Which experiment observation led to the following conclassions ?
a. Atom contains a massive positive center
Size of the nucleus is very small

Answer

To answer the question regarding which experiment led to the conclusions that an atom contains a massive positive center and that the size of the nucleus is very small, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Experiment**: The experiment in question is the Rutherford gold foil experiment, also known as the Rutherford scattering experiment.

2. **Understanding the Setup**: In this experiment, Rutherford directed a beam of alpha particles at a very thin foil made of gold.

3. **Observations Made**:
- Most of the alpha particles passed straight through the gold foil without any deflection.
- A small fraction of the alpha particles were deflected at large angles.
- A very tiny number of alpha particles even bounced back towards the source.

4. **Conclusion from Observations**:
- The fact that most alpha particles passed through indicates that the majority of the atom is empty space.
- The deflections and the few particles that bounced back suggest that there is a very small, dense, positively charged center within the atom, which we now know as the nucleus.

5. **Final Conclusion**: From these observations, Rutherford concluded that:
- Atoms contain a massive positive center (the nucleus).
- The size of the nucleus is very small compared to the overall size of the atom.

### Final Answer:
The experiment that led to the conclusions that an atom contains a massive positive center and that the size of the nucleus is very small is the **Rutherford gold foil experiment**.

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