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Thomson’s atomic model along with his third hypothesis: that “corpuscles” were the sole constituents of atoms was superceded largely by the work of Rutherford – a former student of Thomson.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. ” From these experiments Chadwick had discovered that the beryllium radiation consisted of “particles of mass 1 and charge 0, or neutrons.
” It was Werner Heisenberg who showed, through his own experimentation, that the neutron was in fact a particle unique to the proton and the electron, and not the product of any kind of doublet effect. Chadwick himself deduced this through logical processes based on his understanding of quantum mechanics and the spin of particles, though Heisenberg proved it.