Carbon dioxide (CO2) is a molecular compound, while silicon dioxide (SiO2) is a covalent network solid. The main reason for this difference lies in the structure and bonding properties of the two substances.
In CO2, each carbon atom is double-bonded to two oxygen atoms, forming a linear molecular structure. These molecules interact with each other primarily through weak van der Waals forces, which are much weaker than the covalent bonds that hold the atoms within each molecule together. Because of these weak intermolecular forces, CO2 does not form a continuous three-dimensional network of covalent bonds, which is characteristic of a covalent network solid. Instead, CO2 exists as discrete molecules that can be easily separated from one another.
On the other hand, SiO2 has a three-dimensional tetrahedral structure where each silicon atom is covalently bonded to four oxygen atoms, and each oxygen is bonded to two silicon atoms. This extensive bonding creates a strong and stable lattice that extends throughout the material, giving SiO2 its properties as a covalent network solid. The bonds in SiO2 are strong enough to provide rigidity and stability, making it quite different from the simple molecular structure of CO2.
In summary, while both substances involve covalent bonds, the nature of their structures and the types of forces holding them together determine whether they are classified as molecular compounds or covalent network solids. CO2 is a molecular compound with weak intermolecular forces, whereas SiO2 forms a robust lattice structure typical of covalent network solids.