The plum pudding model is one of several scientific models of the atom. First proposed by J. J. Thomson in 1904 soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model represented an attempt to consolidate the known properties of atoms at the time: 1) electrons are negatively-charged particles and 2) atoms are neutrally-charged.

J J Thomson, in 1904, proposed that there was an equal and opposite positive charge enveloping the electrons in a matrix. This model is called the plum- pudding model after a type of Victorian dessert in which bits of plums were surrounded by matrix of pudding

This model could not satisfactorily explain the results of scattering experiment carried out by Rutherford who worked with Thomson.

The Rutherford model is a model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger-Marsden experiment in 1909 which suggested, upon Rutherford's 1911 analysis, that J. J. Thomson's plum pudding model of the atom was incorrect. Rutherford's new model for the atom, based on the experimental results, contained the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and with this central volume also containing the bulk of the atomic mass of the atom. This region would be known as the "nucleus" of the atom.

`alpha`-Particles emitted by radioactive substance were shown to be dipositive Helium ions (`He^(2+)`) having a mass of `4` units and `2` units of positive charge. Rutherford allowed a narrow beam of `alpha`-particles to fall on a very thin gold foil of thickness of the order of `0.0004` cm and determined the subsequent path of these particles with the help of a zinc sulphide fluorescent screen. The zinc sulphide screen gives off a visible flash of light when struck by an `alpha`- particle, as `ZnS` has the remarkable property of converting kinetic energy of a particle into visible light.

i) Majority of the `alpha`-particles pass straight through the gold strip with little or no deflection.

ii) Some `alpha`- particles are deflected from their path and diverge.

iii) Very few `alpha`-particles are deflected backwards through angles greater than `90^o`.

iv) Some were even scattered in the opposite direction at an angle of `180^o` [ Rutherford was very much surprised by it and remarked that "It was as incredible as if you fired a 15- inch shell at a piece of tissue paper and it came back and hit you"]. There is far less difference between air and bullet than there is between gold atoms and `alpha`-particles assuming of course that density of a gold atom is evenly distributed.

i) The fact that most of the `alpha`- particles passed straight through the metal foil indicates the most part of the atom is empty.

ii) The fact that few `alpha`- particles are deflected at large angles indicates the presence of a heavy positively charge body, for such large deflections to occur `alpha`-particles must have come closer to or collided with a massive positively charged body.

iii) The fact that one in `20,000` have deflected at `180^o` backwards indicates that volume occupied by this heavy positively charged body is very small in comparison to total volume of the atom.

On the basis of the above observation, and having realized that the rebounding `alpha`-particles had met something even more massive than themselves inside the gold atom, Rutherford proposed an atomic model as follows.

i) All the protons (`+ve` charge) and the neutrons (neutral charge) i.e nearly the total mass of an atom is present in a very small region at the centre of the atom. The atom's central core is called nucleus.

ii) The size of the nucleus is very small in comparison to the size of the atom. Diameter of the nucleus is about `10^(-13)` `cm` while the atom has a diameter of the order of `10^(-8)` cm. So, the size of atom is `10^5` times more than that of nucleus.

iii) Most of the space outside the nucleus is empty.

iv) The electrons, equal in number to the net nuclear positive charge, revolve around the nucleus with fast speed in various circular orbits.

v) The centrifugal force arising due to the fast speed of an electron balances the coulombic force of attraction of the nucleus and the electron remains stable in its path. Thus according to him atom consists of two parts (A) nucleus and (B) extra nuclear part.

i) Position of electrons: The exact positions of the electrons from the nucleus are not mentioned.

ii) Stability of the atom: Neils Bohr pointed out that Rutherford's atom should be highly unstable. According to the law of electro-dynamics, any charged particle under acceleration should continuously lose energy. The electron should therefore, continuously emit radiation and lose energy. As a result of this a moving electron will come closer and closer to the nucleus and after passing through a spiral path, it should ultimately fall into the nucleus.

It was calculated that the electron should fall into the nucleus in less than `10^(-8)` sec. But it is known that electrons keep moving outside the nucleus. To solve this problem Neils Bohr proposed an improved form of Rutherford's atomic model. Before going into the details
of Neils Bohr model we would like to introduce you some important atomic terms.