In the previous section, we saw the three dimensional arrangement of atoms. In this section we will learn about Allotropes.
■ Let us see some basic details about diamond:
• It contain only one element, and that is carbon.
• The carbon atoms are held together by covalent bonds. All the covalent bonds are single bonds.
• Each carbon atom is bonded to four other carbon atoms. They form a tetrahedron network. The tetrahedron arrangement in diamond can be seen here.
■ So the diamond is made up of carbon atoms arranged in a 3D space.
• All those atoms are interconnected. The interconnection is by strong covalent bonds. This makes diamonds very hard.
• They are used in cutting tools for glass, brick, concrete etc.,
• Diamonds do not conduct electricity. This is because of the absence of any free electrons.
■ Some more properties of diamonds are listed below:
• Diamonds have high refractive index.
• Pure diamond is transparent.
• When diamonds are cut by experts, they sparkle and reflect light in an attractive way.
• They also give off different colours because of the refraction of white light. So they are used in jewellery.
• Diamonds are good conductors of heat.
Now let us see some basic details about graphite:
• We have seen that, in diamond, the carbon atoms are situated at the corners of a tetrahedron. A tetrahedron is a 'three dimensional' figure.
• But in the case of graphite, the carbon atoms are situated at the corners of a hexagon.
• A hexagon is a 'two dimensional' figure. These 'two dimensional hexagons' are connected to each other. So they form 'two dimensional layers'. This arrangement can be seen here.
• These layers are stacked one above the other. When so many such layers are stacked, we get a solid appearance.
♦ For example, when two or three sheets of paper are stacked, it will still be two dimensional. But when a very large number of sheets of paper are stacked, we get a three dimensional or solid appearance.
• But there is no connection between those individual layers. The layers are kept together only by weak van der waals physical forces. So the layers can slide over one another
Now let us see some properties of graphite:
• Since the layers slide over one another, they are slippery and hence used as solid state lubricants. This is shown here.
• Graphite is a good conductor of electricity. This is because free electrons are present in it. Free electrons are available because, those electrons are not required for bonding.
• So it is used to make electrodes in dry cells
• It is used to make pencils. This is because of it's smoothness and grey colour.
♦ In early days, when graphite was newly discovered, people thought it was black lead. So pencils made of them got the name lead pencils
• Graphite is lustrous and non volatile
♦ In diamond carbon is in one form
♦ In graphite carbon is in another form
• There are a few other forms also for carbon
■ Allotropy is the phenomenon by which the same element exists in different physical forms.
■ Diamond and graphite are allotropes of carbon
• We have seen allotropes of oxygen here.
In the next section, we will learn about Isotopes of Carbon.
■ Let us see some basic details about diamond:
• It contain only one element, and that is carbon.
• The carbon atoms are held together by covalent bonds. All the covalent bonds are single bonds.
• Each carbon atom is bonded to four other carbon atoms. They form a tetrahedron network. The tetrahedron arrangement in diamond can be seen here.
■ So the diamond is made up of carbon atoms arranged in a 3D space.
• All those atoms are interconnected. The interconnection is by strong covalent bonds. This makes diamonds very hard.
• They are used in cutting tools for glass, brick, concrete etc.,
• Diamonds do not conduct electricity. This is because of the absence of any free electrons.
■ Some more properties of diamonds are listed below:
• Diamonds have high refractive index.
• Pure diamond is transparent.
• When diamonds are cut by experts, they sparkle and reflect light in an attractive way.
• They also give off different colours because of the refraction of white light. So they are used in jewellery.
• Diamonds are good conductors of heat.
Now let us see some basic details about graphite:
• We have seen that, in diamond, the carbon atoms are situated at the corners of a tetrahedron. A tetrahedron is a 'three dimensional' figure.
• But in the case of graphite, the carbon atoms are situated at the corners of a hexagon.
• A hexagon is a 'two dimensional' figure. These 'two dimensional hexagons' are connected to each other. So they form 'two dimensional layers'. This arrangement can be seen here.
• These layers are stacked one above the other. When so many such layers are stacked, we get a solid appearance.
♦ For example, when two or three sheets of paper are stacked, it will still be two dimensional. But when a very large number of sheets of paper are stacked, we get a three dimensional or solid appearance.
• But there is no connection between those individual layers. The layers are kept together only by weak van der waals physical forces. So the layers can slide over one another
Now let us see some properties of graphite:
• Since the layers slide over one another, they are slippery and hence used as solid state lubricants. This is shown here.
• Graphite is a good conductor of electricity. This is because free electrons are present in it. Free electrons are available because, those electrons are not required for bonding.
• So it is used to make electrodes in dry cells
• It is used to make pencils. This is because of it's smoothness and grey colour.
♦ In early days, when graphite was newly discovered, people thought it was black lead. So pencils made of them got the name lead pencils
• Graphite is lustrous and non volatile
■ So we have seen two substances. Diamond and graphite. They have different properties.
• But one thing is common. That is., they contain carbon atoms only. No other atoms are present.
• So we can say that carbon exists in two different forms. ♦ In diamond carbon is in one form
♦ In graphite carbon is in another form
• There are a few other forms also for carbon
■ Allotropy is the phenomenon by which the same element exists in different physical forms.
■ Diamond and graphite are allotropes of carbon
• We have seen allotropes of oxygen here.
Now let us see some other allotropes of carbon
• We have seen diamond in which carbon atoms are placed at the corners of three dimensional tetrahedron
• We have seen graphite in which carbon atoms are placed at the corners of two dimensional hexagon
■ Now we will see fullerene.
• In fullerene, carbon atoms are placed at the corners of two dimensional hexagons and pentagons.
• These hexagons and pentagons are connected together.
♦ Just like hexagons are connected together to form a layer in graphite
• But in fullerene, the interconnected hexagons and pentagons do not remain as a layer.
• Instead, they form a spherical shape as in a foot ball. The sphere is hollow inside. So it is like a spherical cage. A figure can be seen here.
♦ One spherical cage is one molecule of fullerene.
♦ They are also known as Bucky balls
• If, instead of forming a sphere, the sheet rolls into a cylinder, we get a cylidrical cage.
♦ Then they are known as Bucky tubes
■ Another allotrope of carbon is graphene. Let us see some of it's basic details:
• We have see the details of graphite. Take out one layer of graphite.
♦ This one layer of graphite is graphene.
• So graphene is a layer having a thickness equal to the thickness of one carbon atom. Some images can be seen here.
• Such a one atom thick layer has special uses in nano technology. We will learn more details about graphene in higher classes
In the next section, we will learn about Isotopes of Carbon.
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