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Rutherford and the negatively charged nucleus?!!!

It's amazing the things you can lean on a wet and windy day!

I was browsing through Twitter when I came across a thread of 'things science teachers get wrong'. And apparently one of the things we get wrong is when we say that Rutherford's conclusion to the gold foil experiment was that the atom had a positively charged nucleus. (I cant find the thread again now, so I'm afraid I can't credit the source for this)

Now that is something I have certainly said over the years, so I was a little perturbed to see this listed as mistake, and that lead me on an internet deep-dive into the experiment and how it lead to the modern concept of the atom. And it turns out the twitter post was (sort of) right. In his first publication on the deviation of the alpha particles he said that 'such a remarkable deviation in the path of a massive charged particle could only be achieved if most of the mass of, say, an atom of gold and most of its charge were concentrated in a very small central body.'

That's pretty much the way I always understood it. But note he did not say that that 'very small central body' (not yet dubbed a nucleus) had to be positively charged. And it was some years before that part of the story fell into place. How could this be? How could a negatively charged nucleus account for the alpha particles rebounding on themselves? I found the best account of it all here.

If I understand it correctly, Rutherford initially thought that the alpha particle might have been behaving like a comet as it passes the sun: drawn towards it initially and then swinging around it at very high velocity - in a motion often compared to a slingshot. Which makes sense to me now that it's pointed out. Though it would never have occurred to me. Apparently it took further experiments involving beta particles to rule out that possibility and to lead to the idea of the positive nucleus.

Among the other things I learned was:

  • It was also Rutherford who managed to divide the radioactivity described by Becquerel and others into alpha and beta particles, and gamma rays

  • it was also Rutherford who discovered the positive charge of the alpha particle and the connection with helium, which he had to do before using them to probe the atom.

  • Chadwick spent much of WW1 interned in Germany - where he had been working with Geiger, before returning to England to work with Rutherford and eventually discover the neutron

  • there was another brilliant member of the team, Henry Moseley, who died at Gallipoli.

  • and loads more...

Now, when I read the whole thing, it seems to me that Rutherford did eventually conclude that the nucleus had to be positive, so we're not exactly wrong when we say that. But it is interesting to learn (yet again) how much more complicated everything is when you learn the details,

Anyway, the article is here, and well worth a read. assuming you're as ignorant of these details as I was until about half an hour ago...

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