Here's why:

* Silicon (Si) is a metalloid with a valency of 4, meaning it can form four bonds.

* Oxygen (O) is a nonmetal with a valency of 2, meaning it can form two bonds.

To achieve a stable compound, one silicon atom (Si) forms bonds with two oxygen atoms (O), resulting in the formula SiO2.

SiO2 is a very common compound found in nature, primarily as quartz.