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

### Step-by-Step Solution:

1. **Understanding the Halogens**: The halogens include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). As we move down the group in the periodic table, the atomic and molecular masses of these elements increase.

2. **Molecular Mass and Shells**: The increase in molecular mass is due to the addition of more electron shells as we go from fluorine to iodine. Each subsequent halogen has more electrons, which contributes to a larger atomic size and mass.

3. **Intermolecular Forces**: The primary intermolecular force present in halogens (F2, Cl2, Br2, I2) is the London dispersion force (also known as van der Waals forces). These forces arise due to temporary dipoles that occur when electron density fluctuates around the molecule.

4. **London Dispersion Forces**: London dispersion forces are directly proportional to the size of the electron cloud around the molecules. Larger molecules with more electrons have stronger London dispersion forces. Thus, as the size and mass of the halogen molecules increase down the group, the strength of these forces also increases.

5. **Effect on Melting and Boiling Points**: The melting and boiling points of a substance are influenced by the strength of the intermolecular forces. Since the London dispersion forces increase with molecular mass, the melting and boiling points of halogens increase down the group.

6. **Conclusion**: Therefore, the increase in melting and boiling points of halogens down the group can be attributed to the increase in molecular mass, which leads to stronger London dispersion forces.

### Final Answer:
The melting and boiling points of halogens increase down the group due to the increase in molecular mass, which enhances the strength of London dispersion forces.