Why is +3 the dominant oxidation state for holmium?

Holmium has the ground-state configuration [Xe] 4f¹¹ 6s². Losing the two 6s² electrons is energetically favorable because these are the outermost, highest-energy electrons. A third electron is also removed (from the 4f subshell) to reach the very stable trivalent lanthanide state, Ho³⁺, which is characteristic of the entire lanthanide series. The energy cost of a fourth ionization is prohibitively high, so +4 is not observed under normal conditions.

What is the electron configuration of Ho³⁺?

Neutral holmium is [Xe] 4f¹¹ 6s². The Ho³⁺ ion loses the two 6s² electrons and one 4f electron, giving [Xe] 4f¹⁰. The 4f¹⁰ configuration has 4 unpaired electrons (by Hund's rule: 5 orbitals, first 5 electrons spin-up, next 5 fill paired — so 10 electrons leave 4 singly occupied), contributing to holmium's strong paramagnetic and magnetocaloric behavior.

Why does holmium have the highest magnetic moment of any element?

Holmium's neutral atom has the electron configuration [Xe] 4f¹¹ 6s². The partially filled 4f subshell contains 4 unpaired electrons in Ho³⁺ (configuration [Xe] 4f¹⁰). However, it is the neutral metal in specific magnetic alloys — particularly Ho at low temperature — where the combination of large spin and orbital angular momentum produces the record effective magnetic moment of about 10.6 Bohr magnetons per atom, the highest measured for any element under cryogenic conditions.

What is Ho²⁺ and where does it occur?

Ho²⁺ would have configuration [Xe] 4f¹¹ — retaining 11 f-electrons after losing only the two 6s electrons. It is very rare because losing a third electron to reach Ho³⁺ is energetically favorable and yields the stable trivalent lanthanide state. Ho²⁺ has been observed in a small number of solid-state compounds such as HoI₂ under reducing conditions, but it is not encountered in normal aqueous chemistry.

What are Ho:YAG lasers used for?

Ho:YAG (holmium-doped yttrium aluminum garnet) lasers emit at approximately 2.1 µm in the mid-infrared. Water absorbs strongly at this wavelength, making the laser effective for tissue ablation. Clinical uses include ureteroscopy (kidney stone fragmentation), orthopedic surgery (cartilage reshaping), and soft-tissue procedures. The laser's precise water absorption minimizes collateral thermal damage compared with CO₂ or Nd:YAG lasers.

How is holmium used in nuclear reactors?

Natural holmium is essentially monoisotopic (¹⁶⁵Ho, 100% abundance) and has a significant thermal neutron absorption cross-section (~64 barns). It is used as a neutron absorber (burnable poison) in some reactor designs and as a control rod material. Neutron capture produces ¹⁶⁶Ho, a beta emitter with a 26.8-hour half-life; ¹⁶⁶Ho-labeled microspheres are investigated for targeted radioembolization of liver tumors.

Holmium Oxidation States

Name: Holmium Oxidation States
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Author: Nham Vu

Reference for holmium oxidation states: the dominant +3 state, rare +2, key compounds (Ho₂O₃, HoCl₃, Ho(NO₃)₃), electron configuration [Xe] 4f¹¹ 6s², and applications in permanent magnets, MRI, and lasers.

Atomic # 67 Ho Holmium

Atomic Mass

164.930 u

Group

Lanthanide

Period

Block

f-block

Electronegativity

1.23 (Pauling)

Oxidation States

+3 (dominant), +2 (rare)

Holmium is an f-block lanthanide with ground-state configuration [Xe] 4f¹¹ 6s². Losing the two 6s² electrons and one 4f electron yields Ho³⁺ ([Xe] 4f¹⁰) — the dominant and most stable oxidation state across all lanthanide chemistry. The +2 state is very rare and confined to a few solid-state compounds. +4 is not observed: removing an additional f-electron after +3 costs far more energy than the system gains.

State	Name	f-electrons	Config (after Xe)	Stability	Notes
0	Elemental	11 (+ 6s²)	[Xe] 4f¹¹ 6s²	Elemental metal	Silvery-white, soft metal; highest magnetic moment of any element at low T
+2	Holmium(II)	11	[Xe] 4f¹¹	Very rare / unstable	Observed in HoI₂ and a few other solid-state compounds under reducing conditions
+3	Holmium(III)	10	[Xe] 4f¹⁰	Dominant / stable	4 unpaired electrons; pale yellow Ho₂O₃; basis of magnets, lasers, and Ho-166 therapy

Why lanthanides favor +3
For all lanthanides, the 6s electrons are removed first (lowest ionization energy), followed by one 4f or 5d electron. The resulting M³⁺ configuration is the thermodynamic sink: the fourth ionization would pull another f-electron out at much higher cost. Holmium follows this pattern exactly, giving Ho³⁺ with [Xe] 4f¹⁰ as the universal form in solution and compounds.

Magnetic properties
Ho³⁺ has 4 unpaired 4f electrons. The neutral metal at low temperatures shows an effective magnetic moment of ~10.6 Bohr magnetons per atom — the highest of any element — arising from the combined spin and orbital angular momentum of the 4f¹¹ configuration. This drives its use in strong permanent magnets and magnetocaloric refrigeration.

Virtually all holmium compounds contain Ho³⁺. The ion typically forms pale yellow or colorless solids. Key compounds span from simple oxides and halides to specialized laser hosts and medical radiopharmaceuticals.

Compound	Formula	Ho State	Appearance	Notes
Holmium(III) Oxide	Ho₂O₃	+3	Pale yellow powder	Most common Ho compound; used as neutron absorber, yellow pigment in glass/ceramics, and starting material for other Ho compounds.
Holmium(III) Chloride	HoCl₃	+3	Yellow crystals	Soluble in water; common laboratory reagent; used to synthesize holmium coordination compounds and optical dopants.
Holmium(III) Nitrate	Ho(NO₃)₃	+3	Pale yellow crystals	Highly soluble; precursor for solution-phase synthesis and as a dopant source in sol-gel processes.
Holmium(III) Fluoride	HoF₃	+3	White solid	Very low solubility; used in optical fiber production and as a fluoride glass component for IR-transmitting optics.
Holmium(III) Iodide	HoI₂	+2	Dark solid	Rare divalent example; formed under strongly reducing conditions; Ho²⁺ is [Xe] 4f¹¹.
Ho:YAG Crystal	Ho:Y₃Al₅O₁₂	+3	Colorless crystal	Ho³⁺-doped yttrium aluminum garnet; lases at ~2.1 µm (mid-IR); used in urological and orthopedic surgery.
Holmium(III) Acetate	Ho(C₂H₃O₂)₃	+3	Pale yellow solid	Used in sol-gel synthesis; precursor for holmium oxide thin films and coatings.
Ho-166 Microspheres	Ho-166 PLLA	+3	Submicron spheres	¹⁶⁶Ho-labeled poly-L-lactic acid microspheres; beta emitter (t½ = 26.8 h); used in selective internal radiation therapy for liver tumors.

Ho:YAG laser mechanism
In a Ho:YAG crystal, Ho³⁺ ions replace a small fraction of Y³⁺ sites in the garnet lattice. Pumped by a Tm:YAG or diode laser, Ho³⁺ undergoes a ⁵I₇ → ⁵I₈ transition at 2.09–2.12 µm. Water absorbs strongly in this range (absorption coefficient ~1 cm⁻¹), so tissue is ablated precisely at the surface with minimal thermal spread — ideal for stone fragmentation and soft-tissue cutting in minimally invasive surgery.

Ho (neutral)

4f orbitals

↑ ↓

11 f-electrons

Full config: [Xe] 4f¹¹ 6s²

11 electrons in the 4f subshell; 7 orbitals, so 5 are spin-up and 4 are spin-up with a paired spin-down — leaving 3 unpaired from a simple Hund's count in the neutral atom. With 6s² also present, total valence electrons = 13.

Ho³⁺

4f orbitals

↑ ↓

10 f-electrons

Full config: [Xe] 4f¹⁰

Loses 6s² and one 4f electron. [Xe] 4f¹⁰: 7 orbitals — 5 singly occupied (spin-up), 5th orbital gets a second spin-down, then 4th and 3rd orbitals each get a paired electron, leaving 4 unpaired. μ_eff ≈ 10.6 BM in the metal.

Ionization energy pattern
The first two ionizations remove the 6s² electrons at relatively low energy. The third ionization pulls a 4f electron — still feasible, producing the stable Ho³⁺. A fourth ionization would remove another f-electron, requiring a large jump in energy because 4f electrons are poorly shielded and tightly bound. This large gap between IE₃ and IE₄ explains why +3 is the ceiling for holmium in normal chemistry.

Ionization	Energy (kJ/mol)	Electron removed from
IE₁ → Ho⁺	581	6s (outermost electron)
IE₂ → Ho²⁺	1140	6s (second 6s electron)
IE₃ → Ho³⁺	2204	4f (first f-electron removal)
IE₄ → Ho⁴⁺	~4100	4f (large jump — not observed in normal chemistry)

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Summary

How it works

Click 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 holmium compounds with formula, appearance, and use notes.
The Electron Config panel shows orbital diagrams for Ho and Ho³⁺ 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 learning f-block (lanthanide) chemistry and why lanthanides almost exclusively adopt +3.
Chemistry teachers explaining the relationship between 4f electron count and magnetic properties.
Materials scientists researching holmium-based permanent magnets and magnetocaloric alloys.
Medical physicists studying holmium microspheres (Ho-166) used in radioembolization therapy.
Anyone revising for exams covering f-block chemistry, inorganic chemistry, or lanthanide behavior.

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Last updated: 2026-07-23 · Reviewed by Nham Vu