Silicon is an element , its symbol is Si and its atomic number is 14 . As a mass it is the eighth most abundant element in the world but it is rarely found in pure form in nature. It is mainly in the form of dust , sand asteroids and planets in the form of silicon oxide (silica) or silicate. About 90% of the Earth's crust is composed of silicate compounds, making it the second most abundant element in the Earth's crust after oxygen (about 27% by mass). [4]