Answer:
At the time of Rutherford's experiment, the accepted model for the atom was the Thomson plum-pudding model of the atom, in which the atom consists of a "sphere" of positive charge distributed all over the sphere, with tiny negative particles (the electrons) inside this sphere.
In his experiment, Rutherford shot alpha particles towards a very thin sheet of gold foil. He observed the following things:
1- Most of the alpha particles went undeflected, but
2- Some of them were scattered at very large angles
3- A few of them were even reflected back to their original directions
Observations 2) and 3) were incompatible with Thomson model of the atom: in fact, if this model was true, all the alpha particle should have gone undeflected, or scattered at very small angles. Instead, due to observations 2) and 3), it was clear that:
- The positive charge of the atom was all concentred in a tiny nucleus
- Most of the mass of the atom was also concentrated in the nucleus
So, Rutherford experiment lead to a change in the atomic model of the atom, as it was clear that the plum-pudding model was no longer adequate to describe the results of Rutherford's experiment.
