Moseley, Henry Gwyn Jeffreys (1887-1915) was a British physicist who first established the atomic numbers of the elements by studying their X-ray spectra. This led to a complete classification of the elements, and also provided an experimental basis for an understanding of the structure of the atom. Moseley was born at Weymouth on 23 November 1887. He came from a family of prestigious scientists, his father being Professor of Anatomy at Oxford and a member of the great Challenger expedition that surveyed the world's oceans from 1872 to 1876. Moseley was educated at Eton and in 1906 entered Trinity College, Oxford, where he obtained a degree in physics in 1910. Immediately after graduation, he was appointed lecturer in physics in the laboratory of Ernest Rutherford (1871-1937) at Manchester, becoming a research fellow in 1912. Moseley remained with Rutherford until 1913. He then worked privately at Oxford with a view to obtaining a professorship there, and in 1914 he visited Australia. On the outbreak of World War I, he returned home and immediately enlisted in the army. Fearing for his safety, Rutherford endeavoured unsuccessfully to secure him scientific duties. Moseley was sent to the Dardanelles and was killed at Gallipolli on 10 August 1915. Moseley was a researcher only from 1910 until 1914, and yet he made discoveries which were of fundamental importance to the development of both physics and chemistry. When he joined Rutherford's group in 1910, Rutherford was researching the phenomena associated with natural radioactivity. Moseley at first helped Rutherford in this work, but when reports of the diffraction of X-rays by Max von Laue (1879-1960) reached him in 1912, Moseley persuaded Rutherford to allow him to study X-ray spectra. He received instruction in X-ray diffraction from Lawrence Bragg (1890-1971) and in 1913 Moseley introduced X-ray spectroscopy to determine the X-ray spectra of the elements. In a series of brilliant investigations, Moseley allowed the X-rays produced from various substances used as a target in an X-ray tube to be diffracted by a crystal of potassium ferrocyanide. The glancing angles were measured accurately and the position of the diffracted beams determined to obtain the wavelengths and frequencies of the X-rays emitted. Moseley examined metals from aluminium to gold and he found that their X-ray spectra were similar but with a deviation that changed regularly through the series. He found that a graph of the square root of the frequency of each radiation against the number representing the element's position in the periodic table gave a straight line. He called this number the atomic number of the element, which has since been shown to be the positive charge on the nucleus and thus the number of protons in the nucleus. Since the atom is electrically neutral, the atomic number is also the number of electrons surrounding the nucleus. It was as a direct result of this work that atomic numbers were placed on a sound experimental foundation. Moseley found that when the elements are arranged in the periodic table according to their atomic numbers, all irregularity caused in the older system of grouping elements by their atomic weight disappeared. Now that the elements were numbered, the rare earth elements could be sorted out, a process that Moseley began at Oxford towards the end of his life. The numbering system also enabled Moseley to predict that several more elements would be discovered, namely those with atomic numbers of 43, 61, 72, 75, 87 and 91. These were all found in due course. Although the number of elements which Moseley was able to examine was limited, the equation relating the square root of the frequency to the atomic number has been found to hold in all cases. The equation is known as Moseley's Law. It has enabled scientists to identify a total of 105 elements in a continuous series of atomic numbers. Any further elements that might be produced by nuclear reactions can only have greater atomic numbers. In 1913 and 1914, the young physicist published his findings in two remarkable papers in the Philosophical Magazine and entitled them The High-Frequency Spectra of the Elements. Moseley's discovery told how many electrons were present in any element, and tied in nicely with the quantum theory of the hydrogen atom which was published in 1913 by Niels Bohr (1885-1962). Moseley's fundamental discovery was a milestone in our knowledge of the constitution of the atom, and we are left to ponder on what this great brain might have discovered had he not been so tragically killed at so young an age. Author not available, Moseley, Henry Gwyn Jeffreys (1887-1915). , The Hutchinson Dictionary of Scientific Biography, 01-01-1998. |