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ATOM IN MODERN SCIENCE 21 tional to the square of the distance between them and ile electrons describe the circular and elliptical trajectories around the nucleus It can thus be seen that most of the matter in the universe is concentrated in the nuclei of the atoms The density of the matter in the nucleus is such that a paise would weigh 600 million tons if its atoms were as ightly packed as the particles in the nucleus
It was found by Rutherford that in the natural sequence of elements arranged in the order of increasing weights, there is a consistent increase of one atomic electron in each element in the sequence Thus an atom of hydrogen has one electron, an atom of helium 2, lithium 3, benllium 4, and so on upto the heaviest natural element, uranium which has altogether 92 clectrons
The numerical designation of an atom is usually known as its atomic number and coincides with its positional number in the atomic table Thus all the physical and chemical properties of any given element can be characterised simply by one figure giving the number of electrons rotating around the central nucleus
It was noticed that some of the properties of the elements begin to repeat themselves after a definite number of steps when arranged in a natural sequence Since each step along the sequence of elements corresponds to one additional electron, the observed periodicity must be due to the recurrent formation of certain stable configuration of atomic electrons, or "electronic shells" It has been established that the first completed shell consists of 2 electrons, the next two shells of 8 electrons each and all the following shells of 18 each 10
We can now, having the picture of an atom, turn 10 Sce Appendix
