RUTHERFORD’S MODEL OF AN ATOM – Evidence for Nucleus and Arrangement of Particles.
Though the nature of the proton was well established by 1900, the role of this positive particle in the structure of atom was uncertain. A clue to the arrangement of positive and negative particles within atoms was finally provided by the experiments of Rutherford.
Rutherford and his co-workers performed several experiments on the scattering of α-particles by thin films. In one of such experiments α -particles emitted from polonium bombarded on thin gold foil, it was found that most of α -particles penetrated the foil and emerged undeflected on the other side. However, one particle in 8000 suffered a deflection greater than 90° and remerged on the same side. A number of particles suffered deflections of varying degrees.
To explain this scattering of α -rays, Rutherford made some basic assumptions:
(a) The mass of the atom is concentrated in its nucleus, the dimensions of which are negligible in comparison with the radius of the atom.
(b) The nucleus carries a positive charge,’+ Ze’
(c) There must be ‘z’ No. of negatively charged electrons outside the nucleus and at fairly large distances, arranged in some manner.
(d) The greater part of the atomic volume comprises of empty space in which the electrons revolve and spin.
On these assumptions, Rutherford derived an expression, giving the number of particles which were deflected through a particular angle.
Chadwick devised experiments to verify these assumptions which were found to be ‘Valid’. Another interesting feature was that in the positive charge ‘Ze’ on the nucleus Z was found to be nearly half the atomic mass.
Weaknesses of Rutherford’s Theory
According to classical theory, any charged particle if accelerated must emit energy. If the revolving electron emits energy, its energy contents would decrease and the radius of the orbit would become smaller. This would go on till the electron falls into the nucleus, charting a spiral path. In actual practice this does not happen. In this respect it is necessary to note that electrons lose energy in the form of radiations. An electron moving through orbital of ever decreasing radii would give rise to radiations of all possible frequencies. In other words, it would give rise to a continuous spectrum. In actual practice, atom gives discontinuous spectrum. Thus according to classical principles of physics, the Rutherford model of the atom could not exist.