Which of the following properties of halogens increase with increasing atomic number?
(I) Ionization energy
(II) Ionic radius
(III) Bond energy of the `X_(2)` molecule
(IV) Enthalpy of vaporisation

Options

A. I,II,III
B. I,III
C. II,IV
D. IV

Answer

To solve the question regarding which properties of halogens increase with increasing atomic number, we will analyze each property one by one.

### Step 1: Analyze Ionization Energy
- **Definition**: Ionization energy is the energy required to remove the most loosely bound electron from an isolated gaseous atom.
- **Trend**: As we move down the group from fluorine to iodine, the ionization energy decreases. This is because with each successive element, an additional electron shell is added, which increases the distance of the valence electrons from the nucleus and increases electron shielding.
- **Conclusion**: Ionization energy **decreases** with increasing atomic number.

### Step 2: Analyze Ionic Radius
- **Definition**: Ionic radius is the measure of an atom's ion in a crystal lattice.
- **Trend**: As we move down the group, the ionic radius increases. This is due to the addition of electron shells, which increases the size of the ion.
- **Conclusion**: Ionic radius **increases** with increasing atomic number.

### Step 3: Analyze Bond Energy of X₂ Molecule
- **Definition**: Bond energy is the energy required to break one mole of bonds in gaseous molecules.
- **Trend**: As we move down the group, the bond length increases due to the larger atomic size, which leads to a decrease in bond strength. For example, F₂ has a weaker bond than Cl₂ due to electron-electron repulsion in the smaller fluorine atoms.
- **Conclusion**: Bond energy **decreases** with increasing atomic number.

### Step 4: Analyze Enthalpy of Vaporization
- **Definition**: Enthalpy of vaporization is the amount of energy required to convert a substance from liquid to gas at constant temperature.
- **Trend**: As we move down the group, the Van der Waals forces between the molecules increase due to larger atomic sizes, which results in higher enthalpy of vaporization.
- **Conclusion**: Enthalpy of vaporization **increases** with increasing atomic number.

### Final Conclusion
Based on the analysis:
- (I) Ionization energy: **Decreases**
- (II) Ionic radius: **Increases**
- (III) Bond energy of X₂ molecule: **Decreases**
- (IV) Enthalpy of vaporization: **Increases**

Thus, the properties that increase with increasing atomic number are:
- **(II) Ionic radius**
- **(IV) Enthalpy of vaporization**