During 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 were repelled by the electrons surrounding the gold atoms.
• C. They collided with the dense, positively charged nucleus of gold atoms.
• D. They were absorbed and re-emitted by the gold atoms.

Answer: (C)

Verified step by step guidance

1. Recall the setup of Rutherford's gold foil experiment, where 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. Analyze why this happened: alpha particles are positively charged and would be repelled by other positive charges, so the deflection occurs due to the presence of a dense, positively charged center within the atom.
5. Conclude that the alpha particles bounced back because they collided with the dense, positively charged nucleus of the gold atoms, which contains most of the atom's mass and positive charge.