Question

What proof did Rutherford have that showed that the positive charge of an atom was in the middle?

Answer 1

He had no proof...he had evidence and supposition...

Explanation:

Rutherford fired heavy `alpha"-particles"` at a gold sheet, the which was only a few atoms thick (gold is extremely malleable! and `alpha"-particles"` were massive, positively charged particles). That some of them (a small proportion) bounced back towards the `alpha"-particle"` emitter led him to propose that the majority of the mass of the GOLD atom, and ALL of the positive charge it contained, were confined to a small, dense nuclear core. And this outline was consistent with these and later experimental data.

And as a matter of fact his assignment of a positively charged nucleus is problematic for chemists. There are many problems in quantum chemistry where you get the right answer but the wrong sign simply because you counted odd instead of even when you count up the electrons.

The point is that electrons and massive, charged nuclear particles (i.e. nuclear protons) have OPPOSITE electric charges; it would have made much more sense to assign a positive charge to electrons, and a negative charge to nuclear protons....