The silicon atom has 14 electrons, but their natural orbital arrangement allows only the outer four of these to be given to, accepted from, or shared with other atoms. These outer four electrons, called "valence" electrons, play an important role in the photovoltaic effect.
Large numbers of silicon atoms, through their valence electrons, can bond together to form a crystal. In a crystalline solid, each silicon atom normally shares one of its four valence electrons in a "covalent" bond with each of four neighboring silicon atoms. The solid, then, consists of basic units of five silicon atoms: the original atom plus the four other atoms with which it shares its valence electrons. In the basic unit of a crystalline silicon solid, a silicon atom shares each of its four valence electrons with each of four neighboring atoms.
The solid silicon crystal, then, is composed of a regular series of units of five silicon atoms. This regular, fixed arrangement of silicon atoms is known as the "crystal lattice."