Henry Moseley – Periodic Table
Henry Gwyn Jeffries Moseley was born in Dorset in November 1887. His parents were Anabel Gwyn Jeffreys, daughter of biologist John Gwyn Jeffreys, and Henry Nottidge Moseley, biologist, anatomist and physiologist who was part of the Challenger expedition of the 1870s, which circumnavigated the world and discovered thousands of previously unknown marine species.
After spending his early years at Summer Field School, Henry (known as Harry to his friends) was offered a scholarship to Eton, where he went on to win Chemistry and Physics prizes in 1906 before successfully gaining entry to Oxford later that year. Sadly, as a result of a severe bout of hayfever, he did not perform as well as he had hoped in his final exams, graduating in 1910 with a second class honours rather than the First he had anticipated. Nonetheless, he was offered a graduate teaching assistant position at the University of Manchester under eminent experimental physicist Ernest Rutherford, who had been awarded the Nobel Prize for Chemistry in 1908, for his work in Nuclear Physics and his discovery of the concept of a radioactive half-life, proving that radioactivity involved the nuclear transmutation of one chemical element to another which contributed to what we know of how elements and radioactive substances work.
Moseley was not comfortable with the teaching side of his work, and after discussions with Rutherford, was able to drop the teaching in favour of research. In 1913, Rutherford offered him a fellowship, which he turned down, choosing instead to return to Oxford in the hope that a more suitable position was to become vacant. He was given laboratory space in which to work, but was to self-fund his research. The previous year, he had discovered through experimentation with energy beta particles that using a radioactive source of radium would produce high potentials, enabling him to invent the first atomic battery. This work would form the basis of later development which contributed to long life radium batteries used for spacecraft and cardiac pacemakers amongst others. His goal had actually been to prove or disprove Einstein’s special theory of relativity – that mass increases through velocity.
Moseley moved on from this area of work, to experimenting further with X-ray spectra. He was interested in the as yet unproven hypothesis submitted by Antonius Van den Broek, that the atomic number of elements on Mendeleev’s Periodic Table were equal to the amount of charge in the nucleus of the atom. Until his theory, the atomic number simply represented the position Mendeleev had attributed it on the table. Van den Broek had no way of proving his theory. Moseley took up the work. By using Bragg’s diffraction law, formulated by father and son team William Henry and William Lawrence Bragg in 1913, and by bouncing high energy electrons off solids, such as metals, X-rays could be produced.
In his lab, he developed his own equipment and quickly devised an experiment with Van den Broek’s hypothesis in mind, and proceeded to bounce similar high energy electrons at the different elements, and by measuring the wavelengths and frequencies of the resulting X-rays, was able to realise that each was unique; further when he worked out the square root of these different frequencies, and plotted them on a graph, the produced a straight line. These properties equaled the atomic numbers of the elements on the Periodic table. Van den Broek’s theory was confirmed. Further each element had a positive charge increasing one unit, therefore showing that each had an atomic number identical to the number of protons it contained; therefore, each element is defined by its number of protons. This mathematical relationship became known as Moseley’s law. By restructuring Mendeleev’s Periodic table using his results, Moseley was further able to show that four elements were missing, 43, 61. 72 and 75. These would be discovered over the next twelve years.
In August 1914, the Great War broke out. Despite his parents begging him not to, and the offer of continuing his research with Rutherford and others, Moseley left his studies and enlisted in the Army. They were initially reluctant to take such a pioneer in science; however, Moseley was adamant. Given a commission as 2nd Lieutenant, he was to find himself in April 1915 in Gallipoli, where on August 10th, whilst telephoning orders, he was shot in the head by a Turkish sniper, killing him instantly. Following his death, Rutherford lobbied Parliament until they passed a ban on allowing important and promising scientists to join up. The following year, 1916, the Nobel Prizes for both Physics and Chemistry went unawarded. It was felt that should he have lived, Moseley would definitely have received one of them. Henry Moseley was one of many young men whose remains were unidentified following the Battles of Gallipoli. He is commemorated on a memorial, known as “The Farm”, little visited on a steep hillside overlooking the ocean.
Bragg and Bragg would go on to receive a joint award of the Nobel Prize for Physics in 1915. In 1914 Max von Laue received the Prize for Physics for his discovery of the diffraction of X-Rays by crystals; Charles Barkla was awarded the Prize for Physics in 1917 for continuing the work into X-rays on crystalline elements, and Karl Siegbahn who also developed Moseley’s work was awarded the Prize again for Physics, in 1924. In a sad twist, Robert Charles Bragg, son/brother of Williams Sr and Jr was also killed at Gallipoli just three weeks after Moseley. Dmitri Mendeleev was nominated twice for his own Nobel Prize but was never to win the award.