Harry Moseley: Numbering the Elements
Redefining the Periodic Table
His scientific output was modest – just eight short papers – but in a career spanning only four years, Harry Moseley profoundly changed our understanding of the nature of matter. As a graduate student in Ernest Rutherford’s physics laboratory at the University of Manchester in England, Moseley used newly discovered X-rays to redefine the Periodic Table, showing that it was actually organized by atomic number – the number of protons in an atom’s nucleus – rather than by atomic weight, as chemists had believed for nearly 50 years.
“Here was a man who at the age of 25 was doing experiments so profound that they would almost certainly have won him the Nobel Prize within a year or two,” says Brandeis University chemist Gregory Petsko. “But when World War I broke out, he enlisted in the army. He was sent to Europe, and he died from a sniper’s bullet at the battle of Gallipoli.” After Moseley’s death, tributes poured in from around the world, none more moving than that of American physicist Robert Millikan, who had met Moseley on a visit to Rutherford’s lab:
"A young man twenty-six years old threw open the windows through which we can glimpse the sub-atomic world with a definiteness and certainty never 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 most hideous and most irreparable crimes in history."
Born With Science In His Blood
Henry Gwyn Jeffreys Moseley had been born with science in his blood. Both his grandfathers had been members of England’s leading scientific organization, the Royal Society, and his father was a famous naturalist and Oxford University professor. But after Moseley earned a degree in physics at Trinity College Oxford, he elected to pursue graduate studies not in Oxford, Cambridge or London but 200 miles to the north, in Manchester. He was drawn there by the brightest star in physics – an irrepressible New Zealander named Ernest Rutherford, who had already won the Nobel Prize for his research on radioactivity.
Rutherford Probes the Atom
Moseley, 22, arrived in Manchester in the fall of 1910, just as Rutherford was beginning a series of groundbreaking discoveries about the structure of the atom. Thirteen years earlier, Rutherford’s Cambridge University mentor, J.J. Thomson, had discovered the electron, a tiny, negatively charged particle that he believed was a piece of every atom. Once other scientists got over their initial skepticism, the electron set off a race to identify the rest of the atom’s pieces and learn how they fit together.
Two of Rutherford’s other graduate students, Ernest Marsden and Hans Geiger, had been trying to answer this question by bombarding ultra-thin sheets of gold foil with alpha particles, the positively charged particles that Rutherford had discovered pouring out of radium during radioactive decay. Most of the time, the alpha particles would fly straight through the gold foil. But every once in a while, Marsden and Geiger observed an alpha bouncing almost straight back in their faces. “It was the most incredible thing that has ever happened to me,” Rutherford said. “It was almost as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you!”
A New Vision of the Atom
After pondering these surprising results, Rutherford came into the lab one day, shortly after Moseley’s arrival, and announced he knew what the atom looked like: All of its positive charge and most of its mass were concentrated in a tiny central core, with the electrons orbiting this nucleus, separated by empty space. “Today we take this picture that Rutherford put together for granted,” says MIT historian David Kaiser. “But it was really pretty new in its day. I think the feeling in those hallways, the laboratories of Manchester, must have been one of great excitement. They could sense that Rutherford and his team had literally cracked open a new view of matter.”
Turning to X-raysWhile all this excitement was going on around him, Moseley was consigned to repeating someone else’s radioactivity research for Rutherford. But in the spring of 1912, when a piece of his radioactivity equipment broke, he used his down time to strike out in a new direction. Working with Charles G. Darwin, grandson of the great biologist, Moseley followed up on the recent German discovery that X-rays can be “diffracted” in much the way light can be broken into a spectrum of colors with different frequencies. Moseley discovered that each element has a unique X-ray spectrum – a sort of fingerprint that can be used to identify that element. Even more surprising, he found there was a simple relationship between an element’s spectrum and its “atomic number.”
Up to then, atomic number had just referred to the number of an element’s box in the Periodic Table. But Moseley’s research showed it was actually a measure of the positive charge on an atom’s nucleus. Building on Moseley’s work, Rutherford would soon discover the subatomic particle responsible for this charge – the proton. These discoveries put the Periodic Table in a whole new light. Its author, Dmitri Mendeleev, had relied on atomic weight in building the table, but Moseley and Rutherford showed the table’s foundation is actually atomic number: Each element in the table has one more proton in its nucleus than the element before it. “Moseley and atomic number – that’s where we find out what an element really is,” says historian Lawrence Principe of Johns Hopkins University.
Knowing How Many Elements God Created
Using his X-ray spectroscope. Moseley could quickly determine whether a chemical sample contained a new element or was simply a mixture of old ones. “He could distinguish between types of matter, with a brand new technique, not dependent on their chemical properties, but by measuring their atomic numbers based on these X-rays,” Kaiser says.
This powerful tool enabled Moseley to clear up much of the confusion surrounding a group of elements called the “rare earths,” whose properties were so similar that they had confounded chemists for decades. “Moseley, who couldn’t tell one rare earth from another, had this wonderful machine that could tell whether or not the specimen in question had the right credentials to be a new element,” says biographer John Heilbron.
Even more surprising, he could tell exactly how many elements remained to be discovered – and where they would fall in the Periodic Table. “The idea that somebody could know how many elements God created – that was terrific,” Heilbron says.
A Scientist at the Front
Moseley’s scientific work was interrupted when England was drawn into World War I. Feeling he had a duty to serve, he enlisted in the Royal Engineers and was sent to Turkey by the British Army. In August 1915, during an ill-advised campaign to take of control a critical waterway called the Dardanelles, the 27-year-old communications offers was shot in the head and killed. Moseley’s death helped persuade Britain and other countries to rethink the role of scientists in war. Never again would men and women of Moseley’s scientific ability be called upon to serve at the front. Instead, they would be asked to advance the nation’s cause through research conducted far from the fighting. All the later scientists who spent their war years researching things like sonar, radar, code-breaking and the atomic bomb have Moseley to thank.
The Nobel That Might Have Been
For his remarkable use of X-rays to study the elements and redefine the Periodic Table, Moseley was nominated for the Nobel Prize in 1915. “But before the committee could consider the nominations, he was killed,” says Oxford University chemist Russell Egdell. “Nobel Prizes can be awarded posthumously only if the committee has considered the nomination before the person gets killed. So he was certainly nominated. He was a front runner. But because he was killed, he couldn’t take the prize.”
Between 1914 and 1917, three Nobel prizes were awarded for the use of X-rays to study matter – to Max Von Laue (1914); William Henry Bragg and his son William Lawrence Bragg (1915), and Charles Barkla (1917). In 1916, the year for which Moseley had been nominated, the Nobel committee awarded no prize in physics.
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