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Atom in Modern Science
In the planetary model of the atom, the nucleus contains 99.97 percent 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; berillium 4; and so on upto 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.
BOHR'S THEORY OF ATOM
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 "eletronic shells“.
1. 97.87 percent of the total mass of the solar system is concentrated in the
sun
