Here's why:
* The experiment: Rutherford fired alpha particles (positively charged) at a thin sheet of gold foil.
* Expected outcome: Based on the "plum pudding model" of the atom, where positive charge was thought to be evenly distributed, the alpha particles should have passed through with only minor deflections.
* Actual outcome: Most alpha particles passed straight through, but a small number were deflected at large angles, and some even bounced back!
* Conclusion: Rutherford concluded that the positive charge of an atom must be concentrated in a tiny, dense region he called the nucleus. The large deflections and rebounds were due to the alpha particles encountering this massive, positively charged core.
The experiment did not support the idea that atoms are mostly empty space (although this is a consequence of the nucleus being so small). It also did not support the "plum pudding model" which was disproven by the experiment.
