The Three Forms of Carbon

Location:

Concepts Shown:

structures and allotropes, material's properties

Equipment:

three models of carbon- diamond, graphite, buckminsterfullerene obtained from Klinger Educational Products, 1 800 522 6252

Procedure:

1) Show the three models of carbon, and pass them around.

2) Discuss the following allotropes of carbon. Allotropes are different crystalline structures of the same chemical substance. Use the overheads of each form to explain the bonding differences (see end of format sheet), while the actual models are being passed around the classroom.

Graphite:

Graphite contains layers (x-y planes) in which carbon atoms are held together with sigma bonds. Every carbon atom in this layer is bonded to three other carbon atoms to form sp2 hybridized orbitals and trigonal plane geometry. These sigma bonds are strong, and the layers contain delocalized electrons as a result of the hybridization. The connection between the layers, in the z-direction, are pi bonds which are much weaker than sigma bonds.

Therefore, graphite has good electrical and thermal conductivity in the x-y plane due to the delocalized electrons in the hybridized orbitals, but poor conductivity in the z-direction that connects the layers. Graphite has good lubricating properties- the weak pi bonds between the layers permit the layers to slide when sheared. This softness can be useful for certain applications. The strong sigma bonds within the layers give graphite a melting temperature of more than 2200 oC.

Diamond:

Diamond is sp3 hybridized, which corresponds to each carbon atom bonded to four other carbon atoms. This produces the tetrahedral geometry as seen in the model. These bonds are covalent (sharing electrons) and are very strong.

The hardness of diamond is a result of each carbon atom bonding to four neighboring atoms, each of which are bonded to four neighbors, and this creates a rigid three-dimensional structure. Diamond is not electrically conductive because there are no delocalized electrons. However, diamond is a very good thermally conductive material because of its low ionicity.


Buckminsterfullerene:

A newly discovered allotrope of carbon, its formula is C60. The structure is an icosohedron, or soccer ball shape, with twelve pentagons and twenty hexagons per molecule. The radius of this molecule is about 3.54 angstroms.

Because of its perfect symmetry and size, this allotrope is an extremely stable and inert material. When doped with alkali materials (Na, K), the material becomes a superconductor with a transition temperature around 19 K for K3C60.

Science:

Everything is basically explained in the presentation section. The main concepts are allotropic structures and how they change the resulting properties. A review of chemical bonding may be necessary.

References:

Author:
Roberta Dean
Credits:


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