Which of the following decreases in going down the halogen group ?

Options

A. ionic radius
B. atomic radius
C. ionisation potential
D. boiling point

Explanation

To solve the question "Which of the following decreases in going down the halogen group?", we will analyze the properties of the halogens (Group 17 elements: fluorine, chlorine, bromine, iodine) and determine which property decreases as we move down the group.

### Step-by-Step Solution:

1. **Identify the Halogens**:
- The halogens are the elements in Group 17 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

2. **Understand the Properties**:
- We need to evaluate the following properties: ionic radius, atomic radius, ionization potential (ionization energy), and boiling point.

3. **Evaluate Ionic Radius**:
- The ionic radius increases as we move down the group. This is because each successive element has an additional electron shell, which increases the size of the ion.
- **Conclusion**: Ionic radius does not decrease.

4. **Evaluate Atomic Radius**:
- Similar to ionic radius, the atomic radius also increases down the group due to the addition of electron shells.
- **Conclusion**: Atomic radius does not decrease.

5. **Evaluate Ionization Potential**:
- Ionization potential (or ionization energy) is the energy required to remove an electron from an atom. As we move down the group, the ionization potential decreases. This is because the outermost electrons are further from the nucleus and experience less nuclear attraction, making them easier to remove.
- **Conclusion**: Ionization potential decreases.

6. **Evaluate Boiling Point**:
- The boiling point of halogens increases down the group due to increased molecular size and van der Waals forces.
- **Conclusion**: Boiling point does not decrease.

7. **Final Answer**:
- The only property that decreases as we go down the halogen group is **ionization potential**.

### Final Answer:
**Ionization potential decreases down the halogen group.**

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