HOLMIUM: HISTORY, PROPERTIES, CHEMICAL STRUCTURE AND USES - CHEMISTRY - 2024

The holmium is a metallic element belonging to the f block the periodic table, specifically to the period of the lanthanides. It is therefore a member of the rare earths, together with erbium, yttrium, dysprosium and ytterbium. All these make up a series of minerals (xenotime or gadolinite) that are difficult to separate by conventional chemical methods.

Its chemical symbol is Ho, having an atomic number of 67, and being less abundant than its neighbors dysprosium (₆₆ Dy) and erbium (₆₈ Er). It is then said that it obeys the Oddo-Harkins rule. Holmium is one of those rare metals that almost no one knows or suspects of its existence; even among chemists, it is rarely mentioned very often.

Ultrapure sample of metallic holmium. Source: Hi-Res Images of Chemical Elements

In the fields of medicine, holmium is known for the use of its laser in surgeries to combat prostate diseases. It also represents the promising material for the manufacture of electromagnets and quantum computers, due to its unusual magnetic properties.

The trivalent holmium compounds, Ho ³⁺, have the particularity of exhibiting a color dependent on the light with which they are irradiated. If it is fluorescent, the color of these compounds changes from yellow to pink. In the same way, it happens with its solutions.

History

The discovery of holmium is attributed to two Swiss chemists, Marc Delafontaine and Jacques-Louis Soret, who in 1878 detected it spectroscopically while analyzing rare earth minerals in Geneva. They called it element X.

Just one year later, in 1879, the Swedish chemist Per Teodor Cleve managed to separate holmium oxide starting from erbia, erbium oxide (Er ₂ O ₃). This oxide, contaminated by other impurities, showed a brown color, which he named 'holmia', which means Stockholm in Latin.

Also, Cleve obtained another green material: 'thulia', which is thulium oxide. The problem with this discovery is that none of the three chemists were able to obtain a sufficiently pure sample of holmium oxide, as it was contaminated by atoms of dysprosium, another lanthanide metal.

It was not until 1886 that the hardworking French chemist, Paul Lecoq de Boisbaudran, isolated holmium oxide by fractional precipitation. This oxide was subsequently subjected to chemical reactions to produce holmium salts, which were reduced in 1911 by the Swedish chemist Otto Holmberg; and thus, the first samples of metallic holmium appeared.

However, today holmium ions, Ho ³⁺, are extracted by ion exchange chromatography, instead of resorting to conventional reactions.

Holmium properties

Physical appearance

Silvery, soft, ductile and malleable metal.

Atomic number

67 (₆₇ Ho)

Molar mass

164.93 g / mol

Melting point

1461 ºC

Boiling point

2600 ºC

Density

At room temperature: 8.79 g / cm ³

Just when it melts or melts: 8.34 g / cm ³

Heat of fusion

17 kJ / mol

Heat of vaporization

251 kJ / mol

Molar heat capacity

27.15 J / (mol K)

Electronegativity

1.23 on the Pauling scale

Ionization energies

First: 581.0 kJ / mol (Ho ⁺ gaseous)

Second: 1140 kJ / mol (Ho ²⁺ gaseous)

Third: 2204 kJ / mol (Ho ³⁺ gaseous)

Thermal conductivity

16.2 W / (m K)

Electrical resistivity

814 nΩ m

Oxidation numbers

Holmium can occur in its compounds with the following numbers or oxidation states: 0, +1 (Ho ⁺), +2 (Ho ²⁺) and +3 (Ho ³⁺). Of all of them, the +3 is by far the most common and stable. Therefore, holmium is a trivalent metal, forming compounds (ionic or partially ionic) where it participates as a Ho ³⁺ ion.

For example, in the following compounds, holmium has an oxidation number of +3: Ho ₂ O ₃ (Ho ₂ ³⁺ O ₃ ^2-), Ho (OH) ₃, HoI ₃ (Ho ³⁺ I ₃ ^-) and Ho ₂ (SO ₄) ₃.

Ho ³⁺ and its electronic transitions are responsible for the compounds of this metal to appear brown-yellow colors. However, when these are irradiated with fluorescent light, they turn pink. The same goes for their solutions.

Isotopes

Holmium occurs in nature as a single stable isotope: ¹⁶⁵ Ho (100% abundance). However, there are man-made radioisotopes with long half-lives. Between them we have:

- ¹⁶³ Ho (t _1/2 = 4570 years)

- ¹⁶⁴ Ho (t _1/2 = 29 minutes)

- ¹⁶⁶ Ho (t _1/2 = 26,763 hours)

- ¹⁶⁷ Ho (t _1/2 = 3.1 hours)

Magnetic order and moment

Holmium is a paramagnetic metal, but it can become ferromagnetic at a temperature of 19 K, exhibiting very strong magnetic properties. It is also characterized by having the largest magnetic moment (10.6 μ _B) among all chemical elements, as well as an unusual magnetic permeability.

Reactivity

Holmium is a metal that does not rust too quickly under normal conditions, so it takes time to lose its shine. However, when heated with a lighter, it turns yellowish, due to the formation of an oxide layer:

4 Ho + 3 O ₂ → 2 Ho ₂ O ₃

Reacts with dilute or concentrated acids to produce their respective salts (nitrates, sulfates, etc.). However and surprisingly, it does not react with hydrofluoric acid, since a layer of HoF ₃ protects it from its degradation.

Holmium also reacts with all halogens to produce their respective halides (HoF ₃, HoCl ₃, HoBr ₃ and HoI ₃).

Chemical structure

Holmium crystallizes into a compact hexagonal structure, hcp (hexagonal close-packed). In theory, Ho atoms remain cohesive thanks to the metallic bond formed by the electrons of their 4f orbitals, according to their electronic configuration:

4f ¹¹ 6s ²

Such interactions, as well as the energy arrangement of its electrons, define the physical properties of holmium. No other allotrope or polymorph is known to this metal, not even under high pressure.

Applications

Nuclear reactions

The holmium atom is a good neutron absorber, which is why it helps to control the development of nuclear reactions.

Spectroscopy

Holmium oxide solutions are used to calibrate spectrophotometers, because their absorption spectrum remains constant almost always, regardless of the impurities it contains. It also shows very characteristic sharp bands associated with the holmium atom, and not with its compounds.

Colorant

Holmium atoms are capable of providing reddish coloration to glass and artificial cubic zirconia gems.

Magnets

At extremely low temperatures (30K or less), holmium exhibits interesting magnetic properties, which are used to make powerful electromagnets, where it helps to concentrate the resulting magnetic field.

Such magnetic materials are intended for nuclear magnetic resonance; for the development of hard drives, with memories that oscillate in the order of the petabytes or terabytes; and for possibly the manufacture of quantum computers.

Holmium laser

A yttrium-aluminum garnet (YAG) crystal can be doped with holmium atoms to emit radiation with a wavelength of 2 µm; that is, we have a holmium laser. Thanks to it, tumor tissue can be precisely cut without causing bleeding, since the energy supplied cauterizes the wounds immediately.

This laser has been used repeatedly in prostate and dental surgeries, as well as to eliminate cancer cells and kidney stones.

References

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4. Dr. Doug Stewart. (2020). Holmium Element Facts / Chemistry. Recovered from: chemicool.com
5. Steve Gagnon. (sf). The Element Holmium. Recovered from: education.jlab.org
6. The Editors of Encyclopaedia Britannica. (April 03, 2019). Holmium. Encyclopædia Britannica. Recovered from: britannica.com
7. Judy Lynn Mohn Rosebrook. (2020). Holmium. Recovered from: utoledo.edu