What oxidation states does samarium exhibit?

Samarium shows +3 (dominant, stable in aqueous solution and most compounds) and +2 (accessible in solid-state compounds and as SmI2 in THF solution). The +2 state is unusually stable for a lanthanide because Sm2+ achieves a half-filled 4f7 configuration, gaining exchange-energy stabilization analogous to Mn2+ in the d-block.

Why does samarium form a stable +2 oxidation state?

Samarium has the configuration [Xe] 4f6 6s2. The Sm2+ ion has the [Xe] 4f6 configuration, and Sm3+ has [Xe] 4f5. However, the half-filled 4f7 of Eu2+ (the next element) is far more stable. For Sm, the +2 state is accessible because IE3 (2260 kJ/mol) is relatively low compared to other lanthanides, making SmI2 a kinetically viable reductant in organic chemistry even though Sm3+ is thermodynamically favored.

What is SmI2 and why is it important?

Samarium(II) iodide (SmI2, "Kagan's reagent") is a mild, selective single-electron reductant widely used in organic synthesis. It mediates ketone-aldehyde couplings, reductive cyclizations, and Barbier-type reactions under mild conditions. It reduces Sm2+ to Sm3+ while delivering one electron to the substrate, and its reduction potential can be tuned by adding HMPA or water.

What is samarium's electron configuration?

Ground state: [Xe] 4f6 6s2 (full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f6 5s2 5p6 6s2). Sm3+ loses both 6s electrons and one 4f electron: [Xe] 4f5. Sm2+ loses only the 6s electrons: [Xe] 4f6.

How does samarium compare to other lanthanides in terms of oxidation states?

Most lanthanides are exclusively +3. Only Ce (+4), Pr (+4), Tb (+4), Eu (+2), Sm (+2), and Yb (+2) commonly deviate. Eu2+ is the most stable +2 lanthanide (half-filled 4f7); Sm2+ and Yb2+ are the next most accessible. SmI2 is preparatively useful precisely because Sm2+ is reducing enough to be reactive but stable enough to handle in solution.

What are the most important samarium compounds?

SmCl3 (samarium trichloride) and Sm2O3 (samarium oxide) are the principal +3 compounds. SmI2 (samarium diiodide) is the key +2 compound and an essential reagent in organic synthesis. Sm2Co17 and SmCo5 are permanent magnet alloys with very high coercivity.

Samarium Oxidation States

Name: Samarium Oxidation States
Availability: InStock
Author: Nham Vu

Reference for samarium (Sm, element 62) oxidation states: +3 is dominant, +2 is accessible and useful in synthesis, with electron configuration, ionization energies, and key compounds explained.

Atomic # 62 Sm Samarium

Atomic Mass

150.36 u

Group

Lanthanides (f-block)

Period

Block

4f-block

Electronegativity

1.17 (Pauling)

Oxidation States

+3 (dom.), +2

Samarium has two known oxidation states: +3 and +2. The +3 state is the thermodynamically stable form in aqueous solution and the vast majority of solid compounds. The +2 state is accessible in anhydrous conditions — most importantly as samarium(II) iodide (SmI₂) in THF, a single-electron reductant central to modern organic synthesis. Sm²⁺ has the [Xe] 4f⁶ configuration; gaining the half-filled 4f⁷ shell of Eu²⁺ is not possible for Sm, but the relatively low third ionization energy (2260 kJ/mol) makes the +2 state accessible under reducing conditions.

Oxidation State	Stability	Notes
+3	Dominant	Universal state in aqueous solution and most solid compounds. Sm loses both 6s and one 4f electron, giving the [Xe] 4f⁵ configuration. Stabilized by high lattice and hydration energies.
+2	Accessible (anhydrous)	Sm²⁺ = [Xe] 4f⁶. Stable in solid SmI₂, SmF₂, SmCl₂. In solution (THF), SmI₂ is a widely used single-electron reductant. Sm²⁺ is slowly oxidized by water.
0	Elemental only	Samarium metal. Used in SmCo₅ / Sm₂Co₁₇ permanent magnets and as a neutron absorber in nuclear reactors.
+4	Not observed	No stable +4 compounds are known for samarium under ordinary conditions. Unlike cerium, Sm lacks the driving force of achieving a stable 4f⁰ configuration via +4 ionization.

Ionization Energies
IE₁ = 544.5 kJ/mol | IE₂ = 1070 kJ/mol | IE₃ = 2260 kJ/mol | IE₄ ≈ 3990 kJ/mol
The relatively low IE₃ (2260 kJ/mol) compared to neighboring lanthanides allows the +2 state to be accessed under reducing conditions, making SmI₂ a viable and controllable reagent.

Most samarium compounds feature Sm in the +3 oxidation state. However, a handful of important +2 compounds exist — particularly SmI₂, which has made samarium uniquely valuable in synthetic chemistry. All Sm²⁺ compounds must be handled under inert atmosphere due to slow oxidation by moisture and air.

Compound	Formula	Sm State	Notes
Samarium(III) Oxide	Sm₂O₃	+3	Most stable Sm compound; cream-yellow solid; used in optical glasses, ceramsite, and as a neutron absorber.
Samarium(III) Chloride	SmCl₃	+3	Anhydrous salt used as a Lewis acid catalyst and precursor for other Sm compounds.
Samarium(III) Iodide (sol.)	SmI₃	+3	Formed by oxidation of SmI₂ with I₂; not commonly isolated.
Samarium(III) Fluoride	SmF₃	+3	Used in optical coatings; very low solubility in water.
Samarium(III) Bromide	SmBr₃	+3	Hygroscopic salt; used as a Lewis acid and Sm precursor.
Samarium(III) Nitrate	Sm(NO₃)₃	+3	Soluble salt; used in nuclear applications as a neutron poison.
Samarium(III) Sulfate	Sm₂(SO₄)₃	+3	Soluble at room temperature; exhibits inverse solubility — less soluble when heated.
Samarium(II) Iodide	SmI₂	+2	Kagan's reagent — the key synthetic tool. Dark blue-green in THF; single-electron reductant. E° ≈ −1.33 V vs NHE.
Samarium(II) Chloride	SmCl₂	+2	Pale yellow solid; reducing; prepared by reduction of SmCl₃ with Sm metal at high temperature.
Samarium(II) Fluoride	SmF₂	+2	Distorted fluorite structure; more ionic than SmI₂; less commonly used in synthesis.
Samarium Cobalt Alloy	SmCo₅	0	Intermetallic compound and one of the strongest permanent magnet materials known; high coercivity, used in motors and sensors.

Ground State Configuration

[Xe]

Xenon core (1s–5p fully filled)

Valence — 6 of 14 electrons filled

Empty in ground state

Valence — lost in all ionizations

Notation

Noble gas: [Xe] 4f⁶ 6s²

Sm²⁺ ion: [Xe] 4f⁶

Sm³⁺ ion: [Xe] 4f⁵

Ionization Steps

[Xe] 4f⁶ 6s²

Neutral atom — 8 valence electrons

Sm⁺

[Xe] 4f⁶ 6s¹

Remove one 6s electron (IE₁ 544.5 kJ/mol)

Sm²⁺

[Xe] 4f⁶

Remove second 6s electron (IE₂ 1070 kJ/mol) — Sm²⁺ state; used in SmI₂

Sm³⁺

[Xe] 4f⁵

Remove one 4f electron (IE₃ 2260 kJ/mol) — dominant stable state

Sm⁴⁺

[Xe] 4f⁴

Remove second 4f electron (IE₄ ≈ 3990 kJ/mol) — not accessible ordinarily

The 4f Orbital and Lanthanide Contraction Samarium's 4f electrons are shielded from the nucleus by the outer 5s/5p shells, making them chemically inert in most reactions — the lanthanide series chemistry is dominated by the 6s/5d electrons and the ion radius shrinkage across the series (lanthanide contraction). The partially filled 4f⁶ of Sm²⁺ and 4f⁵ of Sm³⁺ give these ions their characteristic magnetic and spectroscopic properties.

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Summary

Reference for samarium (Sm, element 62) oxidation states: +3 is dominant, +2 is accessible and useful in synthesis, with electron configuration, ionization energies, and key compounds explained.

How it works

Click a tab — Oxidation States, Compounds, Electron Config, or Physical Props — to explore each section.
The Oxidation States panel explains why +3 dominates and when +2 is accessible, with a stability table.
The Compounds panel lists common Sm compounds with formulas and oxidation state assignments.
The Electron Config panel shows the 4f orbital filling and ionization steps to Sm3+ and Sm2+.
The Physical Props panel provides atomic and material data for quick reference.
Click any monospace table cell to copy its value to your clipboard.

Use cases

Students studying lanthanide chemistry and 4f-block trends.
Organic chemists using SmI2 (Barbier-type reactions, reductive cyclizations) who need oxidation state context.
Chemistry teachers preparing material on rare-earth elements and the inert pair concept.
Researchers in materials science working with samarium-cobalt (SmCo5) permanent magnets.
Anyone preparing for exams covering d- and f-block chemistry.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu