occur naturally on Earth. 3

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The atom

Pm
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 23
5 (O) 8
6 (P) outermost 2
Total61

Configuration [Xe] 4f⁵ 6s²

4f ↑ ↑ ↑ ↑ ↑
6s ↑ ↓
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 61 protons. The outermost shell is shown in red. Electrons do not travel on rings: they fill orbitals, regions where an electron is likely to be found. The boxes are those orbitals for the electrons outside the Xe core, with one arrow per electron.

Related elements

Lanthanides
La Lanthanum Ce Cerium Pr Praseodymium Nd Neodymium Sm Samarium Eu Europium Gd Gadolinium Tb Terbium Dy Dysprosium Ho Holmium Er Erbium Tm Thulium Yb Ytterbium Lu Lutetium

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Sources

  1. 1

    IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024. Radioactive Elements .

    Mass number and half-life of the longest-lived isotope of the 34 elements with no standard atomic weight.

  2. 2

    National Center for Biotechnology Information (PubChem), 2026. Periodic Table of Elements .

    Electron configuration, melting and boiling points, density and discovery year for all 118 elements.

  3. 3

    Royal Society of Chemistry, 2026. Promethium - Element information, properties and uses .

    • A radioactive metal. ... Promethium’s longest-lived isotope has a half-life of only 18 years. For this reason it is not found naturally on Earth.