How do melting and boiling points vary in a (a) period and (b) group in periodic table?

Explanation

To understand how melting and boiling points vary in a group and period of the periodic table, we can break down the explanation into two parts: (a) for groups and (b) for periods.

### (a) Variation in a Group:
1. **Atomic Size Increases**: As you move down a group in the periodic table, the atomic size of the elements increases due to the addition of electron shells.
2. **Van der Waals Forces**: The increase in atomic size leads to an increase in the Van der Waals forces (intermolecular forces) between the atoms. These forces are responsible for holding the atoms together in a substance.
3. **Boiling Point Increases**: As the Van der Waals forces increase, the boiling point of the elements also increases. This is because more energy is required to overcome these stronger forces during the phase change from liquid to gas.
4. **Conclusion**: Therefore, in a group, as we move down, the boiling point generally increases due to the increase in atomic size and the corresponding increase in Van der Waals forces.

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