Friday, May 7, 2010

Divisible Part 3

Are You Positive? 

With the discovery of the electron it was assumed that there must be some particle responsible for a positive charge as atoms were not ionized. In many texts the discovery of the proton is attributed to Ernest Rutherford (1871-1937), but it was discovered prior to his work by German physicist Eugen Goldstein (1850-1930). Goldstein began to study if there were ‘rays’ emitted by the anode in the discharge tube as most studies at the time had been concerned with the ‘rays’ emitted by the cathode. The charge/mass ratios of these emissions were also measured but the results differed greatly from the charge/mass of the electron. It was found that the charge/mass ratio depended on the gas in the tube and was much smaller than that of the electron. When the gas in the tube was hydrogen the charge/mass ratio was found to be greatest. This snag of the charge/mass being dependant on the gas put Goldstein off, what he actually was seeing was ions.

For the definitive discovery of what we now know as the proton it would take several years of work by Rutherford. In 1911 he experimented on a thin gold foil sheet which he bombarded with alpha particles (this experiment was carried out by two of his students Geiger and Marsden). The vast majority of these particles passed through the foil with no interference but several were deflected by varying amounts. It was found that the majority of the deflected particles were deflected by small angles while some were deflected by angles greater than 900 and some even ricocheting back along the path they came. From this he concluded that the gold atom contained within it a very small central mass which was positively charged (the alpha particles have a positive charge so it was assumed that they would be repelled by a like charge at the centre of the atom).
At the time the diameter of the atom was generally considered to be about 10-10 m but Rutherford found the nucleus to be of the order of 10-15 m effectively making the atom comprised of mostly empty space. So the model of the atom was now a densely packed central core of positively charge particles with electrons surrounding it. This model of the atom took over from the view put forward earlier by Thompson. There were several snags with this model but these will be discussed later in the essay.

Rutherford and his colleagues were bombarding nitrogen gas with alpha particles in 1918 and found that there was hydrogen present in the tube. This could only be possible if the nitrogen nuclei contained hydrogen nuclei. This led to a suggestion that the hydrogen atom was a fundamental particle. We now know that the hydrogen atom is a proton paired with an electron.


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