Consider the following statement
I.Electron gain enthalpy becomes more negative with increase in atomic number across a period
II.Effective nuclear charge increases from left to right across period
III.Electron gain enthalpy becomes less negative as we go up a group
Choose the correct option

Options

A. only I
B. Only II
C. I and II
D. All of these

Answer

To solve the question, we need to analyze each of the statements provided regarding electron gain enthalpy and effective nuclear charge.

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is defined as the amount of energy released when an electron is added to a neutral atom in the gaseous state. A more negative value indicates that energy is released more readily when an electron is added.

2. **Analyzing Statement I**:
- The statement claims that "Electron gain enthalpy becomes more negative with increase in atomic number across a period."
- As we move across a period (from left to right), the atomic number increases, which means the number of protons in the nucleus increases. This leads to an increase in effective nuclear charge (Z-effective), which attracts the added electron more strongly. Thus, the electron gain enthalpy becomes more negative.
- **Conclusion**: This statement is **True**.

3. **Analyzing Statement II**:
- The statement claims that "Effective nuclear charge increases from left to right across a period."
- As we move from left to right across a period, the number of protons increases while the shielding effect remains relatively constant (as electrons are added to the same shell). Therefore, the effective nuclear charge indeed increases.
- **Conclusion**: This statement is **True**.

4. **Analyzing Statement III**:
- The statement claims that "Electron gain enthalpy becomes less negative as we go up a group."
- When moving up a group, the atomic size decreases because the outer electrons are closer to the nucleus. This results in a stronger attraction between the nucleus and the added electron, making the electron gain enthalpy more negative (not less negative).
- **Conclusion**: This statement is **False**.

5. **Final Evaluation**:
- Based on the analysis:
- Statement I: True
- Statement II: True
- Statement III: False
- Therefore, the correct option includes the first two statements.