Holmium (Ho)

• Element Symbol: Ho
• Atomic Number: 67
• Atomic Mass: 164.9303
• Group # in Periodic Table: n/a
• Group Name: Lanthanides
• Period in Periodic Table: 6
• Block of Periodic Table: f-block
• Discovered by: Marc Delafontaine, Jacques-Louis Soret, Per Teodor Cleve (1878)

Holmium is a rare earth metal. This places it in the lanthanide series of elements in the periodic table. This series of elements ranges from lanthanum, with an atomic number of 57, through lutetium, with an atomic number of 71. The name of the series derives from the fact that the lighter elements react chemically in ways similar to lanthanum.

Holmium was discovered two separate times in 1878, first by Marc Delafontaine and Jacques-Louis Soret of Switzerland, who were studying spectrographic absorption bands to differentiate elements. The two men recognized that previously unobserved bands belonged to an unknown element that they temporarily dubbed "element X."

Later that year, Per Teodor Cleve became the second scientist to discover the element, while studying ores containing erbium oxide. He encountered it as one of the impurities in his sample. He named the brown-colored substance "holmia," after the Greek name for Stockholm, where he lived. The pure metal was isolated by Otto Holmberg in 1911.

At first holmium was incorrectly placed in the periodic table under the atomic number 66. The physicist Henry Moseley had been given a sample of holmium to analyze that turned out to be extremely contaminated with the element dysprosium, which had not yet been placed in the periodic table. Moseley had examined the x-ray emission lines for both elements but had thought that the main ones belonged to holmium instead of to dysprosium.

Physical Properties

Holmium is a solid in its standard state at 298 kelvins (K). It is a relatively soft metal with a bright, silver-white luster that is not easily seen outside of laboratories because the element rusts easily. While stable in dry air at standard temperature and pressure, it quickly oxidizes in moist, hotter conditions to form holmium oxide. Under different lighting conditions, the perceived color of holmium oxide can vary from tannish yellow to fiery orange-red. Holmium has hexagonal, closely packed crystals and is very malleable and ductile. This means it can be easily bent and stretched. It is fairly resistant to corrosion. It has a high melting point of 1472 degrees Celsius (°C) and a high boiling point of 2700 °C.

Holmium’s magnetic properties are unusual. It has the highest magnetic permeability of any element found in nature. This means that it is the element most able to support the formation of magnetic fields within itself. At room temperature it is paramagnetic. It is attracted by outside magnetic fields that induce a magnetic field within it, but those magnetic properties fade when the field is removed. If holmium is cooled to −254 °C, it becomes ferromagnetic and is able to retain its magnetic field once one has been induced.

Holmium also strongly absorbs neutrons. This ability makes it very useful in controlling the rates of nuclear reactors.

Chemical Properties

Holmium reacts slowly with cold water and air, causing it to tarnish. At high temperatures it reacts with oxygen to form holmium oxide and with water to form holmium hydroxide. It reacts with all halogens, including fluorine, chlorine, iodine, and bromine. It dissolves easily in acids, forming ions with a +3 oxidation, which means the atom has gained three electrons. These ions can become fluorescent and emit unique light wavelengths. The ease with which holmium reacts with other elements means that it cannot exist as a pure element in nature.

Holmium has one stable isotope. One metastable radioactive isotope has a half-life of 1,200 years, whereas most of the others have half-lives of less than three hours. When the metastable isotope does decay, it emits a spectrum of gamma rays.

Applications

The spectrum of gamma rays released by the metastable isotope of holmium can be used to calibrate the gamma-ray spectrometers that are used in nuclear research both on Earth and in outer space. A solution of holmium oxide and perchloric acid is also used to calibrate optical spectrophotometers.

Holmium is used in the solid-state lasers of microwave equipment that have applications in the fields of medicine and dentistry. These lasers, which also play important roles in fiber optics, are used for many other applications. Another optical application of holmium is to add red and yellow color to glass and cubic zirconia for use in jewelry.

Holmium’s unique magnetic properties make it ideal for creating strong, magnetic fields. It is used as the pole piece around high-strength natural magnets. It acts as a magnetic flux concentrator.

The ability of holmium to absorb neutrons makes it ideal for use in nuclear reactors. It is what is known as a "burnable poison." This means that as it absorbs neutrons, it gradually loses the ability to absorb any more neutrons. This ability makes the element useful for controlling the rate of reaction in nuclear fuel without control rods.

Holmium is found in rare earth minerals such as gadolinite and monazite. It is difficult to extract from these ores, but it can be separated using ion exchange. It is mined around the world in China, Brazil, Australia, India, and the United States. It is the fifty-sixth most abundant element in Earth’s crust, making up 1.4 parts per million by weight. It is rarer in water and air, and it is rare compared to other lanthanides. It makes up only 500 parts per trillion of the universe.

Holmium does not play a role in the biology of plants or animals. It is not easily absorbed by plants. When its presence in vegetables has been measured, its concentration has been found to be very low: only about 100 parts per trillion. Its salts can increase people’s metabolisms, but in a typical year a person only eats about one milligram of holmium. It is toxic only if large amounts of its salts are inhaled, eaten, or injected. The long-term effects of holmium on humans are unknown.

Bibliography

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Gupta, C. K., and Nagaiyar Krishnamurthy. Extractive Metallurgy of Rare Earths. Boca Raton: CRC, 2005. Print.

Hammond, C. R. "The Elements." CRC Handbook of Chemistry and Physics. Ed. William M. Haynes. 96th ed. Boca Raton: CRC, 2015. 4-1–42. Print.

Jiles, David. Introduction to Magnetism and Magnetic Materials. 3rd ed. Boca Raton: CRC, 2016. Print.

MacDonald, Roderick P. "Uses for a Holmium Oxide Filter in Spectrophotometry." Clinical Chemistry 10.12 (1964): 1117–20. Web. 13 Nov. 2015.

Patnaik, Pradyot. Handbook of Inorganic Chemicals. New York: McGraw, 2003. Print.