Answer to Question #154644 in Atomic and Nuclear Physics for hani

Question

When a thin foil is bombarded with a particles, a few of them are scattered back toward the source. Rutherford concluded from this that the positive charge of the atom—and also most of its mass—must be concentrated in a very small “nucleus” within the atom. What was his line of reasoning?

Answer 1

At Rutherford's behest, Geiger and Marsden performed a series of experiments where they pointed a beam of alpha particles at a thin foil of metal and measured the scattering pattern by using a fluorescent screen. They spotted alpha particles bouncing off the metal foil in all directions, some right back at the source. This should have been impossible according to Thomson's model; the alpha particles should have all gone straight through. Obviously, those particles had encountered an electrostatic force far greater than Thomson's model suggested they would. Furthermore, only a small fraction of the alpha particles were deflected by more than 90°. Most flew straight through the foil with negligible deflection.

To explain this bizarre result, Rutherford imagined that the positive charge of the atom was concentrated in a tiny nucleus at its center, which in turn meant that most of the atom's volume was empty space.