Trend 4.4 of the Carbon Group (Group IVA) Trends says that the ability of the elements to form multiple bonds diminishes in the series: C-C > Si-Si > Ge-Ge > Sn-Sn > Pb-Pb Because the pπ-pπ overlaps become less favorable. This has the following manifestations:
a) The elements below C the allotropes which would structurally resemble graphite which has a delocalized two dimensional π-system are not observed.
b) There are no simple analogues of ethane (C2H4) and ethyne (C2H2) and compounds of SI, Ge, and Sn with multiple bonds may only be isolated when there are bulky organic substituents on the group IV atoms. Furthermore, the Ge and Sn compounds do not have planar geometries.
c) The analogues of CO2 and CS2 have polymeric structures rather than triatomic molecular geometries.
d) The heavier elements do not form analogues of carbides with C22- and C32- multiply bonded ions.
Graphite-like allotropes not observed down columnEdit
Graphite is the allotrope of Carbon, where each Carbon forms a single (sigma) bond to three other Carbon atoms. The fourth bond that each carbon bond forms is a pi bond that goes between the layers. In the image on the left this pi bond is shown as the lines connecting the red layers to the blue layers. This pi bond that stabilizes the different layers is referred to as the “delocalized, two dimensional ∏ bonds.
Why does Carbon do this and not any of the others?Edit
Carbon is extremely small. This means that it orbitals are able to form a pi bond, because it is easier to overlap. After we move on from the second row, the elements get so much larger and can no longer form these p-∏ bonds. The overlap of the orbitals is very poor. The graphite structure of Carbon takes advantage of these p-∏ bonds by using them to stabilize the different layers. Because these p-∏ bonds cannot be formed in the other elements of the group, the graphite structure is not observed.
What are the benefits of this structure?Edit
Graphite is a structure that we take advantage of every day. It is what we use to write with when we use a pencil. The reason that it works so well is because of the layered structure. When we write we are sliding the top layer off kilter a little bit, just enough that the pi bonds that is hold that layer to the other layers, break. This allows the layer to be removed from the rest and to stick onto the paper. Graphite is also a great lubricant because of its layered structures. To learn more about Graphite and its uses look here.