Aston , Francis William

Aston , Francis William
(1877–1945) British chemist and physicist
Aston was born in Harborne, England, the son of a metal merchant. He was educated at Mason College, the forerunner of Birmingham University, where he studied chemistry. From 1898 until 1900 he did research under P.F. Frankland on optical rotation. He left Birmingham in 1900 to work in a Wolverhampton brewery for three years. During this time he continued with scientific research in a home laboratory, where he worked on the production of vacua for x-ray discharge tubes. This work came to the notice of J.H. Poynting of the University of Birmingham who invited Aston to work with him. He remained at Birmingham until 1910 when he moved to Cambridge as research assistant to J.J. Thomson. He became a research fellow at Cambridge in 1920 and stayed there for the rest of his life, apart from the war years spent at the Royal Aircraft Establishment, Farnborough. Aston's main work, for which he received the Nobel Prize for chemistry in 1922, was on the design and use of the mass spectrograph, which was used to clear up several outstanding problems and became one of the basic tools of the new atomic physics.
Thomson had invented an earlier form of spectrograph in which a beam of positive rays from a discharge tube passed through a magnetic and an electric field, which deflected the beam both horizontally and vertically. All particles (ions) with the same mass fell onto a fluorescent screen in a parabola. Aston improved the design by using a suitable magnetic field, so that ions of the same mass were focused in a straight line rather than a parabola. Different ions were deflected by different amounts, and the spectrograph produced a photographic record of a series of lines, each corresponding to one type of ion. The deflections allowed accurate calculation of the mass of the ions.
Aston's first spectrograph was ready in 1919 and with it he was soon able to throw light on one outstanding problem about the nature of the elements. In 1816 William Prout had put forward his hypothesis that all elements are built up from the hydrogen atom and that their atomic weights are integral multiples of that of hydrogen. Although receiving considerable support it was eventually rejected when it was found that many elements have non-integral weights (e.g. chlorine: 35.453). Frederick Soddy in 1913 had introduced the idea of isotopes; that is, the same chemical element in different forms having differing weights. Aston established that isotopes are not restricted to radioactive elements but are common throughout the periodic table. He also saw that they could explain Prout's hypothesis. Thus he found that neon was made from the two isotopes 20Ne and 22Ne in the proportion of 10 to 1. This will give a weighted average of 20.2 for a large number of neon atoms. The value of 35.453 for chlorine can be similarly explained. The whole-number rule is his principle that atoms have a mass that is equivalent to a whole number of hydrogen atoms.
Aston then went on to determine as many atomic weights as accurately as his instruments would allow. His first spectrograph was only suitable for gases but by 1927 he had introduced a new model capable of dealing with solids. From 1927 to 1935 he resurveyed the atomic weights of the elements with his new instrument.
In the course of this activity he found some minor discrepancies with the whole-number rule. Thus the atomic weight of hydrogen is given not as 1 but 1.008, of oxygen–16 as 15.9949 and of oxygen–17 as 16.99913. Aston attempted to show why these values are so tantalizingly close to the integral values of Prout – why the isotopes of oxygen are not simple 16 and 17 times as massive as the hydrogen atom. He argued that the missing mass is in fact, by the mass–energy equivalence of Einstein, not really missing but present as the binding energy of the nucleus. By dividing the missing mass by the mass number and multiplying by 10,000, Aston went on to calculate what was later called the ‘packing fraction’ and is a measure of the stability of the atom and the amount of energy required to break up or transform the nucleus.
Thus, contained in Aston's work were the implications of atomic energy and destruction and he believed in the possibility of using nuclear energy – he also warned of the dangers. He lived just long enough to see the dropping of the first atomic bomb in August 1945.

Scientists. . 2011.

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