Silicon dioxide forms the cell walls of
diatoms
insects
plants

Silicon dioxide forms the cell walls of diatoms. These diatoms are a group of algae. Phytoplanktons are the most common types of diatoms that are unicellular. These organisms can form colonies in the shapes of filaments or ribbons, fans, zigzags, and stars that are perfect for monitoring environmental conditions, particularly water quality in the past and present. 

2hydrogen and 1 covalent