Hafnium (Hf)

• Element Symbol: Hf
• Atomic Number: 72
• Atomic Mass: 178.49
• Group # in Periodic Table: 4
• Group Name: Transition metals
• Period in Periodic Table: 6
• Block of Periodic Table: d-block
• Discovered by: Dirk Coster, George Charles de Hevesy (1923)

Hafnium is a stable, solid, metallic element. A transition metal, it is part of group 4 in the periodic table, along with titanium, zirconium, and rutherfordium. All of these elements have four valence electrons available, which makes them highly chemically reactive.

Although its existence was predicted in 1869 by Dmitri Mendeleev, the creator of the periodic table of the elements, hafnium was not definitively identified until 1923. Before its discovery some chemists believed it was a lanthanide or rare earth element. Two chemists, Dirk Coster and George Charles de Hevesy (also known as Georg von Hevesy), followed leads from such other chemists and physicists as Charles R. Bury, Friedrich Paneth, and Niels Bohr, who had suggested that an element would be found that would be closely related to zirconium. Coster and Hevesy studied zirconium ores such as zircon, but they found it difficult to chemically separate a new element from zirconium. They used x-ray spectroscopy and found the spectral lines that identified the predicted element with an atomic number of 72. It is found directly below zirconium in the periodic table. The name "hafnium" comes from Hafnia, the Latin name of the Danish capital, Copenhagen, which is where the element was discovered. Hafnium was one of the last nonradioactive elements ever discovered.

Physical Properties

Hafnium is a ductile metal, which means it can be stretched. It has a natural shiny, silver color, but it often appears to be gray because it reacts easily with the oxygen in the air. This reaction results in the corrosion-resistant layer of a hafnium oxide. The element is resistant to both acids and alkalis.

While it is closely related to zirconium chemically, hafnium is twice as dense. Hafnium has a very high melting point, 2233 degrees Celsius (°C), and a very high boiling point, 4603 °C. This means its standard state at 298 kelvins (K) is solid. In addition, the element has closely packed hexagonal crystals. It has a moderate score of 5.0 on the Mohs hardness scale.

Another physical difference between hafnium and zirconium is their ability to absorb neutrons. Hafnium readily absorbs neutrons, which makes it very useful in control rods in nuclear reactors. On the other hand neutrons pass through zirconium atoms; the element rarely absorbs any. This characteristic makes it very useful; many other parts in nuclear reactors, such as the cladding, or covering, around the control rods can be made of this element.

Chemical Properties

Hafnium is so reactive that it does not exist as a free element in nature. It makes up only about 5.8 parts per million of Earth’s upper crust.

If either hafnium or zirconium is ground finely, it can spontaneously ignite in air in a spectacularly fiery display. Due to their similar chemistry, these elements react similarly with various elements. At room temperature, they can be oxidized with the halogens (chlorine, fluorine, bromine, iodine, and astatine) to form tetrahalides. Higher temperatures are needed before they can react with boron, carbon, nitrogen, oxygen, silicon, and sulfur.

There are five stable isotopes of hafnium, but more than thirty-four isotopes total have been isolated. Its standard atomic weight is 178.49, but the weight of the stable isotopes ranges from 176 to 180. The half-life of one of its radioactive isotopes is only 0.025 seconds, whereas its most stable isotope has a half-life of 2.0 petayears—that is, 10¹⁵ years.

Hafnium can be combined with other elements to form various molecules with exceedingly high melting points. The compound with the highest known meting point in the world is tantalum hafnium carbide (Ta₄HfC₅). Its melting point is 4215 °C. Chemists using computer simulations theorize that it is possible to create hafnium-based compounds with even higher melting points.

Applications

Hafnium is rare, and it is difficult to isolate because it is virtually identical chemically to zirconium. It is almost impossible to separate the two of them using purely chemical processes. One way to do so is to repeatedly recrystallize minerals that contain both elements. In 1924 Anton Eduard van Arkel and Jan Hendrik de Boer developed a method to prepare pure metallic hafnium, which involves passing hafnium tetraiodide vapor over a hot tungsten filament. This process is still a commonly used method, but because the presence of both hafnium and zirconium in nuclear reactors made it very important to develop more efficient methods for separating the two elements—any impurities would greatly diminish their effectiveness—a liquid-to-liquid extraction process was developed that uses a wide variety of solvents. After the two elements are separated using this method, additional processing is needed to yield pure hafnium.

Hafnium may be difficult to isolate, but when it is isolated, its physical and chemical properties make it useful in many different fields. Its ability to be used in corrosive environments at high temperatures, for example, makes it ideal for use in the specialized nuclear reactors of military submarines.

Its high melting point and resistance to corrosion mean that the element has many practical applications. It can be used in filaments, vacuum tubes, and electrodes. Its heat resistance combined with its reactivity with nitrogen makes it ideal for use in incandescent bulbs that are filled with that gas. Its oxides are often used in the manufacturing of specialized integrated circuits in which size and the ability to withstand extreme conditions is important—as in satellites. Hafnium is used in alloys of titanium, niobium, and tantalum for the nozzles in rocket engines.

The presence of minute amounts of the long-lived isotopes of hafnium in minerals such as garnet and zircon means that they can be used in geochronometry. That is, they can be used to tell the ages of rocks, even those found in Earth’s mantle.

Hafnium is rarely found on its own. Most zirconium minerals, such as zircon, have minute amounts of hafnium in them. About half of all of the hafnium produced is manufactured as a byproduct during the production of zirconium. This means that the major sources of hafnium are the same as the sources of zirconium. Both are found in heavy mineral sands ore deposits in Brazil and Malawi, so digging for titanium sometimes produces yields of these elements as side products. They can also be found in carbonatite intrusions in western Australia.

Manufacturers have to be careful when working with hafnium because fine particles made during processing can be explosive. The element is not toxic, but it is always safer to be cautious when handling the metallic salts it can be found in.

Bibliography

Coursey, J. S., et al. Atomic Weights and Isotopic Compositions with Relative Atomic Masses. NIST Physical Measurement Laboratory. Natl. Inst. of Standards and Technology, 30 Sept. 2015. Web. 13 Nov. 2015.

Emsley,John. Nature’s Building Blocks: An A–Z Guide to the Elements. 2nd ed. New York: Oxford UP, 2011. Print.

Haynes, William M., ed. CRC Handbook of Chemistry and Physics. 95th ed. Boca Raton: CRC, 2014. Print.

Kaye & Laby Tables of Physical & Chemical Constants. Natl. Physical Laboratory, 2015. Web. 13 Nov. 2015.