Existence of electron, proton and neutron suggested that Dalton’s indivisible atom is composed of sub-atomic particles.

Various atomic models were proposed to explain the distributions of these charged particles in an atom. Some of these models couldn't explain the stability of atoms.

J. J. Thomson proposed this model in 1898.

Assumptions :

(i) An atom possesses a spherical shape (radius `approx` `10^(-10)` m) in which the positive charge is uniformly distributed.

(ii) The electrons are embedded into it in such a manner as to give the most stable electrostatic arrangement (Fig. 2.4).

Other names of this model are Plum Pudding, Raisin Pudding or Watermelon.

This model can be imagined as a pudding or watermelon of positive charge with plums or seeds (electrons) embedded into it.

The mass of the atom is assumed to be uniformly distributed over the atom.

This model was able to explain the overall neutrality of the atom, but was not consistent with the results of later experiments.

Thomson was awarded Nobel Prize for physics in 1906, for the conduction of electricity by gases.

Experiment : Rutherford and his students bombarded very thin gold foil with `alpha`-particles and it is known as α–particle scattering experiment. See fig 2.5.

(i) most of the α– particles went straight through the gold foil undeflected.

(ii) a small fraction of the α–particles was deflected by small angles.

(iii) a very few α– particles (∼1 in 20,000) bounced back (180°).

(i) Since most of the `alpha`-particles went straight through the metal foil, therefore most of the space in the atom is empty.

(ii) A few of the α– particles were deflected from their original paths so it is concluded that the positive charge is concentrated and occupies very small space in the atom.

(iii) Some `alpha`-particles suffered strong deflections which indicates that the nucleus is rigid.

(iv) Calculations by Rutherford showed that the volume occupied by the nucleus is negligibly small as compared to the total volume of the atom. The radius of the atom is about `10^(–10) m`, while that of nucleus is `10^(–15) m`.

(i) The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This was called nucleus by Rutherford.

(ii) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. So, this model resembles solar system.

(iii) Electrons and the nucleus are held together by electrostatic forces of attraction.

(i) According to classical electromagnetic theory, when a charged particle moves under the influence of attractive forces, it loses energy continuously in the form of electromagnetic radiations. So, when electron moves in the attractive field(created by protons), it must emit radiations and as a result loses energy at every turn and move closer to the nucleus following a spiral path. And finally fell into the nucleus making the atom unstable.

Since, the atom is stable, it does not explain the stability of the atom.

(ii) If electrons lose energy continuously, the observed spectrum should be continuous but actual spectrum consists of well defined lines of definite frequencies.

(iii) Another serious drawback of the Rutherford model is that it says nothing about the electronic structure of atoms i.e.,
how the electrons are distributed around the nucleus and what are the energies of these electrons.