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Pm

Promethium

Promethium (Pm) is a lanthanide with atomic number 61. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight; its longest-lived isotope has mass number 145. Its electron configuration is [Xe] 4f5 6s2.

Promethium key data

Symbol Pm
Atomic number 61
Mass number (longest-lived isotope) 145
Protons 61
Neutrons (longest-lived isotope, promethium-145) 84
Electrons (neutral atom) 61
Category Lanthanide
Period 6
Group f-block, no group
Block f-block
Noble gas notation [Xe] 4f5 6s2
Full electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f5 5s2 5p6 6s2
Electrons per shell 2, 8, 18, 23, 8, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.

Measured properties of promethium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown. The first ionisation energy is the value for promethium from the NIST Atomic Spectra Database, in place of PubChem’s 5.55 eV.

Melting point 1315 K (1041.9 °C), 47th of 103 by melting point
Boiling point 3273 K (2999.9 °C), 33rd of 93 by boiling point
Density 7.26 g/cm³, 46th of 96 by density
State at room temperature Solid
Van der Waals radius 236 pm, 22nd of 99 by atomic radius
First ionisation energy 5.5819 eV (538.57 kJ/mol), 88th of 102 by first ionisation energy
Common oxidation states +3
Discovered 1945

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Promethium electron configuration

The full electron configuration of promethium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f5 5s2 5p6 6s2. With the noble-gas core collapsed it is [Xe] 4f5 6s2, and the outermost subshell is 6s2.

Bohr model of promethium (Pm): a nucleus and six shells holding 2, 8, 18, 23, 8 and 2 electrons from the inside out.
Bohr model of promethium (Pm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.
Orbital box diagram of promethium (Pm), [Xe] 4f5 6s2, with the xenon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing five unpaired electrons, in 4f.
Orbital diagram of promethium (Pm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one promethium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

  1. one atom = 145 / (6.02214076 × 10²³)
  2. = 2.4078 × 10⁻²² g

Atoms in one gram

  1. atoms per gram = 6.02214076 × 10²³ / 145
  2. = 4.1532 × 10²¹ atoms

Promethium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of promethium-145, so treat these as nominal.

Promethium on the periodic table

Promethium is in period 6, outside the numbered groups, in the f-block.

The rest of period 6

Work with promethium