Question

Why did Rutherford draw the conclusions that he did from his famous gold-foil experiment?

Answer 1

Well, because he was shooting positively charged `alpha-` particles at the target.

Explanation:

The target, a very thin piece of gold foil, and gold foil is exceptionally malleable, was not expected to provide any deflection to the `alpha-`particles. And nevertheless, while most of the `alpha` particles passed straight thru, some few were deflected, and a smaller quantity bounced straight back at the alpha particle source.

To quote Rutherford :

It was quite the most incredible event that has 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. On consideration, I realized that this scattering backward 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 greater 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 charge.

Rutherford could only account for the deflections if he assumed a tiny nucleus, which contained most of the mass of the atom, and all of the positive charge. The nuclear age was born.

I point out to add in closing that the nucleus was thus shown to have the same electronic charge as the `alpha-`particles, which at that time had been assigned a charge OPPOSITE to that of electrons. It would have made much more sense, and ease of calculation for quantum chemists, had the nuclear charge been defined as negative, and the electronic charge defined as positive. The nucleus and the electrons have equal but opposite charge. C'est la vie.