Discovery of Nucleus (Geiger and Marsden Experiment /Rutherford's atom)

To find out what is inside an atom, Geiger and Marsden at the suggestion of Rutherford, used positively charged
particles called alpha particles to bombard the atom. In their classical experiment, they used a narrow beam of alpha particles emitted from a radioactive source to bombard a thin gold foil. The scattering of the particles from the gold foil was detected by a movable zinc sulphide screen which could be rotated to various positions around the foil. Each time an alpha particle hit the screen, a visible flash of light or scintillation was produced. This was observed by a microscope attached to the screen.

Geiger and Marsden found that most of the alpha particles followed a straight path through the gold foil. But some of them were scattered through large angles and a few were even scattered in the backward direction. As alpha particles are relatively heavy(compared to the electrons), and those used in the experiment travelled at great speeds, very strong forces must have acted on the   deflected alpha particles to cause such marked deviations from their straight paths.

Rutherford commenting on the observation noted : I remember Geiger coming to me in great excitement and saying, We have been able to get some of the alpha particles coming backwards....
It was the most incredible event that has ever happened to me in life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back to hit you. On consideration, I realized that  the backward scattering must be the result of a single collision and when I made calculations, I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greatest part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre carrying a positive charge.

Studying the experimental results, Rutherford (in 1911) deduced that the atom consists largely of empty space, since the majority of the alpha particles passed through the gold foil without being deflected. He explained that the alpha particles which were deflected through large angles or rebounded must have collided with that part of the atom in which the positive charge and mass of the atom were concentrated.
    To account for the observation in the gold foil experiment, Rutherford proposed a nuclear theory of the atom. According to this theory, the atom consists of a positive core called the nucleus, where most of the mass of the atom is contained, the electrons which move around the nucleus.

From the angles through which alpha particles were deflected, Rutherford calculated that the nucleus of an atom would have a radius of about 10^-14m whereas the radius of the atom would be about 10^-10m.
   The model of the atomic structure as put forward by Rutherford has been compared to the solar system. Each atom is a miniature solar system with electrons orbiting the nucleus, like the planets orbiting the sun. Rutherford was later awarded the Nobel Prize for his work on the structure of the atom. Although Rutherford's idea of a central nucleus stood the test of time, his views on the planetary electrons had to be modified as a result of subsequent discoveries.