Experimental set up and observation:
In an experiment a - particles (He2+) was allowed to pass through a thin gold plate (0.0004 mm in thickness). The position of the passing particles was ascertained by the flash produced on a zinc sulfide screen. Most of the particle passes through the layer in straight line (red lines), some of them deflected (blue line) and very few particles reflected back.
Ratherford´s atomic model:
(A) Some particles are deflected and very few particles reflected back : In atom positive charge exists, and it is concentrated at the center of an atom named as nucleus.
(B) Most of the particles pass through : Dimension of the nucleus is extremely small compared to the whole size of the atom, and most part of the atom is vacant.
(C) Electrons are present outside the nucleus and their number is equal to the positive charge of the atom to maintain neutral character of the atom.
(D) The electrons circulate around the positively charged nucleus in circular orbits and the stability of an electron moving in a circular path around the positively charged nucleus arises by the centrifugal force acting outward and inward coulombic attraction by the nucleus.
Merit and Demerit:
Merit: It explained the electrical neutrality in an atom.
Demerit: According to the electromagnetic theory of radiation, an electron moving around the nucleus is an accelerated charge particle. Hence, it should emit radiation energy. Consequently, its distance from the nucleus will decrease continuously and finally it will fall into the nucleus along a spinal path. Thus, the proposed atomic model of Rutherford will not be stable. In short, Rutherford model failed to provide explanation for the stability of an atomic system.
Moreover, the frequency emitted by the moving electron should be continuous as electron approaches to the nucleus along a spinal path. In fact, discontinuous spectrum has been found to be emitted by elements. Therefore, this theory failed to explain the origin of discontinuous spectrum.
Addendum:
An atom is composed of a central nucleus and surrounding electron(s).
The central nucleus consists of positively charged proton(s), and neutral neutron. Therefore, the nucleus is always positively charged. The surrounding electron is negatively charged and it rotates around the nucleus along one or more orbit.
Table 1.1.
It is clear from Table 1A that the mass of an electron is negligible compared to proton or neutron mass. Mass ration electron:proton ≈ 1:1836. Hence, the total mass of an atom depends on the number of protons and neutrons present in it.
Therefore, the mass of on atom which is defined as atomic mass ( m a ) = (N + M) × 1.008 a.m.u
N = no of proton; M = No of neutrons.