The atomic number and the mass number of an atom remains unchanged when it emits

Options

A. gamma ray
B. a neutron
C. `beta-` particle
D. An `alpha-` particle

Answer

To solve the question regarding which type of emission keeps the atomic number and mass number of an atom unchanged, we can analyze the options provided.

### Step-by-Step Solution:

1. **Understanding Atomic Number and Mass Number**:
- The atomic number (Z) represents the number of protons in the nucleus of an atom.
- The mass number (A) is the total number of protons and neutrons in the nucleus.

2. **Types of Emissions**:
- **Gamma Emission**: Gamma rays are high-energy electromagnetic radiation. They have no mass and no charge.
- **Alpha Emission**: Alpha particles consist of 2 protons and 2 neutrons (essentially a helium nucleus). Emitting an alpha particle decreases the atomic number by 2 and the mass number by 4.
- **Beta Emission**: Beta particles are electrons or positrons emitted from a nucleus. Emitting a beta particle increases the atomic number by 1 (due to the conversion of a neutron to a proton) but does not change the mass number.

3. **Analyzing Gamma Emission**:
- When an atom emits a gamma ray, it does not lose or gain protons or neutrons. Therefore, both the atomic number (Z) and mass number (A) remain unchanged.

4. **Conclusion**:
- The only emission that does not change the atomic number or mass number is gamma emission.

### Final Answer:
The atomic number and mass number of an atom remain unchanged when it emits **gamma rays**.