Henry Moseley

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This article is about the physicist; for the naturalist see Henry Nottidge Moseley

Henry Moseley at work. This rare image is the only non-portrait photograph known of the scientist

Henry Gwyn Jeffreys Moseley (November 23, 1887 – August 10, 1915) was an English physicist. His main contribution to science, the quantitative justification of the concept of atomic number in Moseley's law, advanced chemistry and provided independent support for the Bohr model of the Rutherford nuclear atom.

1 Biography
2 Contribution to our understanding
3 Use of X-ray spectrometer
4 Further reading
5 See also
6 References
7 External links

[edit] Biography

Moseley was born in Weymouth, on the south west coast of England, 1887, the youngest of 22 children. Their father was a naturalist, a Professor of Anatomy at Oxford and a member of the Challenger Expedition. <ref>Chemcool bio</ref> He attended Eton College on a King's scholarship <ref>(JSTOR article; permission required)</ref>. In 1906, he entered Trinity College of the University of Oxford, and on graduation from that institution in 1910 went to Manchester University to work with Ernest Rutherford. During his first year at Manchester, he had a full teaching load, but after a year he was relieved of his teaching duties and began full-time research.

In 1913, by using x-ray spectra obtained by diffraction in crystals, he found a systematic relation between wavelength and atomic number, Moseley's law. Previous to this, atomic numbers had been thought of as an arbitrary number, based on sequence of atomic weights, but altered when necessary (for example, by Dmitri Mendeleev) to put an element in the appropriate place in the periodic table. For example, cobalt and nickel had been assigned atomic numbers of 27 and 28, respectively, based on their chemical properties, since they have nearly identical atomic weight (in fact, cobalt's atomic weight is larger than nickel's, which would have reversed them). Moseley's experiments were able to show directly that cobalt and nickel have clearly differing atomic numbers of 27 and 28, and are correctly placed in the periodic table by an objective measure. Moseley's discovery thus showed that atomic numbers were not arbitrary, but have an experimentally measurable basis.

In addition, Moseley showed that there were gaps in the atomic number sequence at numbers 43, 61 and 75 (these are now known, respectively, to be radioactive, non-naturally-occurring, technetium and promethium, and the last discovered naturally-occurring element rhenium). Mendeleev had previously predicted technetium, and Bohuslav Brauner had previously predicted promethium; Moseley confirmed their predictions, predicted one additional undiscovered element, and showed there were no other gaps in the periodic table between aluminum and gold.

In 1914, he resigned at Manchester to return to Oxford to pursue his research, but when World War I broke out, he turned down a job offer and enlisted in the Royal Engineers. He fought at Gallipoli, where he was killed in action by a sniper in 1915, shot through the head while in the act of telephoning an order. Many have since speculated that he could have won the Nobel Prize, but was unable to because it is only awarded to the living. It is speculated that because of Moseley's death in the War that the British and other world governments no longer allowed their scientists to enlist in combat.

Only twenty-seven years old at death, Moseley could in many scientists' opinions have contributed much to the knowledge of atomic structure had he lived. As Niels Bohr once said in 1962, "You see actually the Rutherford work [the nuclear atom] was not taken seriously. We cannot understand today, but it was not taken seriously at all. There was no mention of it any place. The great change came from Moseley."

[edit] Contribution to our understanding

Previous to Moseley and his law, atomic numbers had been thought of as an semi-arbitrary arbitrary number, vaguely increasing with atomic weight but not defined by it. Moseley's discovery showed that atomic numbers were not arbitrary but have a physical basis. He redefined the idea of atomic numbers from its previous status as an around-about approximate numerical tag to help sorting, i.e. in the periodic table, into a real and objective whole-number quantity which was experimentally directly measurable. Furthermore, as noted by Bohr, Moseley's law provided a complete experimental set of data supporting the (at that time new) Rutherford/Bohr concept of the atom, in which atomic number is understood as representing physically the number of positive charges (protons) in a central atomic nucleus. A simple modification of Bohr's formula was found to give Moseley's empirically-derived law for measurement of atomic number.

[edit] Use of X-ray spectrometer

X-ray spectrometers as Moseley knew them worked as follows; electrons are fired at a substance (i.e. element in his case) causing ionisation of a core electron. Decay of the core hole then leads to emission of x-rays which are diffracted by a crystal. Applications of Bragg's law then allows the wave length of the emitted x-rays to be determined.

[edit] Further reading

John L. Heilbron, H. G. J. Moseley: The Life and Letters of an English Physicist, 1887-1915, University of California Press Berkeley and Los Angeles, California, 1974. ISBN 0-520-02375-7.