Elements 72–94

Hafnium

Category: transition metal Atomic number: 72 Colour: silvery grey Melting point: 2,233°C (4,051°F) Boiling point: 4,603°C (8,317°F) First identified: 1923

To explain how hafnium was discovered, we need first to explain a significant breakthrough in our understanding of the periodic table. In 1911, the Dutch amateur physicist Antonius van den Broek suggested (without evidence) that the position of an element in the periodic table might better be defined by the amount of charge in the atom’s nucleus. The young British physicist Henry Moseley had recently joined Ernest Rutherford’s research group at the University of Manchester, where he created the world’s first prototype atomic battery – he became fascinated by van den Broek’s suggestion and set out to investigate it. He knew that when high-energy electrons collided with solids, they emitted X-rays. Returning to Oxford and funding his own research, he put together experimental apparatus that would allow him to fire electrons at various elements and measure the wavelengths and frequencies of the X-rays they emitted.

This led to the crucial discovery that each element emits X-rays at a unique frequency, and that this could be matched up perfectly to the element’s atomic number (how many protons it has). This confirmed van den Broek’s hypothesis and showed that the number of protons completely defined an element. Chemists quickly realized the significance of Moseley’s work, as they could now rearrange the periodic table in a way that resolved any lingering doubts about its anomalies and flaws (and could also use X-rays as a much quicker way to identify elements).

Journey’s End

Henry Moseley was strongly encouraged by his seniors to continue his scientific work after the Second World War broke out, but he insisted on joining the army. He was killed in 1915 at the Battle of Gallipoli. Robert Millikan, the physicist, wrote of him subsequently: ‘a young man but twenty-six years old threw open the windows through which we can now glimpse the subatomic world with a definiteness and certainty never even dreamed of before. Had the European war had no other result than the snuffing out of this young life, that alone would make it one of the most irreparable crimes in history.’

This also meant that new gaps opened up in the table, prompting searches for the elements with sixty-one, seventy-two and seventy-five protons (and confirming that Mendeleev’s final missing element 43 was still to be found). Over the years, the elements 43, 61 and 75 would be shown to be technetium, promethium and rhenium.

Element 72 was discovered in 1923 by two young researchers, Dirk Coster and George de Hevesy, who worked at the institute of the great physicist Niels Bohr in Denmark. There had been ongoing debate about whether element 72 would turn out to be a lanthanide or a transition metal, but Bohr had argued it must be a metal. On this basis, Coster and de Hevesy investigated ores of zirconium, which was the transition metal above element 72 in the revised table. Within a few weeks, they had found traces of hafnium in the ores, using X-rays.

Hafnium has similar properties and uses to zirconium – both are used in nuclear reactors as they absorb neutrons well. It is also used in alloys that need to be strong and have a high melting point, and in plasma welding torches for the same reason.