Wednesday, 2 November 2011

100 years of the atomic nucleus

The atomic nucleus with its orbiting electrons is one of the iconic images of the twentieth century. To some it promised unlimited energy, to others it signified the horrors of nuclear war and to many it means the dangers of radiation.  The concept of a tiny nucleus surrounded by electrons was announced by Ernest Rutherford in 1911 but there is debate about whose idea it was.

J J Thomson discovered the electron in 1897 and suggested that they were embedded in an atom sized blob of positive charge – the “plum pudding” model as it came to be known.  A choc chip muffin is possibly a more up to date analogy.  There were, however, other ideas. 

In 1901, Jean Baptiste Perrin, whose work on Brownian motion proved, to nearly everyone’s satisfaction, that atoms existed, suggested that the electrons may orbit around a sphere of positive charge rather like the planets around the Sun. 

Three years later, Hantaro Nagaoka, a Japanese scientist, imagined electrons arranged in rings around a mass of positive charge like the rings around Saturn.

Neither Perrin nor Nagaoka provided any evidence for their ideas and they were not widely publicised.
Rutherford supported Thomson’s theory.  Born in New Zealand he had been a research student at Cambridge under Thomson from 1895 to 1898.  He had then moved to McGill University in Montreal, Canada where he did Nobel Prize winning work on radioactivity.  In 1907 he became Professor of Physics at Manchester University.  He suggested an experiment to a young German scientist, Hans Geiger, to explore Thomson’s plum pudding model.  Alpha rays would be fired at a piece of gold leaf just a few hundred atoms thick.  The scattered alpha particles would be observed in a detector that Geiger designed.  Each alpha particle showed up as a tiny flash of light. 

Over the next couple of years, Geiger, with student Ernest Marsden, painstakingly observed the sparkles of thousands and thousands of alpha particles.  They showed that, as expected, the small, fast moving, positively charged alpha particles passed through the gold leaf with small deflections caused by the positive “pudding” of the atoms.  Then Rutherford made a surprising suggestion.  He told Marsden to see if any of the alpha particles were deflected at larger angles.  Marsden found that about 1 in 8000 alpha particles bounced back after colliding with the gold leaf. 

Rutherford was astounded.  He later said it was like a cannonball rebounding from a sheet of tissue paper.  He realised that the “plum pudding” model could not explain this observation.  Using Geiger and Marsden’s results he was able to calculate that the positive charge and mass of the atom must be packed into a tiny nucleus at the centre of the atom.  The nucleus must be less than 1x10-14 m in diameter or less than a ten-thousandth of the diameter of the atom itself.  Rutherford noted that the size of the nucleus compared to a “fly in a cathedral”, but the fly had all the mass of the cathedral.

Not much notice was taken of Rutherford’s 1911 paper and it took the work of Nils Bohr to explain why the electrons occupied fixed orbits, but gradually the image of electrons revolving around a central nucleus caught the imagination of artists and writers.  Almost all pictures, however, have the ratio of size of the nucleus and the atom wrong and have more in common with Nagaoka’s vision.

Activities1. Look for images of the nuclear atom.  Measure the ratio of diameters of the nucleus and atom.
2. Various analogies were used by scientists to describe the models of the atoms and their experimental results.  Think of other ways of describing the results of the gold leaf experiment and comparing the size of the nucleus and atom.
3. Is it right that the nuclear atom is attributed to Rutherford?
4. Find out more about the life and work of Ernest Rutherford.
5. Discuss the responses people have to the term “nuclear”.
Peter Ellis

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