Silicon (Si)

Crystalline silicon ([Ne]3s²3p²) (CAS 7440-21-3) is a lustrous, grayish-black color, while the amorphous form is a dark brown powder. Making up 25% of the earth's crust, silicon is the second most abundant element, after oxygen. Silicon is not found free in nature but can form more compounds than any other element, except carbon. It is the most important semiconducting element. Silicon and its many compounds have some of the most useful mechanical, optical, thermal, and electrical properties known.