Explain why silicon dioxide exist as a solid of high melting point whereas Carbon dioxide is a gas even though silicon and carbon belong to the same group.

Carbon dioxide ($\text{CO}_2$) is a simple molecular substance. It consists of discrete $\text{CO}_2$ molecules held together by weak intermolecular forces (van der Waals forces).

Silicon dioxide ($\text{SiO}_2$) has a giant covalent network structure. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is covalently bonded to two silicon atoms, forming a continuous three-dimensional network.

To melt $\text{CO}_2$, only a small amount of energy is required to overcome the weak intermolecular forces, which is why it is a gas at room temperature. In contrast, to melt $\text{SiO}_2$, a large amount of energy is needed to break the strong covalent bonds throughout its giant network structure, resulting in a high melting point and solid state at room temperature.

The difference in structure is due to carbon's ability to form stable double bonds with oxygen, leading to discrete $\text{CO}_2$ molecules, while silicon prefers to form single bonds with oxygen, leading to an extended network.