RUTHERFORD’S EXPERIMENT AND ATOMIC MODEL:
Rutherford’s atomic model shows the existence of nucleus in the atom, nature of charge on the nucleus and the magnitude of charge on the nucleus.
APPARATUS FOR EXPERIMENT
- Alpha particles.
- Gold foil. (0.0004 cm thick)
- Zinc sulphide screen.(ZnS)
In his experiments, Rutherford bombarded alpha particles on very thin metallic foils such as gold foil of 0.0004cm thickness. He obtained alpha particles (He2+) from a radioactive element radium or polonium. A In order to record experimental observations, he made use of circular screen coated with zinc sulphide
Heobserved that most of the alpha particles were pass through the foilundeflected.
CONCLUSIONS DRAWN FROM THESE EXPERIMENTS
From these experiments Rutherford proposed a planetary model for an atom. He drew the following conclusions.
- Since most of the a-particles pass through thefoil undeflected, therefore, most of the space occupied by the atom is empty.
- The deflection of a few particles proved thatthere is a centre of heavy positive charge in an atom which causes repulsion asshown in the figure. The heavy part of the atom is called its nucleus.
- All the space outside the nucleus is occupied bythe fast revolving electron. The massive a-particles are not deflected by theseelectrons.
- The centrifugal force which is produced due torotation of electrons balances the electrostatic force of attraction betweenthe nucleus and electron. This prevents the electrons from falling intonucleus.
RUTHERFORD’S ATOMIC MODEL
- Major portion of the atom is empty.
- The whole mass of the atom is concentrated in the center of atom called nucleus.
- The positively charged particles are present in the nucleus of atom.
- The electrons revolve around the nucleus in different circular orbits.
- Size of nucleus is very small as compare to thesize of atom.
DEFECT IN RUTHERFORD’S THEORY
- According to classical physics, electrons being charged particle should emit energy continuously. The orbit of electron should become smaller and smaller until it falls into the nucleus. But it never falls because if it falls at all, the atomic structure would collapse.
- If the electron emits energy continuously, atom should form continuous spectrum. Actually atoms give line spectrum.