Assertion:On moving down the group the group,ionization enthalpy decreases.
Reason:With decreases in size of the atom,the force of attraction between the nucleus and valence electrons decreases.

Options

A. If both assertion and reason is the correct explanation of assertion.
B. If both assertion and reason is the correct explanation of assertion.
C. If both assertion and reason is the correct explanation of assertion.
D. If both assertion and reason is the correct explanation of assertion.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that "on moving down the group, the group ionization enthalpy decreases."
- **Explanation**: Ionization enthalpy is the amount of energy required to remove an electron from an atom. As we move down a group in the periodic table, the number of electron shells increases. This increase in shells means that the outermost electrons are farther away from the nucleus.

2. **Analyzing the Trend**: As we move down a group, the atomic size increases due to the addition of electron shells.
- **Explanation**: With more shells, the distance between the nucleus and the outermost electron increases, which leads to a weaker attraction between the nucleus and the outermost electron.

3. **Conclusion on Assertion**: Because of the increased distance, it becomes easier to remove the outermost electron, resulting in a decrease in ionization enthalpy.
- **Final Statement**: Therefore, the assertion is true.

4. **Understanding the Reason**: The reason states that "with decrease in size of the atom, the force of attraction between the nucleus and valence electrons decreases."
- **Explanation**: This statement is misleading. As the size of the atom decreases (which typically happens when moving up a group), the distance between the nucleus and the valence electrons decreases, leading to a stronger force of attraction.

5. **Analyzing the Reason**: If the atom size increases (as it does when moving down a group), the distance increases, and thus the force of attraction decreases.
- **Final Statement**: Therefore, the reason provided is false.

6. **Final Conclusion**: The assertion is true, but the reason is false. Hence, the correct answer is that the assertion is true, but the reason is false.

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