Plucker was experimenting with passing an electric current through a vacuum made of evacuated glass vessels. The current produced glowing effects within the tube, the effects changed according to the quality of the vacuum chamber. In 1875 English physicist, William Crookes had devised a better vacuum chamber in which the results could be more easily studied. It seemed that the electric current started at the cathode and travelled to the anode, and created the glow of light. Using a piece of metal, which cast a shadow on the glass opposite the cathode, Crookes demonstrated his observations. At this time physicians didn’t know what an electric current might be and could not tell what was moving from cathode to anode. In 1876, the German physicist Eugen Goldstein named the flow cathode rays. German physicists thought that these rays were waves or a type of electromagnetic radiation; while British physicists thought they were particles. One way of deciding this was to see if the cathode rays were deflected by a magnetic field. Crookes had showed this deflection to exist.
In 1897 English physicist J. J. Thomson was able to show cathode ray deflection in an electric field, now it was decided that these rays were particles with a negative electric charge and was suggested that they were present in all atoms. The amount by which the cathode ray is deflected in a magnetic field of given strength is determined by its mass and by the size of its electric charge. Thomson was therefore able to measure the ratio of the mass to the charge (e/m). The cathode ray was assumed to have the minimum charge observed in ions, so its mass would be only a fraction of that of the hydrogen atom, which was the smallest mass known. By 1911 this was proven by the American physicist Robert A. Millikan, who measured the charge of the particle in 1909. Therefore the first subatomic particle had been discovered.
The second subatomic particle to be found was the proton. After the discovery of the electron physicists were concerned about negatively and positively charged particles (ions). They understood that negatively charged ions could be obtained by adding an electron or more to an atom, however this could not explain positive ions. It was suggested that extracting an electron or two from the atom, which must have been present beforehand, could create a positive charged ion. Atoms in their normal state did not carry any charge. If they contained negatively charge electrons they must have a balancing positive charge. J. J. Thomson suggested the ‘plum pudding’ model of the atom, with the electrons embedded in a sphere of uniform positive charge. J. J. named these positive charge particles positive rays. These particles differed from electrons in more than charge; their masses were also different.
In 1909 members of New Zealand born physicist Ernest Rutherford’s research team, Hans Geiger and Ernest Marsden carried out experiments using alpha particles. The experiment was that a radioactive source shot a stream of alpha particles at a sheet of thin sheets of gold or other metals. They detected little flashes of light where they hit a screen. The alpha particles were expected to pass through the metal with only slight deflections in their paths. However some of the alpha particles were deflected at large angles to the metal. Thomson’s model of the atom could not explain the surprising results. Since some of the alpha particles were substantially deflected, Rutherford concluded that there must be something inside an atom for the alpha particles to bounce off that is small, dense, and positively charged. Rutherford proposed that there is a central core called the nucleus. Rutherford reasoned that there must be particles in the nucleus that cause the positive nuclear charge. In 1920 he and Marsden detected the positively charged particles with the approximate mass of a hydrogen atom. Rutherford named it the proton, from the Greek word protos meaning first.
At first it was thought that the nucleus was made up of protons and electrons however this model was unsatisfactory with electrons larger than the nuclei. Rutherford had deduced the neutron and isolated it because the mass of the proton and electron had been known, but they did not add up to the mass of the atom. By 1920 Rutherford had proposed the existence of neutrons in nuclei, and many physicists were searching for this particle. Thus the planetary model of the atom was established. With protons and neutrons in the nucleus and electrons orbiting the nucleus.
James Chadwick finally discovered neutrons in 1932. He discovered a particle with about the same mass as the proton, but which carried no electric charged at all. As it was electrically neutral, it was named the neutron. He found this particle by bombarding a mixture of beryllium and boron with alpha particles. Others, had done this experiment such as Irene and Frederic Joliot-Curie, but they misinterpreted their results. It was known that some very penetrating radiation was produced which was taught to be some kind of gamma ray. Chadwick found that this radiation was capable of ejecting high-energy protons from hydrogen containing material like paraffin. He was basically detecting protons produced by a neutron beam. Chadwick showed that the evidence was consistent with the hypothesis that the high-energy radiation consisted of the neutrons Rutherford had proposed. Hence the third subatomic particle was found.
Written by Olabisi Showunmi.
