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Two decades of work with IUPAC


What is an atomic weight?
Norman Greenwood Scientist
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There is a movement amongst teachers to want to rename atomic weights, atomic masses. But the fact is that they’re neither weights nor masses. They are a ratio, so they are dimensionless. The… what we call an atomic weight has two components. The first is it is the ratio of the mass of the element that you are looking at compared to the mass of carbon 12 equals 12 – bit of a mouthful – but that is a ratio of masses and therefore it is dimensionless. So the numbers in the atomic weights table are pure numbers. Now why not just call them that? The reason is because an element can have a complete range of atomic weights and what you find in the table is the one that occurs in nature, with some variation perhaps, it is an average, it is the sample that you would pick up. But you have to be alerted to the fact that that number might alter, depending on the variation of the isotopes in the particular sample that you have. So it’s still valid to call it atomic weights and so there was a move to say that we should think of another thing, if it’s not atomic masses and it’s not atomic weights but it’s a pure number. But in fact the Atomic Weights Commission, when I was chairman, have decided and stayed with it, that it is still valuable to call it atomic weights because that is traditionally understood to be the natural average of things. And, indeed, there are many things in physics and chemistry which do not have the dimensions of the name that they have. The resolving power of a microscope hasn’t got the dimensions of power. EMF, electromotive force of a battery is not a force, it is an energy. Resolving power... any of these things, there are many of them; so it’s not particularly worrying that this is just another one such. So that is, I think, something to be said for, about atomic weights and was one of the things that I became very much involved in, and still am.

Norman Greenwood (1925-2012) was born in Australia and graduated from Melbourne University before going to Cambridge. His wide-ranging research in inorganic and structural chemistry made major advances in the chemistry of boron hydrides and other main-group element compounds. He also pioneered the application of Mössbauer spectroscopy to problems in chemistry. He was a prolific writer and inspirational lecturer on chemical and educational themes, and held numerous visiting professorships throughout the world.

Listeners: Brian Johnson

Professor Brian FG Johnson FRS, FRSE, FRS Chem, FAcad Eu, FAS. Professor of Inorganic Chemistry University of Edinburgh 1991-1995, Professor of Inorganic Chemistry University of Cambridge 1995-2005, Master Fitzwilliam College Cambridge 1999-2005. Research interests include studies of transition metal carbonyls, organometallic chemistry, nano- particles and homogeneous catalysis. Professor Johnson is the author of over 1000 research articles and papers.

Tags: Atomic Weights Commission

Duration: 2 minutes, 40 seconds

Date story recorded: May 2011

Date story went live: 25 November 2011