i read in my chem book, it says that SiH4 is molecular compound but SiO2 is covalent network, but why not SiH4 since it has Si which can form network covalent as well?
The reason SiH4 (silane) is considered a molecular compound, while SiO2 (silicon dioxide) is classified as a covalent network, lies in their different structures and bonding patterns.
SiH4 is a tetrahedral molecule in which a central silicon atom is bonded to four surrounding hydrogen atoms. Each bond between silicon and hydrogen is a covalent bond, meaning electrons are shared between the two atoms. However, the bonding between different silane molecules is relatively weak. When SiH4 is in the gas phase or as a pure liquid, the molecules are mostly independent of each other, forming a molecular compound.
On the other hand, SiO2 exists as a solid in its most common form, quartz. In this crystal lattice, each silicon atom is bonded to four oxygen atoms via strong covalent bonds. These silicon-oxygen bonds form a continuous network throughout the entire crystal structure, with each silicon atom being connected to multiple oxygen atoms, and each oxygen atom being bonded to two silicon atoms. This continuous network of covalent bonds gives rise to the solid and highly organized structure of silicon dioxide.
While silicon itself has the ability to form covalent networks, as demonstrated by SiO2, the presence of hydrogen atoms in SiH4 prevents the formation of a solid covalent network. Hydrogen has relatively weak bonding capabilities compared to oxygen, and the Si-H bonds in SiH4 are not sufficient to create a continuous and stable network.
In summary, SiH4 is a molecular compound due to its weak intermolecular forces between individual molecules, whereas SiO2 forms a covalent network due to the strong and continuous covalent bonds between silicon and oxygen atoms throughout the entire solid structure.