Radioactivity is the property due to which a heavy nucleus can decay..................without the help of an external agent.

Answer

**Step-by-Step Solution:**

1. **Understanding Radioactivity**: Radioactivity is a natural process where unstable atomic nuclei lose energy by emitting radiation. This process occurs without any external influence or intervention.

2. **Spontaneous Decay**: The term "spontaneous" indicates that the decay happens on its own. Heavy nuclei, which are often unstable, will decay into lighter nuclei to achieve a more stable configuration.

3. **Nuclear Stability**: The primary reason for radioactivity is to achieve nuclear stability. Heavy nuclei often have an imbalance in the ratio of neutrons (N) to protons (P). For a stable nucleus, this ratio should ideally be greater than 1 and less than 1.6.

4. **Imbalance in N/P Ratio**: When the N/P ratio is outside this range, the nucleus is unstable. To correct this imbalance, the nucleus undergoes decay, emitting particles or radiation in the process.

5. **Emission of Particles**: During the decay process, the nucleus emits alpha particles, beta particles, or gamma rays. This emission helps the nucleus transition to a more stable state.

6. **Conclusion**: Therefore, radioactivity is a self-driven process that allows heavy nuclei to decay and move towards a more stable configuration without needing any external agents or conditions.

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