diamond and graphite

Crystals

A crystalline substance is one in which the atoms or ions that make up the structure are arranged in a very ordered and uniform way. A structure where the atoms or ions are arranged in a random or disordered way is called a non-crystalline or amorphous structure. As a rather simple example we could say a brick wall is crystalline, the brick are laid in a very ordered and regular way, if you knock the wall down then you end up with a large pile of brick, this would be a disordered or amorphous arrangement.

Macromolecular crystals

Carbon is perhaps one of the most remarkable elements in the periodic table, it forms such a wide variety of substances which can range from small to medium sized molecules to others which consist of giant covalent network solids such as diamond and graphite. The structures of these allotropes of carbon are shown below. You can clearly see that the structures of both diamond and graphite are very ordered, that is they are both crystalline in nature. However they have very different properties due to the way in which the carbon atoms are bonded together in each structure.

Graphite

Graphite has a giant covalent structure which consists of flat sheets of carbon atoms arranged a hexagonal arrangement. The carbon atoms in the flat sheet make 3 strong covalent bonds to other carbon atoms. However carbon makes 4 bonds and this leaves 1 spare electron in a p-orbital on the carbon atom. This spare electron forms part of a delocalised sea of electrons between the flat hexagonal sheets. These delocalised electrons account for the fact that graphite is a good electrical conductor.

graphite structure

Between the flat sheets of carbon atoms are weak Van der Waals intermolecular bonds. These are weak bonds and are easily broken. One of the main uses of graphite is as an industrial lubricant. Pencil leads are made of graphiteand if you rub a pencil over a sheet of paper for a few second to leave a thin layer of graphite on the paper then rub your finger over the graphite layer you will feel just how slippery graphite is. You can perhaps imagine that when you rub the graphite pencil lead of the paper the weak intermolecular Van der Waals bond holding the hexagonal layers together break when you push the pencil along, this will leave the flat hexagonal layer intact on the paper, the bonds in this layer will not break since they are strong covalent bonds. The image below shows the layered structure:

layred structure of graphite

Diamond

Diamond like graphi consists of only carbon atoms, but whereas graphite is a soft, flaky material that conducts electricit, diamond is the hardest known natural substance and it is also an electrica insulator. Though like graphite when heated to a very high temperature it tends to sublime before it melts. The reason why diamond has different properties from graphite is due to the bonding within the structure. Diamond has an ordered crystalline structure like graphite but that is where the similarities end. Each carbon atom in diamond makes 4 strong covalent bonds. All the electrons in the carbon atoms valence shell are involved in forming covalent bonds. They are held in these bonds and so are not free to move as they are in graphite, this explains why diamond is an electrical insulator. In the image below you can see that each carbon atom forms 4 covalent bonds which are all the same length, unlike in graphit where the bonds between the layers are much larger than the bonding within the layers. In diamond each carbon atom forms a tetrahedral arrangement; with bond angles of 109.5 with its neighbouring atoms.

3d model showing the structure of diamond

Drawing the structure of diamond

At first glance the structure of diamond looks impossible to draw, however it is fairly easy to draw a simple outline of the basic structure. Remember each carbon atom is in a tetrahedral arrangement but also the whole structure is built up of six membered rings of carbon atoms fused or stuck together. Graphite also consist of 6 membered rings of carbon atoms, but in graphite the rings are flat, in diamond they are puckered or bent into a chair like structure. No matter from which direction you view the diamond structure from you always see the same 6 membered puckered rings of carbon atoms. - that is the structure is highly symmetrical and looks the same from all angles. The diagram below shows you how to draw a basic outline of these fused or puckered rings that build up the diamond structure.

how to draw the repeating unit in the diamond structure

Properties of diamond and graphite



The Fullerenes

In the 1985 scientists discovered a new allotrope of carbon. One in which the carbon atoms are arranged in "football" like structure or structures consisting of hollow cages or tubes. These spheres and tubes are called Fullerenes after Buckminister Fuller, an architect and inventor. Fullerene molecules are often shaped like hollow spheres or footballs, or long thin tubes or even ellipsoid (squashed or flattened spheres). The sizes of these spheres and tubes varies considerably.

Buckyballs

The first fullerene discovered consisted of 60 carbon atoms, C60, covalently bonded together into a football type structure composed of hexagons and pentagons. It was named buckministerfullerene after the American architect Buckminster Fuller. These ball like structures are often referred to as buckyballs. The image below shows the typical structure of a buckyball or fullerene molecule.

Image of a buckyball

Unlike diamond and graphite these newly discovered fullerenes have a molecular structure and not the giant macromolecular structures found in diamond and graphite. The carbon atoms in these football like structures is similar in some ways to that found in graphite. The carbon atoms make 3 covalent bonds and have 1 spare electron which is delocalised over the molecule. However unlike in graphite the delocalisedelectrons are trapped within the molecule and so these buckyballs are electrical insulators. Buckyballs being molecular will dissolve in some organic solvents; this is not the case with giant macromolecular solids.



Key Points



Practice questions

Check your understanding - Questions on diamond and graphite.