Graphite

GRAPHITE

Occurrence: Graphite is a grayish black substance. It occurs in nature mixed with mica, quartz and silica. It is also prepared artificially from carbon in an electric furnace at 2273–2773 K.

Structure of Graphite

Structure of Graphite:

Each carbon is bonded to only three neighboring carbon atoms in the same plane forming layers of hexagonal networks separated by comparatively larger distance. The different layers are held together by weak forces, called vanderwaal forces. The layers can therefore, easily slide over one another. This makes graphite lubricating, soft and greasy to touch.

Within each layer of graphite, every carbon atom is joined to three others by strong covalent bonds. This forms a pattern of interlocking hexagonal rings. The carbon atoms are difficult to separate from one another. So graphite also has a high melting point.

However, the bonds between the layers are weak. The layers are able to slide easily over one another, rather like pack of cards. This makes graphite soft and slippery. When we write with a pencil, layers of graphite flake off and stick to the paper.

Physical properties:

• Low density: Due to wide spacing (340 pm) between the two layers, the carbon atoms in graphite are less closely packed and hence the density of graphite (2.22 g cm–3) is much lower than that of diamond.
• Softness: The various layers of carbon atoms in graphite are held together by weak van der Waals forces of attraction. Therefore, one layer can easily slide over the other. This makes graphite soft and hence a useful dry (solid) lubricant for heavy machinery.
• Electrical and thermal conductivity: Carbon has four valence electrons. But in a graphite crystal, each carbon atom is joined to three other carbon atoms by covalent bonds to form hexagonal rings. Thus, only three valence electrons are used for bond formation and hence the fourth valence electron is free to move. As a result, graphite is a good conductor of heat and electricity.

Use:

• Due to its softness, powdered graphite is used either as a solid or dry lubricant or mixed with petroleum jelly as graphite grease. Since graphite is non-volatile, it can also be used as a lubricant for heavy machinery operating at very high temperatures.
• Graphite is soft and black in colour and marks paper black. Mixed with desired quantity of wax or clay, graphite is used for making the cores of lead pencils.
• Being a good conductor of electricity, graphite is used for making electrodes for dry cellsand electric accessories. The carbon brushes of electric motors are also made up of graphite.

DIFFERENCE BETWEEN DIAMOND AND GRAPHITE:

Some main points of difference between the properties of diamond and graphite are given below: