Chapter 8.7 - Allotropes of Carbon

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

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■ 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.

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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

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In the next section, we will learn about Isotopes of Carbon. 

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Chapter 5.1 - Reaction of Oxygen with Metals

In the previous section, we saw some reactions of oxygen with non-metals. In this section we will see some reactions of oxygen with metals.

A freshly cut surface of metals like aluminium and iron will have a metallic lustre. But this lustre will fade away gradually. This is due to the formation of a new substance on the surface. This new substance is formed because of the reaction of the metal with oxygen.

Reaction with Aluminium:
Aluminium reacts with the oxygen in the atmosphere, and forms aluminium oxide. Let us write the equation:

• One molecule of aluminium is Al. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of aluminium oxide is Al₂O₃. We will write it on the right side

• So the skeletal equation is Al + O₂ → Al₂O₃ . This is not a balanced equation

• The steps for balancing the equation are given below:

So the balanced equation is: 4Al + 3O₂ → 2Al₂O₃

Reaction with Iron:

Iron reacts with the oxygen in the atmosphere, and forms iron oxide. Let us write the equation:

• One molecule of iron is Fe. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of iron oxide is Fe₂O₃. We will write it on the right side

• So the skeletal equation is Fe + O₂ → Fe₂O₃ . This is not a balanced equation

• The steps for balancing the equation are given below:

• So the balanced equation is: 4Fe + 3O₂ → 2Fe₂O₃

• The iron oxide is commonly known as 'rust'

Reaction with magnesium:

When magnesium burns, it reacts with oxygen. We can do a simple experiment to demonstrate this. [Note that all experiments should be done only under the supervision of teachers. All safety precautions should be taken. In the following experiment, avoid looking directly at the flash]

1. Take a magnesium ribbon about 2 cm long. Clean it's surface using sand paper. This is to remove any impurities. Because we want the reaction to take place directly between magnesium and oxygen. We do not want any impurities in between.

2. Hold the ribbon with a pair of tongs. Burn it using a spirit lamp or burner. Note that it should be burnt while keeping it as far away as possible from the eyes.

3. The ribbon will burn with bright white light. Do not look directly at the light. When the burning is complete, we will see that the ribbon has changed into a white powder. This powder is magnesium oxide.

• One molecule of magnesium is Mg. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of aluminium oxide is MgO. We will write it on the right side

• So the skeletal equation is Mg + O₂ → MgO . This is not a balanced equation.

• The steps for balancing the equation are given below:

So the balanced equation is: 2Mg + O₂ → 2MgO

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We have seen some of the reactions of oxygen with metals as well as non-metals. Next we will discuss about Ozone layer.

■ Oxygen (O₂) is found as a diatomic molecule. That is., each molecule of oxygen will have two atoms of oxygen. 

■ But some times, three atoms of oxygen will combine together to form a single molecule. That molecule is entirely different from oxygen in physical and chemical properties. It is called Ozone.

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■ Some elements are found in two or more physical forms. This is called allotropy. The different physical forms are called allotropes of the element. Example:

• Diamond and graphite are two allotropes of carbon.

• Oxygen (O₂) and Ozone (O₃) are two allotropes of oxygen

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Ozone is present mostly in the stratosphere of the atmosphere. Let us see the steps in the formation of Ozone:

1. The ordinary oxygen (O₂) molecules absorbs high energy ultraviolet radiation, and dissociates into individual oxygen atoms.

2. These individual oxygen atoms combine together to form molecules of ozone (O₃).

3. Ozone molecules so formed, absorb low energy ultraviolet radiations and decompose into oxygen molecules (O₂).

4. The O₂ molecules so formed will again undergo the process in step (1)

■ So we see that it is a cyclic process. The cycle is shown in the fig.5.2 below:

Fig.5.2

Let us see the important points to be noted in the cycle:

• In (1), the O₂ absorbs high energy ultraviolet radiation. But in (3), the O₃ absorbs low energy ultraviolet radiation.

• In (1), the O₂ dissociates into individual atoms. But in (3), the O₃ decomposes into O₂.

Importance of the ozone cycle:

We see that ultraviolet radiations are absorbed during the cyclic process. As a result, those harmful radiations do not reach the surface of the earth. Such radiations can cause diseases like cancer. So the ozone layer is very important for sustaining life on earth.

Ozone layer depletion

The proper functioning of Ozone layer is under threat. Let us see why:

• Compounds known as CFC or chlorofluorocarbons, are special compounds that contain chlorine, fluorine and carbon. 

• They have a special property: They are normally in the gaseous state. But they can be easily brought into liquid state by applying pressure. 

• Once they are in the liquid state, they absorb heat from the surroundings. Using this heat, they evaporate and gets converted back into gaseous state. 

• When they absorb heat, the surroundings are cooled. So they are used in refrigerators and air conditioners.

• After long periods of usage, the appliances like refrigerators, air conditioners etc., become worn out. They are then abandoned. 

• The CFC in them will escape into the atmosphere. They reach the stratosphere. 

• The molecules of CFC will break down under the action of ultraviolet radiations. When they break down, individual chlorine atoms become available. 

• These chlorine atoms react with ozone molecules. This reaction will cause the decomposition of ozone molecules. The ozone molecules thus decomposed will be out of the ozone cycle. 

• So if more CFC escapes into the atmosphere, more chlorine will become available, and so more molecules of ozone will be decomposed. Gaps will begin to appear in the ozone layer. This is known as ozone layer depletion.

•When more molecules of ozone is decomposed, only a lesser quantity of ozone will remain to take part in the cycle. This remaining ozone will not be able to absorb all the harmful radiations coming from the sun. As a result, much of those radiations will reach the surface of the earth.

■ In order to create awareness about the need to preserve the ozone layer, September 16^th is celebrated as International Ozone Day. 

■ Now a days many countries have introduced strict rules to avoid using harmful CFC. Safer substances should be used instead. We must all abide by the rules so that, the ozone layer can be restored back to it's original state.

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We have completed our present discussion on oxygen. In the next section, we will discuss about Nitrogen. 

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