Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
With the creation of the planetary model of atom by E. Rutherford, 'proton' and 'electron' emerged as the smallest units of matter. The questions asked by the physicists were: Are these the "basic building blocks" - the ultimate units of matter? Can we subdivide them still further into smaller and simple particles? Would it be possible to reduce all different atoms to perhaps a few really simple particles?
In the middle of the last century, William Prout, an English chemist put forth a hypothesis that the atoms of all elements are made up of various numbers of hydrogen atoms. This hypothesis was based on the fact that the atomic weights of various elements were in most cases very nearly exact multiples of that of hydrogen. Thus atoms of oxygen, which are sixteen times heavier than those of hydrogen, must be composed of 16 hydrogen atoms, somehow stuck together. But the facts found at that time were unfavourable to the acceptance of this bold hypothesis. Isotopes were not discovered and the chemical atomic weight of chlorine for instance, being 35.5 was in direct contradiction to Prout's hypothesis. He died without ever learning how right he actually was.
In 1919, British Physicist F.W. Aston discovered that ordinary chlorine was actually a mixture of two different kinds of chlorine possessing identical chemical properties but having different atomic weights: 35 & 37. Further study revealed the striking fact that a mixture of several components, identical in chemical properties but differing in atomic weights, made up most of the elements. They were called isotopes. Prout's forgotten hypothesis was given a new life and was reformulated by saying that nuclei of various atoms are composed of various number of hydrogen nuclei called protons.
An important step towards a better understanding of nuclear structure of atom was the discovery of neutron as the second constituent of nucleus. It is a particle, which has roughly the same mass as proton, but does not carry an electric charge. Actually the nuclei of various elements are composed of protons as well as neutrons. The existence of neutrons was suggested by Rutherford in 1920, but they were found experimentally only in 1932
Thus, the nucleus of an oxygen atom, which is the eighth element in the atomic table and has 16 units of mass and 8 units of charge must be composed of 8 protons and 8 neutrons, whereas the heavy nucleus of uranium, atomic weight 238, atomic number 92, is formed by 92 protons and 146 neutrons. An important fact to keep constantly in mind about protons and neutrons is that the two are interchangeable. A proton, under certain conditions, loses its positive charge by emitting a positive electron (positron) and thus becomes a neutron. Similarly, a neutron, when agitated, emits a negative electron and becomes a proton. As we shall see, the latter process is taken advantage of in the transmutations of no fissionable uranium into plutonium and of chromium into fissionable uranium 233. The transmutations of all other elements - age-old dream of alchemists - are made possible by the interchange ability of protons into neutrons and vice versa. Protons and neutrons are, thus, two electrical states of the same basic particle called nucleon.
The atoms of the elements have twins, triplets, etc., known as isotopes. The nuclei of these twins etc, all contain the same number of protons and hence all the same chemical properties. They differ, however, in the number of neutrons in their nuclei and hence have different atomic weights. For example, an ordinary hydrogen atom has nucleus of one proton. The isotope of hydrogen - deuterium has one proton plus one neutron in its nucleus. It is, thus, twice as heavy as ordinary hydrogen. We may now say that different combinations of two types of basic particles (called elementary particles), participating in the structure of the material universe, result in the infinite variety of the material world: First of all we have electrons with a negative electric charge and negligible mass. Then, we have nucleons, which represent the basic material particles. They are either neutral, called neutrons, or positively charged, called protons.
But, as we shall see presently, many more particles have been discovered, and the number of particles increased from three to six by 1935, then to eighteen by 1955, and today we know over 200 "elementary particles".
