Why is +3 the dominant oxidation state for thulium?

Thulium has configuration [Xe] 4f¹³ 6s². Losing both 6s electrons and one 4f electron gives Tm³⁺ with [Xe] 4f¹², the most energetically accessible ionization pathway. The +3 state is standard across almost all lanthanides because the 4f electrons are well shielded and the first three ionization energies remain accessible. Removing a fourth electron from the contracted 4f shell requires far more energy, so +4 is not observed in practice.

Why is the +2 state more accessible for thulium than other lanthanides?

In its neutral form, Tm has 4f¹³ — just one electron short of a completely filled 4f¹⁴ subshell. Tm²⁺ has configuration [Xe] 4f¹³, retaining that near-complete subshell. Because a filled 4f¹⁴ shell is stabilizing, accepting one more electron to reach 4f¹⁴ (i.e., Tm²⁺ → Tm¹⁺, or stabilizing Tm²⁺ in a lattice) is relatively favorable compared to lanthanides with fewer f-electrons. This makes Tm²⁺ accessible in reducing solid-state environments such as TmI₂ and TmS.

What compounds contain Tm²⁺?

Thulium(II) compounds include TmI₂ (thulium diiodide), TmS (thulium monosulfide), and certain selenide and telluride hosts. These are prepared under reducing conditions and are sensitive to oxidation. TmI₂ is structurally analogous to EuI₂ and SmI₂ and has been used as a reducing agent in organic synthesis.

What is thulium used for industrially?

The largest applications involve optical and laser technology. Tm³⁺-doped crystals (e.g., Tm:YAG) and Tm-doped silica fibers are used in ~2 µm lasers, which have medical (tissue ablation, kidney stone removal) and LIDAR applications. Thulium-170 (a radioactive isotope) is used in portable X-ray sources and experimental cancer therapy. Thulium oxides appear in specialty glass, phosphors, and ceramic pigments.

Thulium is the rarest of the stable (non-radioactive) lanthanides, with an estimated crustal abundance of about 0.52 ppm. It is rarer than silver (~0.06 ppm is an older figure; estimates vary) in some measures. Despite being called "rare earth," thulium is still more abundant than gold. It is extracted from monazite and xenotime ores alongside other lanthanides.

What is the electron configuration of Tm³⁺?

Tm³⁺ has configuration [Xe] 4f¹². Starting from neutral Tm ([Xe] 4f¹³ 6s²), the two 6s electrons are removed first (lower ionization potential), then one 4f electron is removed to give the +3 ion. The resulting 4f¹² configuration has two unpaired electrons in some orbital arrangements, giving Tm³⁺ a magnetic moment of approximately 7.56 Bohr magnetons (theoretical free-ion value for ³H₆ ground term).

Thulium Oxidation States

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

Reference for thulium oxidation states: dominant +3, relatively accessible +2, rare +4; electron configuration [Xe] 4f¹³ 6s²; rarest stable lanthanide.

Atomic # 69 Tm Thulium

Atomic Mass

168.934 u

Group

Lanthanide

Period

Block

f-block

Electronegativity

1.25 (Pauling)

Oxidation States

+3 (dominant), +2

Thulium has ground-state configuration [Xe] 4f¹³ 6s². Losing both 6s² electrons and one 4f electron yields Tm³⁺ ([Xe] 4f¹²), the dominant state in aqueous and most solid-state chemistry. The +2 state is notably accessible compared to other lanthanides: Tm²⁺ carries a 4f¹³ configuration, just one electron short of a filled 4f¹⁴ — a near-complete subshell that provides extra stabilization in reducing or low-polarity solid-state environments. A +4 state would require removing a 4f electron from an already contracted shell and is not well characterized.

State	Name	f-electrons	Config (after Xe)	Stability	Notes
0	Elemental	13 (+ 6s²)	[Xe] 4f¹³ 6s²	Elemental metal	Silvery-white metal; one electron short of filled 4f¹⁴
+2	Thulium(II)	13	[Xe] 4f¹³	Rare / reducing cond.	TmI₂, TmS, some chalcogenides; more stable than most Ln²⁺ due to proximity to 4f¹⁴
+3	Thulium(III)	12	[Xe] 4f¹²	Dominant / stable	Standard state; 2 unpaired 4f electrons; μ_eff ≈ 7.56 BM (³H₆ ground term)
+4	Thulium(IV)	11	[Xe] 4f¹¹	Not well characterized	Very high IE₄; no confirmed stable compound under normal conditions

Why Tm²⁺ is relatively accessible
Tm²⁺ has 4f¹³ — one electron short of a fully filled 4f¹⁴ subshell. The filled 4f¹⁴ configuration is stabilizing (similar to d¹⁰ in transition metals). In a sufficiently reducing or low-polarity lattice, accepting one more electron to reach 4f¹⁴ (i.e., Tm⁺) or simply persisting as Tm²⁺ (4f¹³) is thermodynamically more tolerable than for lanthanides further from the filled shell.

Rarest stable lanthanide
Thulium is the least abundant stable lanthanide at ~0.52 ppm in Earth's crust, rarer than iodine and about as abundant as bismuth. All other lanthanides (except radioactive promethium) are more abundant. Despite this, thulium-doped lasers and ¹⁷⁰Tm X-ray sources represent commercially viable niche applications.

Most thulium compounds contain Tm³⁺. The divalent Tm²⁺ appears in a small number of solid-state compounds prepared under reducing conditions. Key compounds are listed below.

Compound	Formula	Tm State	Appearance	Notes
Thulium(III) Oxide	Tm₂O₃	+3	White powder	Most common Tm compound; used as starting material, in specialty glass, and minor phosphor applications.
Thulium(III) Chloride	TmCl₃	+3	White crystals	Soluble in water; standard laboratory precursor for Tm coordination chemistry and organometallic synthesis.
Thulium(III) Fluoride	TmF₃	+3	White solid	Very low solubility; used in specialty optical fibers and fluoride glass hosts for ~2 µm Tm-doped lasers.
Thulium(III) Nitrate	Tm(NO₃)₃	+3	White hygroscopic	Highly soluble; solution-phase precursor; used in sol-gel synthesis of Tm-containing oxide materials.
Thulium(III) Iodide	TmI₃	+3	Yellow solid	Precursor to TmI₂; used in metal halide lamps and as a synthetic intermediate.
Thulium(II) Iodide	TmI₂	+2	Dark solid	Divalent Tm compound; strong reductant; used in organic synthesis analogously to SmI₂; synthesized under inert atmosphere.
Thulium(II) Sulfide	TmS	+2	Dark solid	Metallic-like conductivity; prepared at high temperature; demonstrates Tm²⁺ stability in sulfide lattice.
Tm:YAG Crystal	Tm³⁺:Y₃Al₅O₁₂	+3	Colorless crystal	Laser gain medium; ~2 µm emission; used in medical (tissue ablation) and mid-IR LIDAR applications.

TmI₂ as a reductant
Thulium(II) iodide (TmI₂) is a one-electron reductant with a reduction potential of approximately −2.3 V vs. SHE — slightly less reducing than SmI₂ (−1.33 V) but more accessible than some other divalent lanthanide iodides. Its reactivity profile makes it a useful reagent for ketone and aldehyde reductions, radical cyclizations, and pinacol couplings under mild conditions.

Tm (neutral)

4f orbitals

↑ ↓

13 f-electrons

Full config: [Xe] 4f¹³ 6s²

Neutral thulium follows the regular lanthanide filling pattern: [Xe] core, then 4f fills sequentially. With 13 f-electrons, Tm is one electron short of a completely filled 4f¹⁴ subshell.

Tm³⁺

4f orbitals

↑ ↓

12 f-electrons

Full config: [Xe] 4f¹²

Loses both 6s electrons and one 4f electron. The 4f¹² configuration has 2 unpaired electrons (following Hund's rule for 12 electrons in 7 orbitals). Ground term ³H₆; μ_eff ≈ 7.56 BM.

4f¹³ — near-complete subshell
Thulium's 4f¹³ configuration (neutral atom and Tm²⁺) is the electron-hole complement of 4f¹ (as in Ce³⁺). One electron short of a filled shell, it behaves in some ways like a "hole" state. The proximity to 4f¹⁴ is why Tm²⁺ is more stable in solid-state environments than Nd²⁺ or La²⁺.

Ionization	Energy (kJ/mol)	Electron removed from
IE₁ → Tm⁺	596.7	6s (outermost, lowest IE)
IE₂ → Tm²⁺	1163	6s (second 6s electron)
IE₃ → Tm³⁺	2285	4f (one 4f electron; large jump vs. 6s)
IE₄ → Tm⁴⁺	~4120	4f (estimated; very high; no stable +4 chemistry)

Copied!

Summary

Reference for thulium oxidation states: dominant +3, relatively accessible +2, rare +4; electron configuration [Xe] 4f¹³ 6s²; rarest stable lanthanide.

How it works

Select a tab — Oxidation States, Compounds, Electron Config, or Physical Props — to explore each section.
The Oxidation States panel lists all known states with stability, f-electron count, and key notes.
The Compounds panel shows common thulium compounds with formula, appearance, and uses.
The Electron Config panel shows orbital diagrams for Tm and Tm³⁺ side by side.
The Physical Props panel provides atomic and material data for quick reference.
Click any monospace table cell to copy its value to the clipboard.

Use cases

Students studying f-block lanthanide chemistry and the +2/+3 oxidation state patterns.
Chemistry teachers explaining why thulium's +2 state is more accessible than most lanthanides.
Researchers working with thulium-doped laser crystals or fiber amplifiers.
Anyone preparing for exams covering the lanthanide series, electron configurations, or inorganic chemistry.
Scientists researching divalent lanthanide compounds and solid-state synthesis.

Frequently Asked Questions

Last updated: 2026-07-24 · Reviewed by Nham Vu