Silicon crystal. Coloured scanning electron micrograph (SEM) of cleavage in a silicon crystal. Silicon (symbol Si) is an element of group 4A (IUPAC group 14) of the periodic table. Cleavage is the splitting, or tendency of a crystal to split, along definite crystalline planes to produce a smooth surface. Silicon, in its element state, is used in transistors and other solid state devices. Its oxide (silicon dioxide; SiO2), known as silica, is used in glass and building materials. Magnification: x138 at 6x7cm size.