Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
In 1904, famous British physicist J. J. Thomson successfully proved that the atoms of various chemical elements consist of positively and negatively charged components. They are held together by the forces of electric attraction. He called the negatively charged particles electrons, a large number of which were floating in the interior of a mass, with a uniformly distributed positive charge. The atom on the whole is electrically neutral because the total charge of all negative particles equals the total positive charge. The electrons were assumed to be bound comparatively loosely to the body of the atom and one or several of them could be removed by the process of ionisation. Thomson was able to estimate the mass of an electron, which turned out to be very small indeed. According to his estimate the mass of a whole hydrogen atom is 1840 times the mass of an electron. This indicated that the main portion of atomic mass is contained in its positively charged components. Thomson was, however, very far from the truth concerning the uniform distribution of the positive charge through the body of the atom.
In 1911, another British scientist Ernest Rutherford, called the father of modern atom, realized that so-called alpha particles emanating from radioactive substances could be used as high-speed projectiles to explore the interior of the atom. He bombarded atoms with alpha particles in his now famous experiments called "scattering of the alpha particles in their passage to the matter". He obtained totally unexpected results. It was established that the atoms, far from being solid particles, (they were believed to be since antiquity), consist of extremely small particles and vast empty space. Rutherford also showed that almost the entire mass as well as the positive charge of the atoms is concentrated in a nucleus located in the very centre of the atom. The nucleus is, in fact, thousands of times smaller than the atom itself. Thus the originally widespread positive charge of Thomson's atom shrunk into a tiny one in the centre of the atom, while the electrons rotated outside.
Thus, the model of the atom created by Ernest Rutherford was somewhat akin to our solar system: at the centre of an atom is the nucleus, just as the sun is at the centre of our solar system. Almost all of the mass of the atom is located in the positively charged particles called protons at the centre. Orbiting about the nucleus, as the planets orbit the sun, are electrons, which have almost no mass, compared with the nucleus. Each electron has one negative charge. The number of electrons is always the same as the number of protons, so that the positive and negative charges cancel each other and the atom, as a whole, has no charge. The space occupied by the atom is so huge, compared with the size of its particles, that the electrons orbiting the nucleus are 'like a few flies in a cathedral', according to Rutherford.
The magnitude of atoms is so far removed from our macroscopic scale that it is very difficult to visualize the size of this microscopic entity. The diameter of an atom is about one hundred-millionth of a centimetre. If an object of the size of a cricket ball is magnified to the size of earth, its atoms will then have the size of grapes. An atom, therefore, is extremely small compared to macroscopic objects but it is huge compared to its nucleus in the centre. The diameter of the atom is 1.00.000 times greater than the diameter of nucleus, while its volume is 5.00.000 billion times the volume of nucleus. To see the nucleus, the atom will have to be blown up to the size of the biggest dome in the world. In an atom of that size, the nucleus would have the size of a grain of sand!
In the planetary model of the atom, the nucleus contains 99.97 percent1 of the total atomic mass and the distance between the electrons exceeds their diameter by several thousand times. The electric attraction forces between the nucleus and the electrons obey the mathematical law of inverse square i.e. the forces are inversely proportional to the square of the distance between them and the 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 paisa would weigh 600 million tons if its atoms were as tightly 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; beryllium 4; and so on up to the heaviest natural element - uranium which has altogether 92 electrons.
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.
