Rutherford’s Alpha Scattering Experiment is the experiment that is famous for proving that an atom must have a nucleus. In the experiment Rutherford fired positively charged particles at a thin sheet of gold foil. When he did this, he observed that some of the positive particles went straight through the gold atoms in the foil, while others rebounded back.
This suggested that the nucleus of the atom must have a positive charge, as like charges repel, and that since some of the particles went straight through the atom, he concluded that the atom must be mostly empty space.
The previous model of the atom, the Thompson model, described the atom as a blob with some smaller, more solid pieces in it, sort of like a raisin pudding.
If this were true, then shooting a particle through one of these blobs might cause it to bang around in it a bit and come out the other side at some unspecified angle.
To paraphrase, Rutherford felt that, if they compared the energy of the “raisins” to the energy of the alpha particle “bullets” traveling through, the “raisins” could deflect the “bullets” by only a certain amount.
It’s like rolling a billiard ball through a bunch of ping pong balls.
It turns out that the alpha particles actually tended to even bounce back. This simply did not fit with the Thomson model and indicated that something more substantial must be inside the atom.