The Mystery of Matter

Harry Moseley: Numbering the Elements

Harry Moseley as an undergraduate in the Balliol-Trinity physics laboratory at Oxford UniversityHarry Moseley as an undergraduate in the Balliol-Trinity laboratory at Oxford University Ernest RutherfordIn 1910, Moseley moved to Manchester to study under Nobel laureate Ernest Rutherford. Manchester UniversityGenerously endowed by local industrialists, the University of Manchester featured one of the finest physics laboratories in the world.

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.

1912 the Manchester University In 1912 the University of Manchester physics laboratory headed by Ernest Rutherford (second row center) included Harry Moseley (seated second from left), Hans Geiger (inventor of the Geiger counter), Charles G. Darwin (grandson of the great biologist) and James Chadwick, who would later win the Nobel Prize for discovering the neutron. CLICK TO ENLARGE Moseley in uniformMoseley in uniform in 1915

J.J. Thomson of Cambridge University J.J. Thomson of Cambridge University discovered the electron in 1897, setting off a race to find the rest of the atom’s pieces.

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!”

Film Clip: The Discovery of the Electron – In 1897, physicist J.J. Thomson discovers the first piece of the atom, an impossibly small, negatively charged particle called the electron.
Rutherford's vision of the atomRutherford’s vision of the atom: a dense core called the nucleus containing the atom’s positive charge and most of its mass, surrounded by tiny, negatively charged electrons, orbiting at a much greater distance from the nucleus than this drawing suggests. Manchester UniversityIn 1912, a German team headed by Max Von Laue discovered that X-rays could be diffracted in much the way light can. The result was a symmetrical pattern of spots.

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-rays

While 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.”

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.

Film Clip: The Discovery of the Nucleus – In 1910, Rutherford and two of his graduate students make the stunning discovery that all of the atom’s positive charge, and most of its mass, are concentrated in a tiny central core he calls the nucleus. Video: Moseley’s Brilliant Experiment – While others are using X-ray diffraction to study the spacing of atoms in crystals, Moseley uses it to study the X-ray spectra of the elements. His work leads to the discovery that each element is defined by its atomic number – the number of protons in its nucleus.
Turkish bathing in the ocean.In one of his last letters home to his mother, Moseley wrote that sea bathing made the Turkish heat tolerable.

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.

Video: Mysteries of the Periodic Table, Part 2: The Rare Earths – With his X-ray spectroscope, Moseley can quickly tell if a chemical sample is a new element. He uses his instrument to sort out a confusing group of elements called the rare earths.

Because brilliant scientists like Moseley were considered “too valuable to die,” in future wars they would serve by doing research on things like code-breaking, radar and the atomic bomb.

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.

Want to Learn More?

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Video: Changing the Role of Scientists in War – When Moseley is killed during World War I at the age of 27, it prompts Britain and other countries to rethink the role of scientists in war.

Bragg and his sonWilliam Henry Bragg and his son, William Lawrence Bragg, were among those who won the Nobel Prize for their work on X-ray diffraction. Moseley was nominated but died before he could be considered.