Holmium is a silvery lanthanide metal with the highest magnetic moment of any naturally occurring element. It is soft, malleable, and forms a protective oxide layer in air; used in strong permanent magnets, specialty alloys, and laser materials.
The ground-state configuration of holmium is [Xe] 4f11 6s2. In its most common oxidation state (+3), it loses two 6s electrons and one 4f electron to form Ho3+ with a 4f10 configuration.
Holmium possesses 11 unpaired 4f electrons, giving it a very large total magnetic moment (~10.6 Bohr magnetons per atom). This strong magnetic property arises from the aligned spins of unpaired electrons and the partially filled f-shell.
The predominant oxidation state of holmium is +3 (Ho3+), forming compounds such as Ho2O3 and HoCl3. The +2 oxidation state is rare and unstable, seen only under specific reducing conditions.
Holmium slowly oxidizes in air, forming a yellowish oxide layer of Ho2O3 that protects the underlying metal. It reacts slowly with cold water and more vigorously with hot water to form hydroxide and hydrogen gas:
\(\mathrm{2\,Ho(s) + 6\,H_2O(l) \rightarrow 2\,Ho(OH)_3(s) + 3\,H_2(g)}\)
Key applications include:
Holmium is used as a dopant in Ho:YAG lasers that emit infrared light at about 2.1 µm wavelength. These lasers are important in medical applications (such as tissue ablation and lithotripsy) due to their strong absorption by water and biological tissues.
Holmium(III) compounds usually appear pale yellow to pink. These colors arise from sharp f–f transitions within the 4f shell, typical of lanthanide ions.
Holmium is paramagnetic at room temperature and becomes ferromagnetic below 20 K. Its high magnetic moment allows it to align strongly in magnetic fields, making it useful in magnetic refrigeration and research magnets.
Holmium is extracted from rare-earth minerals such as monazite and bastnäsite. It is separated from other lanthanides using solvent extraction and ion-exchange chromatography due to the close chemical similarity among the lanthanides.
Holmium and its compounds are considered to have low toxicity. However, fine powders can be flammable and irritating. Proper precautions such as using gloves, goggles, and fume hoods should be taken when handling holmium materials.
The oxidation of holmium metal in air can be represented as:
\(\mathrm{4\,Ho(s) + 3\,O_2(g) \rightarrow 2\,Ho_2O_3(s)}\)
This forms holmium(III) oxide, a stable compound used in ceramics and glass coloring.