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.
Eventually Thomson presented three hypotheses: cathode rays are charged particles (corpuscles); corpuscles are constituents of the atom; they are the sole constituents of the atom, and he worked out his “plum pudding” model of the atom. This model basically consisted of a homogenous sphere of uniformly distributed negative corpuscle particles embedded in a positively charged cloud with no considerable mass. Luckily for physics Thomson’s atomic model along with his third hypothesis: that “corpuscles” were the sole constituents of atoms was superceded largely by the work of Rutherford – a former student of Thomson.
Regardless, having discovered one of the atom’s fundamental particles, Thomson had opened the door to more intimate study of the atom, and had left atomic theory only a step away from the discovery of the nucleus and subsequently the proton and the neutron. By 1932 the scientific world had acknowledged the existence of the proton (Rutherford) and the electron (Thomson), and the idea that small particles with no electrical charge might exist had been broached upon several times.
But until Rutherford announced his theory in 1920, which described a neutral particle with the properties of the neutron, as we now understand it, no one had been successful. Despite Rutherford’s extensive descriptions of how this neutral particle might be structured and how it might behave, no experimental evidence could be obtained for proof. It was James Chadwick who, in 1932, proved the existence of the “neutron” after years of research largely focused on atomic disintegration and radiation (predominately of heavy elements).
Specifically, beryllium was the subject of Chadwick’s attention following the “very beautiful experiment of Frederic and Irene Joliot-Curie” which also concerned investigating the properties of beryllium radiation, so Chadwick undertook his own experimentation that eventually culminated in his momentous discovery. As assistant director of Cambridge University’s nuclear physics laboratory, Chadwick and his colleagues (including such names as Rutherford) continually encountered discrepancies between the atomic number (number of protons and equivalent to the charge of the atom) and the atomic mass.
It therefore followed quite logically that since electrons effectively have no significant mass, there must be some additional particle or mass within the nucleus that effectively contributed no charge. Originally it was proposed that this neutral particle could possibly be a proton-electron doublet combined within the nucleus to cancel one another out and give a charge of zero, but a mass effectively equal to one proton – however, such propositions where unfounded by scientific evidence.
Chadwick had taken great interest in the work of Frederic and Joliot-Curie, whose experiment had shown that the behavior of beryllium radiation did not seem congruent with quantum radiation. It was capable of ejecting particles of hydrogen and other light gases that were in its path with great velocity, which suggested that the radiation consisted of particles. Chadwick was greatly excited by these findings and set about the refinement of the process to obtain more accurate results.
He measured the precise distance that various atoms were ejected (namely by the use of an expansion chamber) and showed visibly the degree of movement. Chadwick then applied several formulas to the impact of beryllium radiation on various atoms (namely hydrogen and nitrogen) to calculate the maximum velocity capable of being imparted on a hydrogen atom and a nitrogen atom, and thus (through experiment) was able to deduce the mass of each particle of radiation as being approximately 0. 9 (approximately the mass of a proton).
Furthermore, it was found that particles of beryllium radiation could pass through around 200mm of lead, where a proton fired at the same velocity could only penetrate 1/4mm of the same lead. Chadwick summarised his conclusions in his Nobel lecture, stating that “Since the penetrating power of particles of the same mass and speed depends only on the charge carried by the particle, it was clear that the particle of the beryllium radiation must have a very small charge compared with that of the proton. It was simplest to assume that it has no charge at all. “