Which of the following observations was not correct during Rutherford's scattering experiment?

Options

A. Most of the `alpha`-particles passed through the gold foil undeflected.
B. A small fraction of the `alpha`-particles was deflected by small angles.
C. A large number of the `alpha`-particles were bounced back.
D. A very few `alpha`-particles ( ~1 in 20,000) were bounced back.

Explanation

To determine which observation was not correct during Rutherford's scattering experiment, we will analyze each observation based on the findings of the experiment.

### Step-by-Step Solution:

1. **Observation 1: Most of the alpha particles passed through the gold foil undeflected.**
- This observation is correct. Rutherford found that the majority of alpha particles passed straight through the gold foil without any deflection, indicating that most of the atom is empty space.

2. **Observation 2: A small fraction of alpha particles was deflected by a small angle.**
- This observation is also correct. Rutherford observed that a small number of alpha particles were deflected at small angles, which suggested that they were encountering something positively charged in the atom.

3. **Observation 3: A large number of alpha particles were bounced back.**
- This observation is incorrect. In the experiment, only a very small fraction of alpha particles were actually bounced back (deflected at angles greater than 90 degrees). The statement that a large number were bounced back does not align with the experimental results.

4. **Observation 4: Very few alpha particles were bounced back.**
- This observation is correct. Rutherford concluded that only a small number of alpha particles were reflected back, which led to the conclusion that the positive charge is concentrated in a very small region (the nucleus).

### Conclusion:
The observation that is not correct during Rutherford's scattering experiment is **Observation 3: A large number of alpha particles were bounced back.**

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