Rutherford Scattering

In about 1900, this is how we thought atoms might look…plumpud2J J Thomson called this the ‘plum pudding model’.

democDemocritus, the Greek philosopher, is credited with inventing the word ‘atomos’

– Greek for ‘indivisible’. A solid ‘blob’ which couldn’t be cut in half.

Ernest Rutherford was surprised. So surprised, he said about the results that his longsuffering assistants had taken two years to gather..

‘It was as if a fifteen inch howitzer shell had been fired at a piece of tissue paper and had rebounded straight back’.

He had been firing alpha particles at very thin metal films. The positively charged particles would be deflected, being repelled by positively charged atomic nuclei, how much depending on how closely they passed near to them.

Poor Geiger and Marsden, his assistants, had been sitting in the dark for months on end, counting the flashes caused by the interaction of the alpha particles on a phosphor screen which surrounded the evacuated apparatus, thereby giving away their position.

When the results were analysed, they were remarkable. Gone for ever was Thomson’s ‘plum pudding’ model, where the negative bits were scattered randomly within a positively charged atomic ‘soup’; instead, only about one in 8000 alphas were deflected through a significantly large angle, suggesting close approach was as rare as hen’s teeth. Put simply, most of the atom seemed to be empty space.  This full stop to represent the nucleus in the middle of an atom-sized  cinema is about the right proportion.

Firing 10MeV alphas directly at a gold nucleus transfers all their kinetic energy into electric potential energy.


Calculating r gives a value for about 3×10-14m- the approximate nuclear radius

Here’s the simulation link page Fire a few alphas at the gold…

Another famous quote from ER:

‘There is only physics. Everything else is stamp collecting.’

A bit arrogant perhaps. But, this is the guy who really discovered protons (Gk ‘protos’=first)


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