In Rutherford's gold foil experiment, what observation led to the conclusion that the atom has a small, dense nucleus?

• A. A few alpha particles were deflected at large angles, and some even bounced back toward the source.
• B. Alpha particles caused the gold foil to emit visible light.
• C. All alpha particles were absorbed by the gold foil.
• D. Most alpha particles passed straight through the foil with little or no deflection.

Answer: (A)

Verified step by step guidance

1. Recall the setup of Rutherford's gold foil experiment, where alpha particles were directed at a thin sheet of gold foil to observe their scattering behavior.
2. Understand that if the atom's positive charge were spread out evenly (as in the plum pudding model), alpha particles would pass through with minimal deflection.
3. Note the key observation: while most alpha particles passed through the foil with little or no deflection, a small fraction were deflected at large angles, and some even bounced back toward the source.
4. Interpret this observation to mean that the positive charge and most of the atom's mass must be concentrated in a very small, dense region (the nucleus), which repelled the positively charged alpha particles strongly when they came close.
5. Conclude that the large-angle deflections and backscattering of alpha particles provided evidence for the existence of a small, dense nucleus at the center of the atom.