Postulations regarding the fundamental nature of matter have been undertaken for millennia, but it was around 400 BC that Greek philosopher Democritus laid down the first atomic theory – and for a philosopher more than 2000 years ago, he was surprisingly close. Democritus wrote that all things are composed of minute, indivisible, indestructible particles of pure matter, which move forever in empty space.
What has followed since has effectively been the refinement and proof of this original atomic theory of matter, and though there is a world of difference between scientific knowledge in 400 BC and the present, essentially Democritus, Thomson, Chadwick and Bohr where all talking about the same thing.
The following men have made vast contributions to science, and not only to our understanding of the atom.
Their influence has been so extensive that to account for anything more than a fraction of their contributions would require several independent dossiers, and as such the following paragraphs are designed to give a brief account of some of their more obvious discoveries (namely the achievements for which they were awarded Nobel Prize’s).
The electron is one of the most fundamental particles of the atom, but before Thomson’s extensive experimentation with cathode rays proved or at least strongly suggested the existence of his so-called “corpuscles”, the thought of a particle smaller than an atom was almost ludicrous.
But the evidence was there: the results of the cathode ray tube experiments supported his theory that cathode rays were really streams of miniscule pieces of atoms.
He came to this conclusion by performing three famous experiments, which he summarised during an evening lecture to the Royal Institution on Friday, April 30, 1897. The first experiment was a variation of Jean Perrin’s 1985 experiment which had deduced that cathode rays carry an electrical charge; Thomson set up a cathode ray tube ending with a pair of metal cylinders which, essentially, were connected to an electrometer.
Thomson then deflected the cathode rays with a magnet so that they did not contact the electrometer receptor, and found that when the cathode rays did not enter the electrometer, neither did a charge. From this Thomson concluded that the cathode rays and the charge were inseparable, which should therefore imply that the rays were the source of the charge. The second experiment that Thomson carried out was to retest the effect of and electric field on the cathode rays.
Until Thomson refined the experimentation method, attempts to bend the rays with electric fields had all failed. Thomson had considered this, and suspected that traces of gas not removed by the original pump where preventing the cathode rays from bending as they should (charged particles bend in electric fields unless surrounded by a conductor). So he labored to extract all the air from the tube, and eventually succeeded in bending the rays with an electric current.
These two experiments indicated to Thomson that cathode rays “are charges of negative electricity carried by particles of matter”, but he was still unable to deduce what order of classification the cathode rays constituted – whether they were “atoms, or molecules, or matter in a still finer state of subdivision”. And so Thomson continued to investigate the properties of cathode rays (particles) in his third experiment, which attempted to derive the ratio of the charge of a cathode particle to its mass (e/m).
He did this not by measuring that actual mass and charge of a particle, but by measuring the angular effect of a magnetic field in bending the cathode rays, and the amount of energy the particles carried. The results of this particular experiment were groundbreaking; Thomson reaffirmed Emil Wiechert’s finding earlier that year that the charge-to-mass ratio of a corpuscle was around 1700 times larger than hydrogen. As Thomas explained in his 1906 Nobel Lecture “the charged atom of hydrogen …
had the greatest known value of e/m … for the corpuscle in the cathode rays the value of e/m is 1,700 times the value of the corresponding quantity for the charged hydrogen atom. ” He went on to describe the possible reasons for such a vast discrepancy, stating that “the very large value of e/m for the corpuscles, as compared with that of hydrogen, is due to the smallness of m the mass, and not to the greatness of e the charge. ” Thomson’s initial proof for this fact came by performing an experiment based on the work of C.
T. R. Wilson involving the exposure of air saturated with water to radium radiation, and manipulation of the resultant drops of water by charged plates to determine charge on one drop and thus the charge on one corpuscle. He also experimented on how cathode rays penetrated certain gases, and proved if not entirely unambiguously, that corpuscles must have a mass far smaller than any known atom, as each corpuscle had a charge equal to that of hydrogen.
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