Elemental Carbon, Silicon, Germanium
Physical PropertiesYou've learned that the properties of elements in the periodic table change across a row and down any column. We can see trends in properties from top to bottom of group 14, but carbon is remarkably different from all of the elements below it.
Carbon is the only element that doesn't melt, even at very high temperature. Melting points decrease from silicon to germanium to tin. Lead is a little higher than tin but it also has a different solid state structure (see below).
Carbon also has a significantly smaller radius, higher electronegativity, and a higher ionization energy.
Silicon and germanium are very much alike in all of their properties, as are tin and lead.
Solid State StructuresThe most common solid state structure for this group is the diamond structure (a), with tetrahedral symmetry around each atom. Carbon in the form of diamonds, silicon, germanium, and one form of tin pack in this form. While the heaviest element, lead, has a cubic close-packed structure (b).
Electrical ConductivityElectrical conductivity is a measure of a materials ability to allow the flow of electrons through it. Electrons can easily pass through materials when those materials have molecular orbitals extending through the material that are half filled (with one electron).
Each atom of these elements uses 4 valence orbitals and 4 electrons to form bonding and antibonding molecular orbitals. If there are n atoms in a material, there will be 2n bonding molecular orbitals and 2n antibonding orbitals. The bonding orbitals are all filled with 2 electrons each and the antibonding orbitals are empty.
Because n is a very large number, the energy gap between each of the bonding orbitals is too small to measure. They become a band of orbitals, called the valence band. The antibonding orbitals form a band called the conduction band.
The energy gap between the bands depends on the strength of bonds between atoms. Remember that this is the energy difference between the bonding and antibonding orbitals. Carbon forms very strong C-C bonds so it has a very large band gap. Thermal energy available at room temperature isn't enough to excite any electrons from the filled band to the empty band.
Silicon and germanium have significantly weaker bonding between their atoms. This results in a smaller band gap. At room temperature, some of the electrons have enough energy to move into the conduction bands. This means that there are some orbitals in the valence band and in the conduction band that hold only one electron. Electrons can travel through the material through these orbitals.
Tin, like other metals, has no band gap. At room temperature, many of the electrons in the valence band have sufficient energy to move into the conduction band. Because there are many half-filled molecular orbitals, it is easy for electrons to travel through the material.
Graphite doesn't have the same bonding mode as diamond. This material has pi molecular orbitals that extend over each layer. There is no band gap between the pi bonding orbitals and the pi antibonding orbitals so many electrons have enough energy at room temperature to migrate to the pi antibonding levels. This makes graphite a conductor with electrical conductivity of 1 x 105 S/m.