Niels Bohr's theory led to the discovery of hafnium

Person: Niels Bohr
Lived: 1885–1962
Field: Physics
From: Danish

The story

After Henry Moseley's X-ray studies around 1914 showed how to number the elements, chemists could see exactly which places in the periodic table were still empty. One gap was element 72. The French chemist Georges Urbain claimed to have found it among the rare earth elements, a family so alike that chemists struggled to tell its members apart, and he called it celtium.

Niels Bohr's theory of how electrons are arranged in shells around the nucleus pointed somewhere else. By early 1923 Bohr, agreeing with a suggestion made by the chemist Charles Bury in 1921, held that element 72 should not be a rare earth at all, but should behave chemically like zirconium. It was a bold claim, because it set the new physics of the atom against the established methods of the chemists.

Knowing what to look for made the search far simpler. Working at Bohr's institute in Copenhagen, the Dutch physicist Dirk Coster and the Hungarian chemist George de Hevesy went through mineral samples from the city's Museum of Mineralogy, looking for a zirconium-like element. Using X-ray spectroscopy, they soon found it in zircon from Norway in 1923, proving Bohr right and Urbain wrong. They named it hafnium, after Hafnia, the Latin name for Copenhagen. It turned out to be more common than gold.

The discovery was a striking confirmation of Bohr's ideas about atomic structure and helped persuade sceptical chemists that quantum theory could predict real chemistry. Hafnium was one of the last two stable elements to be found, and the University of Copenhagen's Faculty of Science still uses a stylised hafnium atom on its seal.

The element itself proved useful. Because hafnium absorbs neutrons extremely well, it is used in control rods that regulate nuclear reactors, including those in naval submarines, and its oxide is used in making some of the smallest modern computer chips. Decades later another element, bohrium, number 107, was named in Bohr's own honour.

Why it matters

The discovery showed that quantum theory could predict real chemistry, and hafnium went on to serve in nuclear reactor control rods and modern microchips.