In Rutherford's gold foil experiment, why did some alpha particles bounce back after striking the foil?

• A. They lost energy due to interactions with the gold foil and reversed direction.
• B. They collided with the dense, positively charged nucleus of gold atoms.
• C. They were absorbed and re-emitted by the gold atoms.
• D. They were repelled by the electrons surrounding the gold atoms.

Answer: (B)

Verified step by step guidance

1. Recall the setup of Rutherford's gold foil experiment: alpha particles (which are positively charged) were directed at a very thin sheet of gold foil.
2. Understand that most alpha particles passed through the foil with little or no deflection, indicating that atoms are mostly empty space.
3. Recognize that a small fraction of alpha particles bounced back or were deflected at large angles, which was unexpected based on the plum pudding model of the atom.
4. Identify that this backward scattering occurs because alpha particles encountered a very small, dense, and positively charged region within the atom, which is the nucleus.
5. Conclude that the repulsion between the positively charged alpha particles and the positively charged nucleus caused some alpha particles to bounce back, revealing the nuclear structure of the atom.