Which of the following are evidences for the existence of neutrons in an atom?

Options

A. The ratio of the charge to the mas of atom's nucelus is smaller than that of a hydrogen nucleus
B. An atom is always electrically neutral
C. Isotopes are present
D. Some atoms are radioactive in nature

Answer

To determine the evidence for the existence of neutrons in an atom, we can analyze the provided options step by step.

### Step-by-Step Solution:

1. **Understanding Charge to Mass Ratio**:
- The charge to mass ratio of an atom's nucleus is smaller than that of a hydrogen nucleus.
- For hydrogen, which has one proton, the charge is \(1.6 \times 10^{-19} \text{ C}\) and the mass is approximately \(1 \text{ u}\).
- For other elements, as we add protons (which contribute to charge), we also add neutrons (which contribute to mass but not charge).
- If neutrons did not exist, the charge to mass ratio would remain constant across all elements, but it does not. This indicates that there are neutral particles (neutrons) contributing to the mass without adding to the charge.

**Conclusion**: This provides evidence for the existence of neutrons.

2. **Electrical Neutrality of Atoms**:
- An atom is electrically neutral, meaning the total positive charge (from protons) equals the total negative charge (from electrons).
- However, this neutrality does not provide direct evidence for neutrons. The presence of one proton and one electron can also result in neutrality without needing a neutron.

**Conclusion**: This does not provide evidence for the existence of neutrons.

3. **Presence of Isotopes**:
- Isotopes are atoms of the same element that have the same number of protons but different masses due to varying numbers of neutrons.
- The existence of isotopes implies that there are particles with mass (neutrons) that do not carry charge, as the number of protons (and thus the charge) remains constant while the mass varies.

**Conclusion**: This provides evidence for the existence of neutrons.

4. **Radioactivity**:
- Some atoms are radioactive, but radioactivity does not necessarily indicate the presence of neutrons.
- Radioactivity can occur due to various reasons, including unstable configurations of protons and neutrons, but it does not directly prove the existence of neutrons.

**Conclusion**: This does not provide evidence for the existence of neutrons.

### Final Answer:
The evidences for the existence of neutrons in an atom are:
- The ratio of charge to mass of an atom's nucleus is smaller than that of hydrogen nucleus.
- The presence of isotopes.