Why does promethium have only the +3 oxidation state?

Promethium has the electron configuration [Xe] 4f⁵ 6s². The 4f electrons are deeply shielded by the outer 5s, 5p, and 6s shells and experience high effective nuclear charge, making them energetically inaccessible for bonding under ordinary conditions. All lanthanides share this pattern — the 6s² and one 4f electron (or 5d¹) are used to reach +3, and going beyond that requires far more energy than is chemically available. A +2 state has been reported in molten-salt systems but is not stable in aqueous chemistry.

Does promethium occur naturally?

Essentially no. Promethium has no stable isotopes; all nuclei are radioactive. Trace amounts exist transiently in uranium ores via spontaneous fission, and ¹⁴⁷Pm is continuously produced in nuclear reactors as a fission product of uranium-235. All practical quantities of promethium are synthetically produced in nuclear reactors. It was first isolated in 1945 from uranium fission products at Oak Ridge National Laboratory.

Which promethium isotope is most commonly used?

¹⁴⁷Pm is the workhorse isotope. It has a half-life of 2.6234 years, emits beta particles (0.224 MeV, no significant gamma), and is produced in gram quantities in nuclear reactors. It was used in luminescent paints and nuclear batteries (betavoltaic cells). ¹⁴⁵Pm (17.7-year half-life) and ¹⁴⁶Pm (5.53 years) are also studied but harder to produce in quantity.

What compounds of promethium are known?

Because promethium is radioactive and difficult to produce, only a handful of compounds have been characterized: the trichloride (PmCl₃), tribromide (PmBr₃), trifluoride (PmF₃), trioxide (Pm₂O₃), and nitrate Pm(NO₃)₃. All feature Pm in the +3 state. Their properties are inferred largely from neighboring lanthanide homologs (neodymium and samarium) due to the scarcity of promethium.

What is the lanthanide contraction and how does it affect Pm?

Across the lanthanide series (La → Lu), nuclear charge increases by one proton per element while 4f electrons are added. The 4f shell shields poorly, so each added proton pulls the outer electrons slightly closer. This causes the atomic radius to decrease gradually from La to Lu — the lanthanide contraction. Promethium sits in the middle of the series (4f⁵) and has a typical lanthanide radius (~185 pm metallic), with properties interpolating between neodymium (Z=60) and samarium (Z=62).

Why was promethium the last natural element to be discovered?

Elements with atomic numbers 1–92 were all identified from natural sources by the 1940s — except element 61. The reason is its nuclear instability: every promethium nucleus decays on a timescale much shorter than Earth's age, so no primordial Pm survived. The element was confirmed in 1945 by Marinsky, Glendenin, and Coryell via ion-exchange chromatography of uranium fission products, earning element 61 the name Promethium after the Titan Prometheus who brought fire to humanity.

Promethium Oxidation States

Name: Promethium Oxidation States
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Author: Nham Vu

Reference for promethium (Pm, Z=61): exclusively +3 oxidation state, electron configuration, radioactive/synthetic nature, and key isotopes.

Atomic # 61 Pm Promethium

Atomic Mass

145 u (most stable ¹⁴⁵Pm)

Group

Lanthanide (f-block)

Period

Block

f-block

Electronegativity

1.13 (Pauling, estimated)

Oxidation States

+3 (only stable state)

Radioactive synthetic element. Promethium has no stable isotopes. All quantities are produced in nuclear reactors. No primordial Pm exists on Earth.

Promethium exhibits one chemically accessible oxidation state: +3. Its ground-state configuration is [Xe] 4f⁵ 6s². The three electrons used in bonding come from the 6s² pair and one 4f electron, leaving a Pm³⁺ core with a [Xe] 4f⁴ configuration. The remaining 4f electrons are shielded by the 5s² and 5p⁶ shells and do not participate in bonding under ordinary conditions. A +2 state has been reported in molten-salt matrices but is thermodynamically unstable in aqueous solution.

Oxidation State	Stability	Notes
+3	Stable — only state	The universal lanthanide state. Pm³⁺ has a [Xe] 4f⁴ configuration. Forms PmCl₃, PmF₃, Pm₂O₃, Pm(NO₃)₃, and other classic trivalent lanthanide salts. Properties interpolate between Nd³⁺ and Sm³⁺.
+2	Rare — non-aqueous only	Reported in molten-salt and solid-state systems under strongly reducing conditions. Not stable in water — Pm²⁺ reduces H₂O spontaneously. Not relevant to standard aqueous or synthetic chemistry.
0	Elemental metal only	Promethium metal (silver-colored, reactive) has been produced in microgram quantities. It tarnishes rapidly in air. The metallic state is purely academic — no structural or engineering use due to radioactivity and scarcity.

Why all lanthanides default to +3
Lanthanide chemistry is dominated by the +3 state because removing the first three electrons (6s² + one 4f or 5d¹) releases enough lattice or hydration energy to compensate the ionization cost. The 4f electrons beyond the first few are too tightly bound and well-shielded to be removed chemically. Across Ce–Lu, only Ce(+4), Sm(+2), Eu(+2), Tm(+2), and Yb(+2) deviate notably — Pm sits firmly in the standard +3-only group alongside La, Nd, Gd, Tb, Ho, Er, and Lu.

All known promethium compounds feature Pm in the +3 oxidation state. Because promethium is produced only in nuclear reactors and is intensely radioactive, experimental data come from work with microgram-to-milligram quantities under strict radiation controls. Properties are confirmed by analogy with the neighboring lanthanides neodymium (Nd, Z=60) and samarium (Sm, Z=62).

Compound	Formula	Pm State	Notes
Promethium(III) Chloride	PmCl₃	+3	Yellow crystals; most studied Pm compound. Prepared from ¹⁴⁷Pm and HCl. Isomorphous with NdCl₃. Used in early characterization work at Oak Ridge (1945–1950s).
Promethium(III) Fluoride	PmF₃	+3	Insoluble pink solid; lanthanide trifluoride structure. Pm³⁺ luminescence at ~930 nm (near-infrared) distinguishes it from neighboring lanthanides.
Promethium(III) Bromide	PmBr₃	+3	Pale rose solid; isostructural with other lanthanide tribromides. Prepared by bromination of the metal or oxide. Data mainly inferred from Nd/Sm analogy.
Promethium(III) Oxide	Pm₂O₃	+3	White solid; the most stable binary compound. Cubic or monoclinic structure depending on temperature. Forms as a final product when Pm solutions are calcined.
Promethium(III) Nitrate	Pm(NO₃)₃	+3	Soluble salt; used in aqueous radiochemistry and ion-exchange separation. Typical lanthanide nitrate coordination chemistry with nine-coordinate Pm³⁺.
Promethium(III) Sulfate	Pm₂(SO₄)₃	+3	Sparingly soluble; used in early isolation and purification of Pm from fission product mixtures. Classic behavior mirrors Nd₂(SO₄)₃ and Sm₂(SO₄)₃.

Ground State Configuration

Five 4f electrons; shielded, non-bonding in Pm(III)

Empty in ground state (no 5d¹ unlike La/Ce/Gd)

6s² pair — both electrons lost to reach +2/+3

Notation

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

Noble gas: [Xe] 4f⁵ 6s²

Pm³⁺: [Xe] 4f⁴

Ionization Steps

[Xe] 4f⁵ 6s²

Neutral atom — two 6s and five 4f valence electrons

Pm²⁺

[Xe] 4f⁵

Lose both 6s² electrons → unstable in water, reduces H₂O

Pm³⁺

[Xe] 4f⁴

Lose 6s² + one 4f electron → only stable ion; all Pm chemistry here

4f shielding explained
The 4f orbitals in Pm are radially contracted inside the 5s² and 5p⁶ shells. Those outer shells screen the 4f electrons from the ligand field almost completely — this is why lanthanide 4f transitions give sharp, atom-like emission lines rather than broad d-d bands. For Pm³⁺ the remaining four 4f electrons give a ⁵I₄ ground term (Hund's rules: L=6, S=2, J=4).

Promethium has no stable isotopes. Mass numbers 128–163 have been observed; the table below lists the most significant ones. All are radioactive. ¹⁴⁷Pm is produced in the largest quantities (kilogram scale total over decades of reactor operation) and is the only isotope with practical applications.

Isotope	Half-life	Decay Mode	Notes
¹⁴⁵Pm	17.7 years	Electron capture → ¹⁴⁵Nd	Longest-lived Pm isotope. No beta emission makes handling somewhat easier. Not produced in significant reactor quantities.
¹⁴⁶Pm	5.53 years	EC (66%) + β⁻ (34%) → ¹⁴⁶Nd/Sm	Second-longest-lived. Mixed decay modes. Produced in smaller reactor yields than ¹⁴⁷Pm.
¹⁴⁷Pm	2.6234 years	β⁻ (0.224 MeV) → ¹⁴⁷Sm	Most important isotope. Pure beta emitter (no significant gamma). Fission product of ²³⁵U; produced in gram-scale totals in power reactors. Used in betavoltaic batteries and luminescent paint (now replaced by tritium/LEDs due to radioactivity concerns).
¹⁴⁸Pm	5.368 days	β⁻ → ¹⁴⁸Sm	Short-lived; produced in reactors but decays quickly. Used in short-duration tracer studies.
¹⁴⁸ᵐPm	41.29 days	β⁻ (IT minor) → ¹⁴⁸Sm	Metastable isomer; longer-lived than ground state. Emits higher-energy beta particles.
¹⁴⁹Pm	53.08 hours	β⁻ → ¹⁴⁹Sm	Fission product; decays to stable ¹⁴⁹Sm within days. Important in reactor neutronics (Sm-149 is a significant neutron poison).

Discovery context
Element 61 was formally confirmed in 1945 by Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell at Oak Ridge National Laboratory, using ion-exchange chromatography to separate fission products of ²³⁵U from an irradiated uranium pile. Earlier claimed discoveries (1926 "illinium," 1934 "florentium") were not confirmed. The name Promethium was proposed by Grace Mary Coryell, referencing the Titan Prometheus — element 61 was literally "stolen from the fire" of a nuclear reactor.

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Summary

Reference for promethium (Pm, Z=61): exclusively +3 oxidation state, electron configuration, radioactive/synthetic nature, and key isotopes.

How it works

Click a tab — Oxidation States, Compounds, Electron Config, or Isotopes — to explore each section.
The Oxidation States panel explains why +3 is the only state and how the 4f shell drives lanthanide chemistry.
The Compounds panel lists known promethium compounds with their formulas and notes on radioactive handling.
The Electron Config panel shows the orbital filling diagram and the Pm³⁺ ion configuration.
The Isotopes panel lists the most important Pm isotopes with half-lives and decay modes.
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Last updated: 2026-07-24 · Reviewed by Nham Vu