Based on the experimental observations, different models were proposed for the structure of the atom. In this section, we will discuss two such models namely Thomson model and Rutherford model.

Thomson model

All matter is made of atoms and all the atoms are electrically neutral. We have just seen that all atoms contain the electrons. Based on these facts, Thomson concluded that there must be an equal amount of positive charge present in the atom. He proposed that an atom could be considered as a sphere of uniform positive charge in which electrons are embedded. This is shown below in Fig.3.3.

This model is similar to a water-melon according to which an atom can be thought of as a sphere of positive charge in which the electrons are embedded like seeds. This model is also called plum pudding model or raisin pudding model because the electrons resembled the raisins dispersed in a pudding (an English dessert).
During this period only, the phenomenon of radioactivity was also being studied by the scientists. This phenomenon of spontaneous emission of rays from atoms of certain elements also proved that the atom was divisible and it contained sub –atomic particles. Ernest Rutherford and his coworkers were also carrying out experiments which revealed that the radiation could be of three types: a(alpha), b(beta) and g(gamma). You will study more about them in lesson 14.
In 1910, Rutherford and his co-workers performed an experiment which led to the downfall of the Thomson model. Let us now study about the contribution of Rutherford.