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Atomic weights issues in Chemistry of the Elements


My unique chapter in Chemistry of the Elements
Norman Greenwood Scientist
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A second aspect – and this I think it’s fair to say was unique to the book – was it struck me as being strange in all my lecturing experience, and it had been fairly considerable by that time, students had never asked questions like, where do the atoms come from? Why are there only 90 or so elements occurring in nature? Why do they have the properties that they have?

So it seemed to me, if you’re going to talk, and expect students to spend three years of their life thinking about inorganic compounds, it might be useful to give them some background. So I put this as a preliminary chapter, The Origin of the Elements.

It’s of course a chapter of chemistry or nuclear physics, whichever way you like to look at it, which couldn’t have been written 50 years ago because it wasn’t understood. But from the mid-1930s onwards, and this is a thing worth remembering, that there was no idea why the sun was hot... 50 years ago before I was writing it, in the early ‘30s let’s put it that way - 70 years ago.

Why is the sun hot? Well, Gamow and then others said that a possible thing was nuclear reactions, which by then were known to give high energy, though the processes weren’t really fully understood of course. But how does it get from the sun to the earth, and in particular, on the simple nuclear reactions that were then known, you would make up to the most stable, which was iron as it turns out, but after that how do the elements form?

And it turns out that they are in fact formed in interstellar space and they are formed, as I very briefly mentioned before, in supernovae explosions. And that became clear only a few decades ago.

So astonishingly, although there’s a wealth of these elements forming this myriad of compounds, and everything that we can see in the world is made of these 90 elements – they all came from supernova or stars which then develop particular types – not every star turns into a supernova, but those that do spew out this stuff and the process of aggregation starts again.

So the concept of the different types of stars is important. So... and as you will know, there are different sorts of stars, so I went through the types of stars and what their history is, how many billions of years they were, whether they’re new or young. And the other very important thing, that the further out in space you see, the further back in time you go. Speed of light is important, is enormously fast, but the universe is an enormous size, it is also expanding.

And one of the fascinating things about the ingenuity of human knowledge is that we know what the elements are in the Sun. In fact helium, which is named  from Helios for Sun, was discovered in the Sun before it was discovered on Earth. Think about it, how do we know? We know because of spectroscopy, and I’ve been banging on about spectroscopic techniques as well, and that’s one of the reasons why they’re important. We know the components, many of the components, are parts of the stars.

It was then found, and we talked in Mössbauer spectroscopy about the Doppler shift, we know that all stars are receding from the earth. That doesn’t make the earth the centre of the universe. It means that everything is expanding wherever you are, everything – this is simplified – is moving away from us. And that’s only been known for the last few decades.

But the point I want to make is that, and the reason why the Hubble spaceship and other astronomical telescopes and devices are there is, the longer, the further we see in space, the longer back in time we go. And so we’re not only looking at stars as they are now, what we’re seeing happened before. And so we’re actually, in reverse, getting a history of the development of the universe, and that is one of the major ways of finding out how the universe developed, you can turn the clock around.

So they were some of the ideas that were going around in my mind.

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: Chemistry of the Elements, Mössbauer spectroscopy, Doppler Effect, Hubble Space Telescope, George Gamow

Duration: 5 minutes, 18 seconds

Date story recorded: May 2011

Date story went live: 25 November 2011