Silicon (Si) is the chemical element of atomic number 14. It is the most abundant element in the earth's crust after oxygen, or 25.7% of its mass, but it is comparatively only present in relatively small quantities in the material constituting the living.
It is a semi-metal, ie it does not meet all the criteria to be defined as being a metal.
It has four free electrons in the last orbit, so it is able to form four covalent bonds. (the carbon can form 4 covalent bonds but it is not a semi-metal)
