Hi! I study semiconductors and I am confused with the bonding picture for semiconductor crystals. For example, it is said that in Si crystal one can find covalent bonds. In the most books this bonds are shown like that: 2D crystal structure where lines between atoms are a covalent bonds, each line stands for one electron (like here http://www.aplusphysics.com/courses/honors/microe/silicon.html). At the same time it is well known that in Si or in Ge there are only 4 electrons on the upper shell: two s electrons and two p electrons, only these ones can form bonds. Therefore it follows that for such 2D pictures of Si or Ge crystals we are just running out of electrons to make covalent bonds for the third dimension (usual crystal has 3D)! Can you please explain what's going on with the third dimension. If such representations which i mentioned above is not true (or partially true), then what is complete 3Dimentional picture for bonding in semiconductors should look like?