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					Slide 1
Forms of Carbon, relating structure to property.

Carbon is one of the most common elements on earth.
It exists in three different forms: diamond, graphite and Fullerene. As we
can see these materials are very different in form and function even though
they are all made of only carbon atoms. In this video we will learn that like
with all materials, the properties of these three types of carbon arise at the
nanoscale, where carbon atoms are arranged differently.

Slide 3
Before we talk about the different structures of carbon it is important to
understand that crystals are distinct from molecules. Molecules are well-
defined units with a fixed number of atoms (for example in methane, we
have 5 atoms, 4 hydrogen and 1 carbon atom). Crystals are more complex.
They are made by repeating atoms in all directions. The number of atoms is
too large to count, and is never well-defined. Crystals have a unit cell, which
is like a brick from which the crystal is built.
The unit cell of the diamond crystal is shown here. The number of atoms
and links between atoms is not fixed, and can continue indefinitely.

Slide 4
One of the properties of crystals is that they are sharp sided. These sharp
sides are called facets. Crystals of ice are formed when water molecules
arrange themselves in hexagonal patterns which we see in snow flakes.
These hexagonal patterns make themselves known externally in the shapes
of ice crystals, which have facets: flat faces with sharp edges. So, we can tell
if something is a crystal by just looking if it has sharp edges

Slide 5
Let us now look at the crystalline forms of carbon. The first one we look at
is diamond. We know of diamond as a brilliant jewel and also the hardest
known naturally occurring mineral. How can this property be explained?
We need to look at the unit cell of diamond. In this computer model the
black spheres are carbon atoms. The reason that diamond is so hard is
because every carbon in diamond is bonded to 4 other carbon atoms in a 3-
dimensional way, the resulting material is very hard. No other naturally
occurring material can scratch a diamond. Diamonds are not only used in
jewelry but also in cutting and grinding because they are so hard.

Slide 6
The second pure form, or allotrope of carbon, is graphite. Graphite is used
in pencils - your pencil lead is not really lead, it is graphite! Graphite is a
soft material. The structure of graphite is different from the structure of
diamond. In graphite we have a sheet structure as shown in this model. The
sheets are not bonded with each other; they can slide off each other. Each
carbon atom is bonded to only 3 other carbons in the same plane, or flat
sheet, the resulting material is soft. The bonds between each sheet are weak
and easily break; the pencil marks on paper are the flat sheets of carbon.
Slide 7
These very diverse properties of materials which are both composed of pure
carbon arise from differences in structure. The hardness and clarity of
diamond is a result of each carbon bonding covalently to 4 other carbons. In
graphite, each carbon forms a covalent bond with three other carbons.
Slide 8
The third form of carbon is the carbon 60 molecule. It consists of 12
pentagons and 20 hexagons with carbon atoms at each corner forming a
spherical molecule like a soccer ball. It is often called a Buckyball. Here are
some Buckyballs drawn by a computer. The actual molecules are very small
about 1 nanometer. The molecules of C60 are not held together very tightly.
As a consequence we can make a solution out of C60. The purple solution
contains the individual molecules.

Slide 9
How do we know these atoms are connected to form a spherical shape?
Crystal structures can be described in terms of planes of atoms. The
perpendicular distance between planes is the interplanar distance d. Here are
two different d-spacings in graphite. These distances can be studied using x-
ray diffraction, the same x-rays used to take pictures of the bones of our

Slide 10
The effects of diffraction can be seen in everyday life. The most colorful
examples of diffraction are those involving light; for example, the closely
spaced tracks on a CD or DVD form the familiar rainbow pattern we see
when looking at a disk.

Slide 11
When x-rays go through crystals, they diffract - the x-rays split in different
directions. The different planes of atoms diffract X-rays in different ways,
and in a manner that is specific to the particular crystal structure.

Slide 12
An x-ray diffraction experiment measures these different planes. The signals
in the pattern correspond to different planes of atoms in the structure. This
is how we gain information about the structure of a material.

Slide 13
This technique can provide us with “fingerprints” of the different materials.
Each x-ray diffraction pattern for different materials is very distinct. Here
we see the fingerprint of Carbon60, graphite and diamond. They are very
different from each other