J.J. Thomson proposed that the nucleus of an atom contains only nucleus.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Statement**: The statement claims that J.J. Thomson proposed that the nucleus of an atom contains only the nucleus. We need to analyze this statement to determine its truthfulness.

2. **J.J. Thomson's Contribution**: J.J. Thomson is known for his discovery of the electron, which he identified as a negatively charged subatomic particle. He conducted experiments using cathode ray tubes, which led to this discovery.

3. **Clarifying the Concept of Nucleus**: The nucleus of an atom is the central part that contains protons and neutrons. It is important to note that Thomson did not propose the existence of the nucleus as we understand it today.

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