There are five polyhedra that are basic building blocks of crystals.
Lets look first at the basic tetrahedral unit.
The Si - O combination has a radius ratio of 0.30, which means that the silicon ion fits nicely into a tetrahedral polyhedron. An exploded schematic diagram of this polydedron is shown at left. We could talk about electron orbitals and discuss just why this happens, but lets keep the discussion simple. Lets just say that the Silicon ion shares its charge equally between the four oxygen ions, leaving each oxygen with an excess charge of negative one. This anion tends to react readily with alkali and alkali earth ions.