The Golden Foil Experiment
The discovery Earnest Rutherford is most frequently credited with, and which contributed most to the understanding of atomic theory, was that of atoms being composed mostly of empty space and also having a very small, but positively charged nucleus at the centre[1].
Rutherford came to these discoveries because he wanted to test the ‘plum pudding’ atomic model of his mentor, J.J Thomson. The model being called such because it asserted that there were many electrons that were dotted in a sphere of positive charge, not unlike the currents in a plum pudding[2].
Rutherford, along with Hans Geiger and Ernest Marsden, conducted an experiment in which they bombarded a sheet of gold foil with alpha particles, watching for changes in their deflection when they emerged. This was achieved by observing ‘scintillations’ (small flashed of light) with the use of a microscope, and whilst in a darkened laboratory.
They expected the alpha particles to deviate a little from their trajectories; however, to their surprise, they found that most emerged unperturbed from their original course. Moreover, less frequently some particles were deflected at great angles. The conclusion which Rutherford drew was that Thomson’s model must being wrong, as it assumed that charge was equally distributed throughout, and so would not cause such great deflections. Instead, most of the atom appeared to be made up of empty space, because most alpha particles did not deviate from their course the little they were expected to. The great deflections seen also indicated the presences of a small positively charged mass; this being needed to deflect a positively charges alpha particle and by such great angles[3]. He called this mass a ‘the nucleus’.
Radioactivity and the Proton
Rutherford then happened across the proton particle in 1918 during experiments in the field of radioactivity. One subject he was exploring was transmutation – where one element is changed into another. During an experiment which involved the bombardment of nitrogen gas with alpha particles (radiation) it was observed that hydrogen nuclei and oxygen were produced[4]. Rutherford correctly deduced that the hydrogen nuclei must have come from within the nitrogen atoms and therefore nitrogen must contain hydrogen nuclei. This means that, not only are atoms divisible, but the number of hydrogen nuclei determines what element an atom represents[5].
By the same experiment that his mentor, Thomson, used to discover the electron, Rutherford found that his hydrogen nuclei were positively charged. He fired a stream of hydrogen nuclei past magnets and noted that their movement was that of a positively charged particle. He then named these hydrogen nuclei protons.
Neutron
Although Rutherford did not actually discover the neutron, he did postulated its existence, believing it necessary to explain how protons could sit tightly together in the nucleus without flying apart[6] – acting like a nuclear glue. His hypothesis was later confirmed by his student James Chadwick.
Rutherford’s Legacy
Rutherford was one of the greatest figures in Chemistry, revolutionising atomic theory. In particular, he discovered the presence of a positively charged nucleus at the centre of all atoms, as well as also showing that they are made up mostly of empty space. He was also the first person to transmutate one element into another as well as prove the existence of another subatomic particle, the proton.
Another one of his great achievements, but perhaps still unrecognised, was his idea to bombard particles with one another, so as to understand the atoms better. This is a technique that has carried on to the present day, and which has yielded huge results in the field of particle physics.
He was recognised for his work, receiving a Noble Prize in 1908[7].
Bibliography
Web sources (all Accessed 16/02/11):
Cavendish laboratory, University of Cambridge
Rutgers, School of Art and Sciences
Thinkquest Educational Foundation
Suite101.com
University of Aberdeen
The History of Computing Project
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