Hafnium

The History of Hafnium: The Triumph of Quantum Theory. Hafnium is a bright, silvery, ductile transition metal. It is located just below zirconium (Zr) in the periodic table. This position is the key to its entire history, as hafnium and zirconium are chemically so similar that they are almost indistinguishable. Wherever zirconium is found in nature, hafnium is there too, hidden within it.

This is why hafnium was the second-to-last stable natural element to be discovered. At the beginning of the 20th century, Mendeleev's periodic table was beginning to fill up, but there were still gaps. One of these gaps was at position 72, between lutetium (element 71) and tantalum (element 73). Most chemists thought element 72 must be another "rare earth", extending the lanthanide series after lutetium.

This is where the famous Danish physicist Niels Bohr, one of the fathers of quantum mechanics, intervened. According to Bohr's calculations, the "rare earth" (lanthanide) series had to end exactly with element 71 (lutetium). Therefore, element 72 could not be a rare earth. It had to begin a new electron subshell and behave chemically like the element just above it in group 4: zirconium.

In January 1923, two scientists working at Bohr's institute in Copenhagen took up the challenge: Dutch physicist Dirk Coster and Hungarian chemist George de Hevesy. They took zircon (a zirconium mineral) and used X-ray spectroscopy to identify spectral lines that did not match any known element. They had found it, hidden within zirconium, exactly as Bohr had predicted. To honor the city where the discovery took place, they named the element Hafnium, after Hafnia, the Latin name for Copenhagen.

Discovering hafnium was one thing; separating it from zirconium was another. It was not until the 1940s and 1950s, driven by the needs of the emerging nuclear industry, that efficient industrial processes were developed to obtain pure hafnium.

Hafnium's history takes an ironic turn with the advent of nuclear energy. Although zirconium and hafnium are chemical twins, they are absolute opposites on the nuclear level: Zirconium lets neutrons pass through very easily, while Hafnium absorbs neutrons with incredible voracity. This crucial difference made the separation of the two imperative. Hafnium's ability to absorb neutrons made it the material of choice for nuclear reactor control rods (especially in submarines).

In the 21st century, hafnium has found new glory in the semiconductor industry. Hafnium-based oxides are used in modern processors (such as those from Intel or AMD) to create microscopic insulators (high-k dielectrics), enabling chips to be made smaller and faster.