The intermolecular forces of attraction (i.e., H-bonding and van der waal's forces) exist among polar and non-polar species which effect melting point, boiling point, solubility and viscosity of covalent compounds :
Q. Melting and boling points of halogen increase down the group due to

Options

A. Increase in London dispersion forces
B. Increase in extent of polarity
C. Increase in Molecular mass
D. Both (a) and (b)

Explanation

To understand why the melting and boiling points of halogens increase down the group, we can break down the reasoning into a few key steps:

### Step-by-Step Solution:

1. **Identify the Group**: Halogens are located in Group 17 of the periodic table. The elements in this group include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).

2. **Molecular Mass Increase**: As we move down the group from fluorine to astatine, the molecular mass of the halogens increases. For example, the molecular masses are approximately:
- Fluorine (F2) = 38 g/mol
- Chlorine (Cl2) = 71 g/mol
- Bromine (Br2) = 160 g/mol
- Iodine (I2) = 254 g/mol
- Astatine (At2) = 210 g/mol (approx.)

3. **London Dispersion Forces**: The primary intermolecular forces present in non-polar molecules, such as halogens, are London dispersion forces (also known as Van der Waals forces). These forces arise due to temporary dipoles that occur when electron density fluctuates within the molecules.

4. **Effect of Molecular Mass on London Forces**: The strength of London dispersion forces is directly proportional to the molecular mass of the molecules. As the molecular mass increases, the strength of these forces also increases, leading to higher melting and boiling points.

5. **Bond Length and Polarity**: As we move down the group, the bond length between the halogen atoms increases. Longer bond lengths can lead to an increase in the polarity of the molecules, which can also enhance the strength of the intermolecular forces.

6. **Conclusion**: Both the increase in molecular mass and the increase in bond length (leading to increased polarity) contribute to the increase in melting and boiling points of halogens as we move down the group.

### Final Answer:
The melting and boiling points of halogens increase down the group due to:
- **Increase in molecular mass**, which enhances London dispersion forces.
- **Increase in bond length**, which increases polarity and further strengthens intermolecular attractions.

Thus, the correct answer is that both factors contribute to the observed trend.