Holmium

Holmium
Atomic number	67
Symbol	Ho
Boiling point	2600 °C
Electronegativity	1.23 (Pauling)
Electron configuration	[Xe] 4f11 6s2
Density	8.79 g/cm^3
Main isotopes	165Ho
Melting point	1461 °C
Block	f
Phase STP	Solid
Oxidation states	+3, +2
Wikidata	Q1846

Holmium (chemical symbol Ho, atomic number 67) is a silvery-white, malleable rare-earth metal of the lanthanide series (period 6, f-block). It normally occurs as a solid metal at standard conditions. Holmium metal is fairly corrosion-resistant in dry air (forming a thin oxide surface) but oxidizes in moisture or when heated, burning to Ho₂O₃. The element is essentially monoisotopic: natural holmium is ~100% ^165Ho by abundance Its atomic weight is about 164.93 u In compounds, Ho nearly always adopts the +3 oxidation state (likewise forming Ho³⁺ ions), though +2 and +1 states have been observed in specialized compounds The metal has a high magnetic moment (10.6 μB) – the largest of any naturally occurring element – and correspondingly very high magnetic permeability and saturation. This makes holmium useful in specialized magnet applications. Holmium strongly absorbs neutrons, so it serves in nuclear reactors as a “burnable poison” to control reactivity The name “holmium” comes from Holmia, the Latin name for Stockholm, where it was discovered by spectroscopic analysis in 1878

Atomic Structure and Electron Configuration

Holmium has 67 electrons with the configuration [Xe] 4f¹¹ 6s². All of its 4f and 6s electrons contribute to its chemistry (13 electrons beyond xenon core). Its common valence is +3 (removing the two 6s and one 4f electron). In the periodic table, Ho falls among the late lanthanides, after dysprosium (Dy) and before erbium (Er) Atomic radius reflects the lanthanide contraction: holmium’s metallic radius is about 176 pm (Its Ho³⁺ ionic radius is roughly ~90–100 pm depending on coordination.) Holmium’s first ionization energy is about 6.02 eV (≈582 kJ/mol) in line with other lanthanides. Its electronegativity on the Pauling scale is low (≈1.23) reflecting its electropositive character. Periodic trends show only gradual changes across the lanthanides: holmium’s atomic radius is smaller than lighter rare-earths (due to f-electron shell contraction) but larger than the heaviest (erbium, etc.), and its ionization potential and electronegativity are similarly in the middle of the lanthanide range.

Isotopes and Nuclear Properties

The only stable (primordial) isotope of holmium is ^165Ho so natural holmium is effectively monoisotopic. Although ^165Ho is effectively stable for all practical purposes, theory predicts extremely slow α-decay to ^161Tb with an enormously long half-life. Among synthetic isotopes, ^163Ho (half-life ~4570 y) is the longest-lived radioisotope after ^165Ho Holmium-166 (^166Ho) (t₁/₂ ≈ 26.8 h) is a β^−/γ-emitter widely used in nuclear medicine. Its decay (β max ~1.8 MeV, γ at 80.6 keV) makes ^166Ho useful for targeted radiotherapy (e.g. liver cancer microspheres) and imaging Irradiating ^165Ho in a reactor (thermal neutron capture cross-section ~64 barns produces ^166Ho in high yield (with only trace ^166mHo byproduct). A metastable isomer, ^166mHo (t₁/₂ ≈ 1200 y), emits many γ-rays and is used to calibrate gamma-ray spectrometers

The ground-state nucleus of ^165Ho has spin-parity 7/2⁻ and a nuclear magnetic dipole moment of +4.16 nuclear magnetons (and an electric quadrupole moment +3.58 b). This large nuclear moment (from 10 unpaired f-electrons) contributes to holmium’s extreme paramagnetism (and its MRI visibility at high fields).

Allotropes and Typical Compounds

Holmium metal has a single main crystallographic form under normal conditions, adopting a hexagonal close-packed (hcp) lattice (often called α-holmium) (Like many lanthanides, it may adopt a slightly different structure under extreme high pressures, but no distinct allotropes are commonly noted.) In compounds, holmium behaves as a +3 cation with largely ionic bonding. The principal oxide is holmium(III) oxide, Ho₂O₃ (sometimes called holmia), a stable sesquioxide. Holmium oxide is a pale yellow or reddish powder (its color depends on light conditions) It is the primary form obtained during ore processing. In air, especially if warm, holmium metal tarnishes and forms a yellowish surface layer of Ho₂O₃

All the common trihalides of holmium are known: HoF₃ (pink solid), HoCl₃ (yellow), HoBr₃ (yellow), and HoI₃ (yellow) These form easily when Ho metal burns in halogen gases (2Ho + 3F₂ → 2HoF₃, etc.) Holmium also forms a triiodide and previously known HoF₃ Fluoride is often used to extract the metal. The oxide and halides share the Nd/Sm/Tb chemistry pattern: i.e. high coordination numbers (often 8–9 in crystals), and they dissolve as Ho³⁺ ions in water. For example, dissolving Ho metal in acid yields [Ho(H₂O)₉]³⁺ complexes (yellow solutions) The hydroxide Ho(OH)₃ also forms readily (Ho is a strong base oxide), and soluble salts like holmium nitrate and sulfate are easily prepared.

Other holmium chalcogenides exist (sulfide Ho₂S₃, selenide Ho₂Se₃, etc.), as well as the nitride HoN and hydride (HoH₂). However, there are no extensive organic or covalent holmium compounds; it shows the typical ionic chemistry of late rare earths.

Physical Properties

Holmium is a dense, relatively heavy metal with density ~8.80 g/cm³ (at 20 °C). Its melting point is about 1747 K (1474 °C) and its boiling point around 2873 K (2600 °C) As a solid, it is malleable and has a bright metallic luster. The hcp crystal structure persists from ambient up to near melting. Its thermal conductivity is moderate (about 16 W/m·K at room temperature) and electrical resistivity is high for a metal (~0.000812 Ω·m at 20 °C The specific heat is roughly 160 J/(kg·K).

Spectroscopically, the Ho³⁺ ion has numerous sharp spectral lines, arising from f–f electronic transitions. These give holmium compounds distinctive colors and make them useful in optics. In a typical UV-Vis absorption spectrum, holmium solutions (e.g. holmium perchlorate) exhibit sharp peaks across the visible range. In fact, a holmium-doped glass or aqueous solution is a standard for calibrating spectrophotometer wavelength scales

Magnetically, holmium metal is strongly paramagnetic at room temperature. Below its Curie temperature (~19 K), Ho becomes ferromagnetic At low temperatures a field of a few tesla is enough to align nearly all Ho spins, giving saturation magnetization around 160 Am²/kg (10.6 μB per atom). Above 19 K it is a conventional paramagnet with extremely high susceptibility.

Chemical Reactivity and Trends

Holmium metal is very reactive (as expected for an electropositive rare earth). It tarnishes in air (forming Ho₂O₃) and burns readily in oxygen or halogen fumes. It reacts slowly with cold water but quickly with hot water to yield the hydroxide and hydrogen gas (2Ho + 6H₂O → 2Ho(OH)₃ + 3H₂) It dissolves readily in most dilute acids to give Ho³⁺ salts, liberating H₂. For example, in sulfuric acid: 2Ho + 3H₂SO₄ → 2Ho³⁺ + 3SO₄²⁻ + 3H₂ All trivalent Ho compounds (oxide, hydroxide, halides, sulfates, nitrates, etc.) are quite stable, reflecting the +3 oxidation state preference. Heavier (lower) oxidation states (Ho²⁺, Ho¹⁺) can be obtained only under special reducing conditions.

In terms of periodic trends, holmium’s chemistry closely parallels that of its lanthanide neighbors (dysprosium and erbium). It has low electronegativity (≈1.23) and is a strong Lewis acid as Ho³⁺. Holmium(III) salts are typically colorless or lightly colored, and behave as weak bases (holmium hydroxide is only sparingly soluble). The element’s standard reduction potential (Ho³⁺ + 3e⁻ → Ho) is quite negative (around –2.3 V), meaning Ho metal is a strong reductant when reacting.

Because holmium is so reactive, pure Ho metal must be stored away from air and moisture (it is often sold under inert oil). The metal does not passivate strongly, so it can corrode under aggressive conditions. It is sometimes alloyed with other refractory metals or lanthanides to modify its reactivity and mechanical properties.

Occurrence and Production

Holmium is a relatively rare element in the Earth’s crust (~1.2–1.4 parts per million by mass) (comparable to elements like tungsten). In the cosmos it is extremely scarce (on the order of 0.5 parts per billion by mass) because heavy lanthanides are only produced by rare rapid (r-process) neutron-capture events in supernovae Holmium never occurs in nature as a free element; it is always found mixed with other lanthanides. Important mineral sources include monazite Ce,La,Th)PO₄] and gadolinite Ce,La)(Be₂Fe2+) (SiO₄) O(OH, which contain trace amounts of Ho alongside other rare-earths Bastnäsite Ce,La)F(CO₃ may also supply heavy lanthanides. No mineral is known to contain mostly holmium; it is extracted chemically as part of the mixed rare-earth oxides (so-called "mixed rare earth carbonate/oxide") from these ores.

Worldwide, the largest production of rare-earth concentrates (including holmium) is from China, but other producers include the United States (e.g. Mountain Pass, though that field focuses on lighter rare earths), India, Brazil, and Australia. The ores are usually processed by acid digestion (typically sulfuric or hydrochloric) to separate out a mixed rare-earth solution, from which individual lanthanides are separated by ion-exchange or solvent-extraction chromatography. The separated holmium typically emerges as Ho₂O₃ or a fluoride. Pure holmium metal is then obtained by reducing the oxide or fluoride: for example, finely divided HoF₃ can be mixed with Ca metal and heated under vacuum to yield Ho + CaF₂ Another method uses electrolysis of molten HoCl₃ or HoF₃ salts (often in a fluoride flux). Overall, special handling is needed at each step to keep Ho from oxidizing or reacting.

Applications and Technology

Holmium’s unique properties enable several specialized applications:

• Magnetics: Holmium’s extremely high magnetic moment and permeability make it useful in research magnets. For instance, pieces of solid Ho metal or Ho-containing alloys are used as pole pieces or flux concentrators in high-field electromagnets Small additions of Ho are sometimes alloyed into permanent magnet materials (like Nd–Fe–B) to improve performance at high temperature.

• Lasers: Holmium-doped laser media are widely used. A particularly important example is the Ho:YAG (holmium-doped yttrium aluminum garnet) laser, which emits around 2.1 μm (infrared). Ho:YAG lasers are used in medicine (e.g. urology for kidney and bladder stone lithotripsy, and in orthopedic or ENT surgery) because the wavelength is strongly absorbed by water/tissue, allowing precise cutting. Newer fiber lasers doped with Ho are also emerging for fiber-optic communications and surgery. In general, the narrow atomic transitions of Ho³⁺ (in garnets or glass) make it an excellent mid-infrared lasing ion.

• Nuclear Technology: Because natural ^165Ho readily absorbs neutrons (σ≈64 barns) and its product ^166Ho decays away, holmium is employed as a “burnable poison” in some nuclear reactor fuel assemblies It helps regulate reactivity by soaking up excess neutrons early in the fuel cycle. Other isotopes of Ho (like ^166Ho) are used in nuclear medicine: ^166Ho-labeled microspheres and other compounds are used for internal radiotherapy (e.g. to treat liver tumors or bone metastases) The long-lived ^166mHo is used in calibration of gamma-ray detectors

• Optics and Glass: Holmium oxide has strong sharp absorption bands across the visible spectrum. Holmium-doped filter glasses (often called holmium glass) transmit a mostly neutral visible light but show fine absorption lines. These are used to calibrate wavelength scales of spectrophotometers (standards of wavelength accuracy Holmium glass is also used ornamentally or as a colorant (imparting a pinkish tint). In addition, Ho:YAG or Ho:glass lasers rely on Ho-containing media.

• Biomedical: Beyond lasers and nuclear therapy, holmium has niche biomedical uses. For example, ^165Ho metal or salts are being explored as MRI contrast agents (due to Ho’s strong paramagnetism) and CT contrast (due to its high atomic number) Holmium microspheres (radioactive or not) are used experimentally in radionuclide therapies and as markers. Because of Ho’s stable presence (100% ^165Ho), it has also been used in doped nanoparticles for multimodal imaging.

Overall, holmium’s applications exploit its magnetic and nuclear interactions, as well as the optical properties of Ho³⁺. It has no large-scale commercial uses like bulk metals; instead, it is valued in specialized high-tech and medical devices.

Biology, Environment, and Safety

Holmium has no known essential biological role. Its chemistry in the environment mimics that of other rare earths: it tends to form insoluble oxides and hydroxides. Therefore, Ho released to soil or water tends to precipitate out and bind to particulates. In biological systems, lanthanides are generally of low bioavailability, and Ho in particular does not accumulate significantly in living tissues at normal exposures.

Toxicologically, soluble holmium salts must be handled with care. In general, ingestion or inhalation of lanthanide compounds can irritate mucous membranes, lungs, and eyes Rare-earth element exposure (from mining or processing dust) has been linked to lung and bone effects in animals. Holmium oxide is considered to be very low in acute toxicity, but fine Ho-containing dust can cause mechanical irritation (pneumoconiosis) if inhaled heavily. As with heavy metals, large doses could potentially harm the liver or kidneys, but typical exposures are far below such levels. No specific occupational exposure limit for Ho exists, but general guidelines for rare-earth dust (nuisance dust limits) are applied. Handling holmium metal or compounds requires standard precautions: use of gloves, goggles, and masks as needed.

Environmentally, the mining and refining of holmium (as a rare earth) can cause pollution (stockpiled tailings contain radioactive thorium from monazite, for example). Holmium itself is chemically stable and does not biodegrade. It should not bioaccumulate or undergo long-range transport; it will typically remain near where it enters the environment. However, as a heavy element, it poses some risk if discharged to water bodies (it can be toxic to aquatic organisms at high concentrations). In sum, holmium is not especially dangerous in everyday life, but industry and laboratories handle it with typical heavy-metal precautions.

History and Etymology

Holmium was identified in 1878–79 through spectroscopic and chemical analysis of rare-earth minerals. In 1878 Swiss spectroscopists Jacques-Louis Soret and Marc Delafontaine observed an unknown set of spectral lines (tagged “Element X”) in a sample from the Ytterby region. Independently, Per Teodor Cleve of Sweden isolated an oxide of the same substance in 1879 Cleve named the oxide “holmia” after Holmia (Latin for Stockholm) The name “holmium” for the element derives from this. These discoveries were enabled by earlier work of Carl Mosander, who had separated out erbium and terbium from “yttria” and left unknown oxides behind.

Pure holmium metal proved harder to isolate. It was not until 1911 that Otto Holmberg succeeded in obtaining metallic holmium That same year, Ho₂O₃ purity was finally improved, marking a significant milestone in rare-earth chemistry. (Earlier, chemists confused holmium with its neighbors dysprosium and erbium until spectral and chemical tools improved.) In 1914 Henry Moseley then assigned atomic number 66 to holmium’s spectral lines, but later realized his sample was contaminated; holmium’s correct atomic number was established as 67

The element “holmia” (Ho₂O₃) had been sold in small amounts to researchers by the early 20th century. Industrial preparation of holmium metal began only in the mid-1900s when new reduction and electrolysis methods were developed. Advances since then have made high-purity Ho metal and salts available for research. In recent decades, holmium’s unusual magnetism and spectral properties have sparked interest in areas like quantum computing (e.g. single-atom memories) and medical technologies, though chemically it remains a classic lanthanide.

Data Table

Property	Value
Element name	Holmium
Symbol	Ho
Atomic number (Z)	67
Atomic weight (standard)	164.9303 u
Category	Rare earth metal (lanthanide)
Group/period/block	f-block, period 6
Oxidation states	+3 (common); also +2, +1 (rare)
Phase at STP	Solid (metal)
Crystal structure (RT)	Hexagonal close-packed (hcp)
Density	8.80 g/cm³ (295 K)
Melting point	~1747 K (1474 °C)
Boiling point	~2873 K (2600 °C)
Atomic (metallic) radius	~176 pm
Electron configuration	[Xe] 4f^11 6s^2
Valence electrons	6s²4f^11 (outer 2+11 electrons)
Electronegativity (Pauling)	1.23
First ionization energy	6.02 eV (≈582 kJ/mol)
Magnetic ordering	Paramagnetic (ferromagnetic below 19 K)
Magnetic moment (^165Ho)	+4.16 μN (nuclear dipole)
Melting heat capacity	~160 J/(kg·K)
Applications	Lasers (e.g. Ho:YAG), magnets, neutron absorber, calibration standard
Discovered by	Delafontaine & Soret (1878), Cleve (1879)
Name origin	From Holmia (Latin for Stockholm)