Henry Moseley and a tale of seven elements

By Eric Scerri

This year marks the 100^th anniversary of a remarkable discovery by an equally remarkable scientist. He is Henry Moseley, whose working career lasted a mere four years before he was killed in World War I shortly before his 26^th birthday. Born in 1887 in England, Moseley came from a distinguished scientific family. Both of his grandfathers — a mathematical physicist on his father’s side and an oceanographer on his mother’s side — and his father were fellows of the Royal Society. His own father, who died when Henry was just four years old, had also been a zoologist whose book had been praised by Charles Darwin.

Young Henry Moseley attended Eton College on a scholarship and began to show early academic promise. He then studied natural science at Trinity College, Oxford having again obtained a scholarship. He didn’t think much of his teachers at Oxford, whom he once described as being more interested in fox hunting than science. Moseley wasted no time in contacting Britain’s leading physicist, Ernest Rutherford, who was then at the University of Manchester. Rutherford obviously recognized a kindred spirit, accepting the young graduate even though he had only obtained a second-class degree in physics.

After preliminary experiments involving radioactivity, which were proposed by Rutherford, Moseley began to develop an interest in X-rays, which had by then become a hot topic. After many years of debate as to their nature it was still unclear whether they consisted of waves or particles. Then in 1912, a breakthrough seemed to occur after von Laue in Germany suggested that X-rays might have very small wavelengths and might consequently be diffracted by objects as small as planes of atoms within crystals.  Even though this prediction was quickly confirmed others continue to wonder whether X-rays might still also have particulate properties.

At this point Moseley teamed up with Charles Darwin, the grandson of the Darwin, and produced a paper based on a detailed study of how X-rays behaved when reflected by metal targets. Soon Moseley, who had always had a keen interest in chemistry, began to examine how a sequence of elements following each other in the periodic table might behave when acting as targets for beams of X-rays. He began with experiments on a sequence of ten elements from calcium to zinc inclusive. He omitted scandium which falls immediately after calcium because he was not able to obtain a sample of it.

Nevertheless, the outcome was remarkably clear and simple. If the square roots of the frequencies of the diffracted X-rays were plotted against a series of whole numbers, a smooth graph was obtained. This meant that he had discovered a method for counting the elements and a means of finding which elements, if any, remained to be discovered. In 1914 he extended his study to encompass most of the known elements between aluminum and gold, and still the same simple relationship held out. In the remainder of his short life he immediately set about applying his method to many long-standing problems and some new ones.

First of all, the lightest elements in the periodic table had long been surrounded in mystery. The former use of atomic weights to order the elements suggested that one or perhaps two elements might be missing between hydrogen and helium, the two lightest known elements. Also, some authors had reported new spectral lines, which were attributed to possible missing elements called coronium and nebulium.

Secondly, there had been much confusion about known many rare-earth elements existed in the sixth row of the periodic table and whether some newly reported elements were genuine or not. Moseley personally examined samples of a supposed new element named celtium. By measuring the X-ray frequencies that this sample produced he confidently ruled against the existence of any new element.

Thirdly, and perhaps most importantly, he was able to resolve the long-standing controversy over the order in which the elements cobalt and nickel should be placed in the periodic table. The former approach of using increasing atomic weights to order the elements implied that nickel should come before cobalt. Moseley’s method showed otherwise because cobalt had the lower atomic number associated with its X-ray spectrum.

But alas all this brilliant work was cut short because World War I broke out and Moseley insisted on volunteering to fight in the trenches in spite of efforts to prevent him from doing so by Rutherford among others. He was killed on 10 August 1915 by a bullet to the head at the battle of Gallipoli in Turkey. It was left to others to apply his X-ray method further and it soon became clear that precisely seven elements remained to be discovered between the limits of the periodic table that stood between hydrogen and uranium.

Oddly enough, the fact that the search had been clearly narrowed down to just seven elements with known atomic numbers did not seem to diminish the level of controversy and argumentation over the next thirty or so years before they had all been correctly identified. The seven elements, all rather exotic, are protactinium (1917), hafnium (1923), rhenium (1925), technetium (1937), francium (1939), astatine (1940), and promethium (1945). Three of them, technetium, astatine, and promethium had to be artificially synthesized before their discovery could be confirmed. Almost all of these seven ‘discoveries’ were surrounded by controversy as well as acrimonious disputes of a personal and, in some cases, of a nationalistic nature. Above all the tale of these seven elements continues to affirm the essentially human and frail nature of scientific discovery.

Eric Scerri is a leading philosopher of science specializing in the history and philosophy of the periodic table. He is the author of A Tale of Seven Elements, The Periodic Table: A Very Short Introduction, and The Periodic Table: Its Story and Its Significance. He is also the founder and editor in chief of the international journal Foundations of Chemistry and has been a full-time lecturer at UCLA for the past twelve years where he regularly teaches classes of 350 chemistry students as well as classes in history and philosophy of science. He is also giving the Moseley Centennial Lecture at the American Physical Society April meeting on Monday, 15 April 2013, 10:45 AM–12:33 PM.

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