1. Our atomic age is more than 2000 years old. More than 2000 years ago Greek philosophers discussed the structure of the atom. One of the philosophers, Democritus, suggested that all matter consisted of particles which are invisible and indivisible. The Greeks called this particle the ‘atom.’
2. The idea of atomic structure of matter was later almost forgotten and came to life again at the beginning of the 19-th century when the English scientist, John Dalton, introduced into science the idea of the elements, the basic building particles of matter.
3. Then, after many years of careful research, the discovery of the electron, a part of an atom, was announced. The discovery of the electron was only the first step in the exploration of inner world of the atom. Since it was known that the atom was electrically neutral the physicists now began to search for the positive particles which could balance out the negative charge of the electron.
4. In 1911 another English scientist, Ernest Rutherford, discovered that the atom had a core, or nucleus, in its centre, and that the nucleus was positively charged and contained nearly all the weight of the atom. He showed that the positive charge of the nucleus was caused by particles called «protons».
5. In 1932 the third basic atomic particle, neutron, was discovered. The weights of the parts of the atom were calculated very carefully and it was found that the proton and neutron have almost the same weight, but they are much greater in mass than the electron.