The chemical reactions describing the thermal oxidation of silicon in oxygen or water vapor are given by the following equations:
Si(solid) + O2 ® SiO2 (solid) -------------------DRY OXIDATION
Si(solid) + 2H2O ® SiO2(solid) +2H2 -----------WET OXIDATION
For growth of an oxide of thickness d, a layer of silicon with thickness of 0.44d is consumed. Oxidation proceeds by the diffusion of the oxidizing species through the oxide to the Si-SiO2 interface, where the oxidation reaction occurs.
When bare silicon is exposed to the atmosphere, it reacts almost immediately with oxygen or moisture present in the air and forms a thin (about 10 to 20°A) layer of silicon dioxide, called the native oxide. The thickness of the native oxide is sufficient to stop further oxidation of the silicon at room temperature. 
Fig. 1 Silicon Oxidation Process 
Uses of silicon dioxide