Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding model’ of the atom?
According to Thomson’s plum pudding model, the positive charge in the atom is spread out evenly. Because of this, α-particles were expected to pass straight through the gold foil or be deflected only slightly.
However, Rutherford found that a few α-particles bounced back sharply. This complete reversal in direction means the α-particles encountered a massive, concentrated region of positive charge.
If the positive charge was diffused evenly as Thomson suggested, it could not produce such a strong repulsive force to bounce the heavy α-particles back. Therefore, this single surprising result completely rules out the plum pudding model, proving that the atom's positive charge and mass are concentrated in a tiny central nucleus.
Explanation
The textbook context explains that Thomson's model assumed positive charge was spread out evenly, which would only allow for slight deflections or straight passages of alpha particles. The observation that some alpha particles bounced back sharply indicates a concentrated positive charge capable of strong repulsion, directly contradicting the diffuse charge proposed in the plum pudding model.