Rutherford's alpha scattering reveals concentrated positive charge

Steps:

• Recall that in the alpha-particle experiment, most particles pass straight through the foil, indicating atoms are mostly empty space.
• Observe that a small fraction of alpha particles scatter at large angles, suggesting repulsion by a dense, positively charged core.
• This large-angle deflection implies a tiny, massive nucleus, as diffuse charge would cause only small deflections.
• Conclude that option A directly evidences the nucleus's existence via these rare, sharp deflections.

Why A is correct:

• Large-angle deflections follow Rutherford's scattering formula, where cot(θ/2) relates to impact parameter, proving a point-like, concentrated positive charge (nucleus) repels alphas strongly.

Why the others are wrong:

• B: Unchanged kinetic energy shows elastic collisions but doesn't indicate a nucleus's location or density.
• C: Slower particles deflect more due to longer interaction time, but this relates to velocity, not nuclear existence.
• D: Deflection count varying with thickness reflects scattering probability, not evidence for a central nucleus.

Final answer: A