Decrease in atomic number is observed during :

Options

A. Alpha emission
B. Beta emission
C. Positron emission
D. Electron emission

Explanation

To solve the question regarding the decrease in atomic number during different types of nuclear emissions, we will analyze each type of emission step by step.

### Step 1: Understanding Alpha Emission
- In alpha emission, an atomic nucleus emits an alpha particle (which is a helium nucleus consisting of 2 protons and 2 neutrons).
- The general reaction can be represented as:
\[ \text{X} \rightarrow \text{Y} + \alpha \]
- Here, the atomic number (Z) decreases by 2, and the mass number (A) decreases by 4.
- **Conclusion**: Alpha emission results in a decrease in atomic number by 2.

### Step 2: Understanding Beta Emission
- In beta emission, a neutron in the nucleus is transformed into a proton, and a beta particle (an electron) is emitted.
- The general reaction can be represented as:
\[ \text{X} \rightarrow \text{Y} + \beta \]
- Here, the atomic number (Z) increases by 1, while the mass number (A) remains the same.
- **Conclusion**: Beta emission results in an increase in atomic number, not a decrease.

### Step 3: Understanding Positron Emission
- In positron emission, a proton in the nucleus is transformed into a neutron, and a positron (the antiparticle of an electron) is emitted.
- The general reaction can be represented as:
\[ \text{X} \rightarrow \text{Y} + \beta^+ \]
- Here, the atomic number (Z) decreases by 1, while the mass number (A) remains the same.
- **Conclusion**: Positron emission results in a decrease in atomic number by 1.

### Step 4: Understanding Electron Capture
- In electron capture, an electron from the inner shell is captured by the nucleus, combining with a proton to form a neutron.
- The general reaction can be represented as:
\[ \text{X} + e^- \rightarrow \text{Y} \]
- Here, the atomic number (Z) decreases by 1, while the mass number (A) remains the same.
- **Conclusion**: Electron capture results in a decrease in atomic number by 1.

### Final Conclusion
From the analysis:
- **Alpha emission**: Decrease in atomic number by 2.
- **Beta emission**: Increase in atomic number by 1.
- **Positron emission**: Decrease in atomic number by 1.
- **Electron capture**: Decrease in atomic number by 1.

Thus, the types of emissions that result in a decrease in atomic number are:
1. Alpha emission
2. Positron emission
3. Electron capture

### Answer
The decrease in atomic number is observed during:
- **Alpha emission**
- **Positron emission**
- **Electron capture**