Question #b40f4
1 Answer
The alpha particles that were observed to recoil backward from the gold film were those that collided with the gold nuclei in a "head on" manner.
Explanation:
Rutherford's brilliant experiment was originally predicted - by Rutherford himself! - to yield no large angle scatterings of the alpha particles being fired at the gold foil.
When these were observed, Rutherford was greatly surprised. It was this event that led him to make the famous remark, "It was quite the most incredible thing that ever happened to me in my life. It was almost as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you!"
Rutherford had expected all the alpha particles to pass through the foil with very little deflection, because he (and everyone else) believed the atom to be much as J. J. Thomson had described - a ball of uniform low density. The large angle scatterings force him to rethink the very structure of the atom, and after lengthy calculations aimed at accounting for the scattering he observed, he was able to announce to the world the first nuclear model of the atom.
