NCERT Textbook: Carbon & its Compounds | Science & Technology for UPSC CSE PDF Download

 Page 1


Science
58
Carbon and its
Compounds
4 CHAPTER
I
n the last Chapter, we came to know many compounds of importance
to us. In this Chapter we will study about some more interesting
compounds and their properties. Also, we shall be learning about carbon,
an element which is of immense significance to us in both its elemental
form and in the combined form.
Things made Things made Others
of metal of glass/clay
Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1
n Make a list of ten things you have used
or consumed since the morning.
n Compile this list with the lists made by
your classmates and then sort the items
into the adjacent Table.
n If there are items which are made up of
more than one material, put them into
both the relevant columns of the table.
Look at the items that come in the last column of the above table
filled by you – your teacher will be able to tell you that most of them are
made up of compounds of carbon. Can you think of a method to test
this? What would be the product if a compound containing carbon is
burnt? Do you know of any test to confirm this?
Food, clothes, medicines, books, or many of the things that you listed
are all based on this versatile element carbon. In addition, all living
structures are carbon based. The amount of carbon present in the earth’s
crust and in the atmosphere is quite meagre. The earth’s crust has only
0.02% carbon in the form of minerals (like carbonates, hydrogen-
carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon
dioxide. In spite of this small amount of carbon available in nature, the
importance of carbon seems to be immense. In this Chapter, we will know
about the properties of carbon which make carbon so important to us.
4.1 BONDING IN CARBON – THE COVALENT BOND
In the previous Chapter, we have studied the properties of ionic
compounds. We saw that ionic compounds have high melting and boiling
points and conduct electricity in solution or in the molten state. We also
2024-25
Page 2


Science
58
Carbon and its
Compounds
4 CHAPTER
I
n the last Chapter, we came to know many compounds of importance
to us. In this Chapter we will study about some more interesting
compounds and their properties. Also, we shall be learning about carbon,
an element which is of immense significance to us in both its elemental
form and in the combined form.
Things made Things made Others
of metal of glass/clay
Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1
n Make a list of ten things you have used
or consumed since the morning.
n Compile this list with the lists made by
your classmates and then sort the items
into the adjacent Table.
n If there are items which are made up of
more than one material, put them into
both the relevant columns of the table.
Look at the items that come in the last column of the above table
filled by you – your teacher will be able to tell you that most of them are
made up of compounds of carbon. Can you think of a method to test
this? What would be the product if a compound containing carbon is
burnt? Do you know of any test to confirm this?
Food, clothes, medicines, books, or many of the things that you listed
are all based on this versatile element carbon. In addition, all living
structures are carbon based. The amount of carbon present in the earth’s
crust and in the atmosphere is quite meagre. The earth’s crust has only
0.02% carbon in the form of minerals (like carbonates, hydrogen-
carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon
dioxide. In spite of this small amount of carbon available in nature, the
importance of carbon seems to be immense. In this Chapter, we will know
about the properties of carbon which make carbon so important to us.
4.1 BONDING IN CARBON – THE COVALENT BOND
In the previous Chapter, we have studied the properties of ionic
compounds. We saw that ionic compounds have high melting and boiling
points and conduct electricity in solution or in the molten state. We also
2024-25
Carbon and its Compounds 59
saw how the nature of bonding in ionic compounds explains these
properties. Let us now study the properties of some carbon compounds.
Most carbon compounds are poor conductors of electricity as we
have seen in Chapter 2. From the data
given in Table 4.1 on the boiling and
melting points of the carbon compounds,
we find that these compounds have low
melting and boiling points as compared
to ionic compounds (Chapter 3). We can
conclude that the forces of attraction
between the molecules are not very
strong. Since these compounds are
largely non-conductors of electricity, we
can conclude that the bonding in these
compounds does not give rise to any ions.
In Class IX, we learnt about the
combining capacity of various elements and how it depends on the
number of valence electrons. Let us now look at the electronic
configuration of carbon. The atomic number of carbon is 6. What would
be the distribution of electrons in various shells of carbon? How many
valence electrons will carbon have?
We know that the reactivity of elements is explained as their tendency
to attain a completely filled outer shell, that is, attain noble gas
configuration. Elements forming ionic compounds achieve this by either
gaining or losing electrons from the outermost shell. In the case of carbon,
it has four electrons in its outermost shell and needs to gain or lose four
electrons to attain noble gas configuration. If it were to gain or lose
electrons –
(i) It could gain four electrons forming C
4–
 anion. But it would be difficult
for the nucleus with six protons to hold on to ten electrons, that is,
four extra electrons.
(ii) It could lose four electrons forming C
4+
 cation. But it would require
a large amount of energy to remove four electrons leaving behind a
carbon cation with six protons in its nucleus holding on to just two
electrons.
Carbon overcomes this problem by sharing its valence electrons with
other atoms of carbon or with atoms of other elements. Not just carbon,
but many other elements form molecules by sharing electrons in this
manner. The shared electrons ‘belong’ to the outermost shells of both
the atoms and lead to both atoms attaining the noble gas configuration.
Before going on to compounds of carbon, let us look at some simple
molecules formed by the sharing of valence electrons.
The simplest molecule formed in this manner is that of hydrogen.
As you have learnt earlier, the atomic number of hydrogen is 1. Hence
hydrogen has one electron in its K shell and it requires one more electron
to fill the K shell. So two hydrogen atoms share their electrons to form a
molecule of hydrogen, H
2
. This allows each hydrogen atom to attain the
Table 4.1 Melting points and boiling points of some
compounds of carbon
Compound Melting Boiling
point (K) point (K)
Acetic acid (CH
3
COOH) 290 391
Chloroform (CHCl
3
) 209 334
Ethanol (CH
3
CH
2
OH) 156 351
Methane (CH
4
) 90 111
2024-25
Page 3


Science
58
Carbon and its
Compounds
4 CHAPTER
I
n the last Chapter, we came to know many compounds of importance
to us. In this Chapter we will study about some more interesting
compounds and their properties. Also, we shall be learning about carbon,
an element which is of immense significance to us in both its elemental
form and in the combined form.
Things made Things made Others
of metal of glass/clay
Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1
n Make a list of ten things you have used
or consumed since the morning.
n Compile this list with the lists made by
your classmates and then sort the items
into the adjacent Table.
n If there are items which are made up of
more than one material, put them into
both the relevant columns of the table.
Look at the items that come in the last column of the above table
filled by you – your teacher will be able to tell you that most of them are
made up of compounds of carbon. Can you think of a method to test
this? What would be the product if a compound containing carbon is
burnt? Do you know of any test to confirm this?
Food, clothes, medicines, books, or many of the things that you listed
are all based on this versatile element carbon. In addition, all living
structures are carbon based. The amount of carbon present in the earth’s
crust and in the atmosphere is quite meagre. The earth’s crust has only
0.02% carbon in the form of minerals (like carbonates, hydrogen-
carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon
dioxide. In spite of this small amount of carbon available in nature, the
importance of carbon seems to be immense. In this Chapter, we will know
about the properties of carbon which make carbon so important to us.
4.1 BONDING IN CARBON – THE COVALENT BOND
In the previous Chapter, we have studied the properties of ionic
compounds. We saw that ionic compounds have high melting and boiling
points and conduct electricity in solution or in the molten state. We also
2024-25
Carbon and its Compounds 59
saw how the nature of bonding in ionic compounds explains these
properties. Let us now study the properties of some carbon compounds.
Most carbon compounds are poor conductors of electricity as we
have seen in Chapter 2. From the data
given in Table 4.1 on the boiling and
melting points of the carbon compounds,
we find that these compounds have low
melting and boiling points as compared
to ionic compounds (Chapter 3). We can
conclude that the forces of attraction
between the molecules are not very
strong. Since these compounds are
largely non-conductors of electricity, we
can conclude that the bonding in these
compounds does not give rise to any ions.
In Class IX, we learnt about the
combining capacity of various elements and how it depends on the
number of valence electrons. Let us now look at the electronic
configuration of carbon. The atomic number of carbon is 6. What would
be the distribution of electrons in various shells of carbon? How many
valence electrons will carbon have?
We know that the reactivity of elements is explained as their tendency
to attain a completely filled outer shell, that is, attain noble gas
configuration. Elements forming ionic compounds achieve this by either
gaining or losing electrons from the outermost shell. In the case of carbon,
it has four electrons in its outermost shell and needs to gain or lose four
electrons to attain noble gas configuration. If it were to gain or lose
electrons –
(i) It could gain four electrons forming C
4–
 anion. But it would be difficult
for the nucleus with six protons to hold on to ten electrons, that is,
four extra electrons.
(ii) It could lose four electrons forming C
4+
 cation. But it would require
a large amount of energy to remove four electrons leaving behind a
carbon cation with six protons in its nucleus holding on to just two
electrons.
Carbon overcomes this problem by sharing its valence electrons with
other atoms of carbon or with atoms of other elements. Not just carbon,
but many other elements form molecules by sharing electrons in this
manner. The shared electrons ‘belong’ to the outermost shells of both
the atoms and lead to both atoms attaining the noble gas configuration.
Before going on to compounds of carbon, let us look at some simple
molecules formed by the sharing of valence electrons.
The simplest molecule formed in this manner is that of hydrogen.
As you have learnt earlier, the atomic number of hydrogen is 1. Hence
hydrogen has one electron in its K shell and it requires one more electron
to fill the K shell. So two hydrogen atoms share their electrons to form a
molecule of hydrogen, H
2
. This allows each hydrogen atom to attain the
Table 4.1 Melting points and boiling points of some
compounds of carbon
Compound Melting Boiling
point (K) point (K)
Acetic acid (CH
3
COOH) 290 391
Chloroform (CHCl
3
) 209 334
Ethanol (CH
3
CH
2
OH) 156 351
Methane (CH
4
) 90 111
2024-25
Science
60
Figure 4.4 Figure 4.4 Figure 4.4 Figure 4.4 Figure 4.4
Triple bond between
two nitrogen atoms
Figure 4.3 Figure 4.3 Figure 4.3 Figure 4.3 Figure 4.3
Double bond between
two oxygen atoms
electronic configuration of the nearest noble gas,
helium, which has two electrons in its K shell. We can
depict this using dots or crosses to represent valence
electrons (Fig. 4.1).
The shared pair of electrons is said to constitute a
single covalent bond between the two hydrogen atoms.
A single covalent bond is also represented by a line
between the two atoms, as shown in Fig. 4.2.
The atomic number of chlorine is 17. What would be its electronic
configuration and its valency? Chlorine forms a diatomic molecule, Cl
2
.
Can you draw the electron dot structure for this molecule? Note that
only the valence shell electrons need to be depicted.
In the case of oxygen, we see the formation of a double bond between
two oxygen atoms. This is because an atom of oxygen has six electrons
in its L shell (the atomic number of oxygen is eight) and it requires two
more electrons to complete its octet. So each atom of oxygen shares two
electrons with another atom of oxygen to give us the structure shown in
Fig. 4.3. The two electrons contributed by each oxygen atom give rise to
two shared pairs of electrons. This is said to constitute a double bond
between the two atoms.
Can you now depict a molecule of water showing the nature
of bonding between one oxygen atom and two hydrogen
atoms? Does the molecule have single bonds or double bonds?
What would happen in the case of a diatomic molecule of
nitrogen? Nitrogen has the atomic number 7. What would be
its electronic configuration and its combining capacity? In
order to attain an octet, each nitrogen atom in a molecule of
nitrogen contributes three electrons giving rise to three shared
pairs of electrons. This is said to constitute a triple bond
between the two atoms. The electron dot structure of N
2
 and
its triple bond can be depicted as in Fig. 4.4.
A molecule of ammonia has the formula NH
3
. Can you draw
the electron dot structure for this molecule showing how all
four atoms achieve noble gas configuration? Will the molecule
have single, double or triple bonds?
Let us now take a look at methane, which is a compound
of carbon. Methane is widely used as a fuel and is a major
component of bio-gas and Compressed Natural Gas (CNG). It
is also one of the simplest compounds formed by carbon.
Methane has a formula CH
4
. Hydrogen, as you know, has a
valency of 1. Carbon is tetravalent because it has four valence
electrons. In order to achieve noble gas configuration, carbon
shares these electrons with four atoms of hydrogen as shown
in Fig. 4.5.
Such bonds which are formed by the sharing of an electron pair
between two atoms are known as covalent bonds. Covalently bonded
molecules are seen to have strong bonds within the molecule, but inter-
molecular forces are weak. This gives rise to the low melting and boiling
Figure 4.1 Figure 4.1 Figure 4.1 Figure 4.1 Figure 4.1
A molecule of hydrogen
Figure 4.2 Figure 4.2 Figure 4.2 Figure 4.2 Figure 4.2
Single bond between
two hydrogen atoms
2024-25
Page 4


Science
58
Carbon and its
Compounds
4 CHAPTER
I
n the last Chapter, we came to know many compounds of importance
to us. In this Chapter we will study about some more interesting
compounds and their properties. Also, we shall be learning about carbon,
an element which is of immense significance to us in both its elemental
form and in the combined form.
Things made Things made Others
of metal of glass/clay
Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1
n Make a list of ten things you have used
or consumed since the morning.
n Compile this list with the lists made by
your classmates and then sort the items
into the adjacent Table.
n If there are items which are made up of
more than one material, put them into
both the relevant columns of the table.
Look at the items that come in the last column of the above table
filled by you – your teacher will be able to tell you that most of them are
made up of compounds of carbon. Can you think of a method to test
this? What would be the product if a compound containing carbon is
burnt? Do you know of any test to confirm this?
Food, clothes, medicines, books, or many of the things that you listed
are all based on this versatile element carbon. In addition, all living
structures are carbon based. The amount of carbon present in the earth’s
crust and in the atmosphere is quite meagre. The earth’s crust has only
0.02% carbon in the form of minerals (like carbonates, hydrogen-
carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon
dioxide. In spite of this small amount of carbon available in nature, the
importance of carbon seems to be immense. In this Chapter, we will know
about the properties of carbon which make carbon so important to us.
4.1 BONDING IN CARBON – THE COVALENT BOND
In the previous Chapter, we have studied the properties of ionic
compounds. We saw that ionic compounds have high melting and boiling
points and conduct electricity in solution or in the molten state. We also
2024-25
Carbon and its Compounds 59
saw how the nature of bonding in ionic compounds explains these
properties. Let us now study the properties of some carbon compounds.
Most carbon compounds are poor conductors of electricity as we
have seen in Chapter 2. From the data
given in Table 4.1 on the boiling and
melting points of the carbon compounds,
we find that these compounds have low
melting and boiling points as compared
to ionic compounds (Chapter 3). We can
conclude that the forces of attraction
between the molecules are not very
strong. Since these compounds are
largely non-conductors of electricity, we
can conclude that the bonding in these
compounds does not give rise to any ions.
In Class IX, we learnt about the
combining capacity of various elements and how it depends on the
number of valence electrons. Let us now look at the electronic
configuration of carbon. The atomic number of carbon is 6. What would
be the distribution of electrons in various shells of carbon? How many
valence electrons will carbon have?
We know that the reactivity of elements is explained as their tendency
to attain a completely filled outer shell, that is, attain noble gas
configuration. Elements forming ionic compounds achieve this by either
gaining or losing electrons from the outermost shell. In the case of carbon,
it has four electrons in its outermost shell and needs to gain or lose four
electrons to attain noble gas configuration. If it were to gain or lose
electrons –
(i) It could gain four electrons forming C
4–
 anion. But it would be difficult
for the nucleus with six protons to hold on to ten electrons, that is,
four extra electrons.
(ii) It could lose four electrons forming C
4+
 cation. But it would require
a large amount of energy to remove four electrons leaving behind a
carbon cation with six protons in its nucleus holding on to just two
electrons.
Carbon overcomes this problem by sharing its valence electrons with
other atoms of carbon or with atoms of other elements. Not just carbon,
but many other elements form molecules by sharing electrons in this
manner. The shared electrons ‘belong’ to the outermost shells of both
the atoms and lead to both atoms attaining the noble gas configuration.
Before going on to compounds of carbon, let us look at some simple
molecules formed by the sharing of valence electrons.
The simplest molecule formed in this manner is that of hydrogen.
As you have learnt earlier, the atomic number of hydrogen is 1. Hence
hydrogen has one electron in its K shell and it requires one more electron
to fill the K shell. So two hydrogen atoms share their electrons to form a
molecule of hydrogen, H
2
. This allows each hydrogen atom to attain the
Table 4.1 Melting points and boiling points of some
compounds of carbon
Compound Melting Boiling
point (K) point (K)
Acetic acid (CH
3
COOH) 290 391
Chloroform (CHCl
3
) 209 334
Ethanol (CH
3
CH
2
OH) 156 351
Methane (CH
4
) 90 111
2024-25
Science
60
Figure 4.4 Figure 4.4 Figure 4.4 Figure 4.4 Figure 4.4
Triple bond between
two nitrogen atoms
Figure 4.3 Figure 4.3 Figure 4.3 Figure 4.3 Figure 4.3
Double bond between
two oxygen atoms
electronic configuration of the nearest noble gas,
helium, which has two electrons in its K shell. We can
depict this using dots or crosses to represent valence
electrons (Fig. 4.1).
The shared pair of electrons is said to constitute a
single covalent bond between the two hydrogen atoms.
A single covalent bond is also represented by a line
between the two atoms, as shown in Fig. 4.2.
The atomic number of chlorine is 17. What would be its electronic
configuration and its valency? Chlorine forms a diatomic molecule, Cl
2
.
Can you draw the electron dot structure for this molecule? Note that
only the valence shell electrons need to be depicted.
In the case of oxygen, we see the formation of a double bond between
two oxygen atoms. This is because an atom of oxygen has six electrons
in its L shell (the atomic number of oxygen is eight) and it requires two
more electrons to complete its octet. So each atom of oxygen shares two
electrons with another atom of oxygen to give us the structure shown in
Fig. 4.3. The two electrons contributed by each oxygen atom give rise to
two shared pairs of electrons. This is said to constitute a double bond
between the two atoms.
Can you now depict a molecule of water showing the nature
of bonding between one oxygen atom and two hydrogen
atoms? Does the molecule have single bonds or double bonds?
What would happen in the case of a diatomic molecule of
nitrogen? Nitrogen has the atomic number 7. What would be
its electronic configuration and its combining capacity? In
order to attain an octet, each nitrogen atom in a molecule of
nitrogen contributes three electrons giving rise to three shared
pairs of electrons. This is said to constitute a triple bond
between the two atoms. The electron dot structure of N
2
 and
its triple bond can be depicted as in Fig. 4.4.
A molecule of ammonia has the formula NH
3
. Can you draw
the electron dot structure for this molecule showing how all
four atoms achieve noble gas configuration? Will the molecule
have single, double or triple bonds?
Let us now take a look at methane, which is a compound
of carbon. Methane is widely used as a fuel and is a major
component of bio-gas and Compressed Natural Gas (CNG). It
is also one of the simplest compounds formed by carbon.
Methane has a formula CH
4
. Hydrogen, as you know, has a
valency of 1. Carbon is tetravalent because it has four valence
electrons. In order to achieve noble gas configuration, carbon
shares these electrons with four atoms of hydrogen as shown
in Fig. 4.5.
Such bonds which are formed by the sharing of an electron pair
between two atoms are known as covalent bonds. Covalently bonded
molecules are seen to have strong bonds within the molecule, but inter-
molecular forces are weak. This gives rise to the low melting and boiling
Figure 4.1 Figure 4.1 Figure 4.1 Figure 4.1 Figure 4.1
A molecule of hydrogen
Figure 4.2 Figure 4.2 Figure 4.2 Figure 4.2 Figure 4.2
Single bond between
two hydrogen atoms
2024-25
Carbon and its Compounds 61
These two different structures result in diamond and graphite having very different
physical properties even though their chemical properties are the same. Diamond is
the hardest substance known while graphite is smooth and slippery. Graphite is also
a very good conductor of electricity unlike other non-metals that you studied in the
previous Chapter.
Diamonds can be synthesised by subjecting pure carbon to very high pressure and
temperature. These synthetic diamonds are small but are otherwise indistinguishable
from natural diamonds.
Fullerenes form another class of carbon allotropes. The first one to be identified was
C-60 which has carbon atoms arranged in the shape of a football. Since this looked
like the geodesic dome designed by the US architect Buckminster Fuller, the molecule
was named fullerene.
The structure of diamond
The structure of C-60
Buckminsterfullerene
QUESTIONS
?
1. What would be the electron dot structure of carbon dioxide which has
the formula CO
2
?
2. What would be the electron dot structure of a molecule of sulphur which
is made up of eight atoms of sulphur? (Hint – The eight atoms of sulphur
are joined together in the form of a ring.)
Figure 4.5 Figure 4.5 Figure 4.5 Figure 4.5 Figure 4.5
Electron dot structure for
methane
points of these compounds. Since the electrons are shared between
atoms and no charged particles are formed, such covalent compounds
are generally poor conductors of electricity.
Allotropes of carbon
The element carbon occurs in different forms in nature with
widely varying physical properties. Both diamond and
graphite are formed by carbon atoms, the difference lies in
the manner in which the carbon atoms are bonded to one
another. In diamond, each carbon atom is bonded to four
other carbon atoms forming a rigid three-dimensional
structure. In graphite, each carbon atom is bonded to three
other carbon atoms in the same plane giving a hexagonal array.
One of these bonds is a double-bond, and thus the valency of
carbon is satisfied. Graphite structure is formed by the
hexagonal arrays being placed in layers one above the other.
More to Know!
The structure of graphite
2024-25
Page 5


Science
58
Carbon and its
Compounds
4 CHAPTER
I
n the last Chapter, we came to know many compounds of importance
to us. In this Chapter we will study about some more interesting
compounds and their properties. Also, we shall be learning about carbon,
an element which is of immense significance to us in both its elemental
form and in the combined form.
Things made Things made Others
of metal of glass/clay
Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1 Activity 4.1
n Make a list of ten things you have used
or consumed since the morning.
n Compile this list with the lists made by
your classmates and then sort the items
into the adjacent Table.
n If there are items which are made up of
more than one material, put them into
both the relevant columns of the table.
Look at the items that come in the last column of the above table
filled by you – your teacher will be able to tell you that most of them are
made up of compounds of carbon. Can you think of a method to test
this? What would be the product if a compound containing carbon is
burnt? Do you know of any test to confirm this?
Food, clothes, medicines, books, or many of the things that you listed
are all based on this versatile element carbon. In addition, all living
structures are carbon based. The amount of carbon present in the earth’s
crust and in the atmosphere is quite meagre. The earth’s crust has only
0.02% carbon in the form of minerals (like carbonates, hydrogen-
carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon
dioxide. In spite of this small amount of carbon available in nature, the
importance of carbon seems to be immense. In this Chapter, we will know
about the properties of carbon which make carbon so important to us.
4.1 BONDING IN CARBON – THE COVALENT BOND
In the previous Chapter, we have studied the properties of ionic
compounds. We saw that ionic compounds have high melting and boiling
points and conduct electricity in solution or in the molten state. We also
2024-25
Carbon and its Compounds 59
saw how the nature of bonding in ionic compounds explains these
properties. Let us now study the properties of some carbon compounds.
Most carbon compounds are poor conductors of electricity as we
have seen in Chapter 2. From the data
given in Table 4.1 on the boiling and
melting points of the carbon compounds,
we find that these compounds have low
melting and boiling points as compared
to ionic compounds (Chapter 3). We can
conclude that the forces of attraction
between the molecules are not very
strong. Since these compounds are
largely non-conductors of electricity, we
can conclude that the bonding in these
compounds does not give rise to any ions.
In Class IX, we learnt about the
combining capacity of various elements and how it depends on the
number of valence electrons. Let us now look at the electronic
configuration of carbon. The atomic number of carbon is 6. What would
be the distribution of electrons in various shells of carbon? How many
valence electrons will carbon have?
We know that the reactivity of elements is explained as their tendency
to attain a completely filled outer shell, that is, attain noble gas
configuration. Elements forming ionic compounds achieve this by either
gaining or losing electrons from the outermost shell. In the case of carbon,
it has four electrons in its outermost shell and needs to gain or lose four
electrons to attain noble gas configuration. If it were to gain or lose
electrons –
(i) It could gain four electrons forming C
4–
 anion. But it would be difficult
for the nucleus with six protons to hold on to ten electrons, that is,
four extra electrons.
(ii) It could lose four electrons forming C
4+
 cation. But it would require
a large amount of energy to remove four electrons leaving behind a
carbon cation with six protons in its nucleus holding on to just two
electrons.
Carbon overcomes this problem by sharing its valence electrons with
other atoms of carbon or with atoms of other elements. Not just carbon,
but many other elements form molecules by sharing electrons in this
manner. The shared electrons ‘belong’ to the outermost shells of both
the atoms and lead to both atoms attaining the noble gas configuration.
Before going on to compounds of carbon, let us look at some simple
molecules formed by the sharing of valence electrons.
The simplest molecule formed in this manner is that of hydrogen.
As you have learnt earlier, the atomic number of hydrogen is 1. Hence
hydrogen has one electron in its K shell and it requires one more electron
to fill the K shell. So two hydrogen atoms share their electrons to form a
molecule of hydrogen, H
2
. This allows each hydrogen atom to attain the
Table 4.1 Melting points and boiling points of some
compounds of carbon
Compound Melting Boiling
point (K) point (K)
Acetic acid (CH
3
COOH) 290 391
Chloroform (CHCl
3
) 209 334
Ethanol (CH
3
CH
2
OH) 156 351
Methane (CH
4
) 90 111
2024-25
Science
60
Figure 4.4 Figure 4.4 Figure 4.4 Figure 4.4 Figure 4.4
Triple bond between
two nitrogen atoms
Figure 4.3 Figure 4.3 Figure 4.3 Figure 4.3 Figure 4.3
Double bond between
two oxygen atoms
electronic configuration of the nearest noble gas,
helium, which has two electrons in its K shell. We can
depict this using dots or crosses to represent valence
electrons (Fig. 4.1).
The shared pair of electrons is said to constitute a
single covalent bond between the two hydrogen atoms.
A single covalent bond is also represented by a line
between the two atoms, as shown in Fig. 4.2.
The atomic number of chlorine is 17. What would be its electronic
configuration and its valency? Chlorine forms a diatomic molecule, Cl
2
.
Can you draw the electron dot structure for this molecule? Note that
only the valence shell electrons need to be depicted.
In the case of oxygen, we see the formation of a double bond between
two oxygen atoms. This is because an atom of oxygen has six electrons
in its L shell (the atomic number of oxygen is eight) and it requires two
more electrons to complete its octet. So each atom of oxygen shares two
electrons with another atom of oxygen to give us the structure shown in
Fig. 4.3. The two electrons contributed by each oxygen atom give rise to
two shared pairs of electrons. This is said to constitute a double bond
between the two atoms.
Can you now depict a molecule of water showing the nature
of bonding between one oxygen atom and two hydrogen
atoms? Does the molecule have single bonds or double bonds?
What would happen in the case of a diatomic molecule of
nitrogen? Nitrogen has the atomic number 7. What would be
its electronic configuration and its combining capacity? In
order to attain an octet, each nitrogen atom in a molecule of
nitrogen contributes three electrons giving rise to three shared
pairs of electrons. This is said to constitute a triple bond
between the two atoms. The electron dot structure of N
2
 and
its triple bond can be depicted as in Fig. 4.4.
A molecule of ammonia has the formula NH
3
. Can you draw
the electron dot structure for this molecule showing how all
four atoms achieve noble gas configuration? Will the molecule
have single, double or triple bonds?
Let us now take a look at methane, which is a compound
of carbon. Methane is widely used as a fuel and is a major
component of bio-gas and Compressed Natural Gas (CNG). It
is also one of the simplest compounds formed by carbon.
Methane has a formula CH
4
. Hydrogen, as you know, has a
valency of 1. Carbon is tetravalent because it has four valence
electrons. In order to achieve noble gas configuration, carbon
shares these electrons with four atoms of hydrogen as shown
in Fig. 4.5.
Such bonds which are formed by the sharing of an electron pair
between two atoms are known as covalent bonds. Covalently bonded
molecules are seen to have strong bonds within the molecule, but inter-
molecular forces are weak. This gives rise to the low melting and boiling
Figure 4.1 Figure 4.1 Figure 4.1 Figure 4.1 Figure 4.1
A molecule of hydrogen
Figure 4.2 Figure 4.2 Figure 4.2 Figure 4.2 Figure 4.2
Single bond between
two hydrogen atoms
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Carbon and its Compounds 61
These two different structures result in diamond and graphite having very different
physical properties even though their chemical properties are the same. Diamond is
the hardest substance known while graphite is smooth and slippery. Graphite is also
a very good conductor of electricity unlike other non-metals that you studied in the
previous Chapter.
Diamonds can be synthesised by subjecting pure carbon to very high pressure and
temperature. These synthetic diamonds are small but are otherwise indistinguishable
from natural diamonds.
Fullerenes form another class of carbon allotropes. The first one to be identified was
C-60 which has carbon atoms arranged in the shape of a football. Since this looked
like the geodesic dome designed by the US architect Buckminster Fuller, the molecule
was named fullerene.
The structure of diamond
The structure of C-60
Buckminsterfullerene
QUESTIONS
?
1. What would be the electron dot structure of carbon dioxide which has
the formula CO
2
?
2. What would be the electron dot structure of a molecule of sulphur which
is made up of eight atoms of sulphur? (Hint – The eight atoms of sulphur
are joined together in the form of a ring.)
Figure 4.5 Figure 4.5 Figure 4.5 Figure 4.5 Figure 4.5
Electron dot structure for
methane
points of these compounds. Since the electrons are shared between
atoms and no charged particles are formed, such covalent compounds
are generally poor conductors of electricity.
Allotropes of carbon
The element carbon occurs in different forms in nature with
widely varying physical properties. Both diamond and
graphite are formed by carbon atoms, the difference lies in
the manner in which the carbon atoms are bonded to one
another. In diamond, each carbon atom is bonded to four
other carbon atoms forming a rigid three-dimensional
structure. In graphite, each carbon atom is bonded to three
other carbon atoms in the same plane giving a hexagonal array.
One of these bonds is a double-bond, and thus the valency of
carbon is satisfied. Graphite structure is formed by the
hexagonal arrays being placed in layers one above the other.
More to Know!
The structure of graphite
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62
4.2 VERSATILE NATURE OF CARBON
We have seen the formation of covalent bonds by the sharing of
electrons in various elements and compounds. We have also seen the
structure of a simple carbon compound, methane. In the beginning
of the Chapter, we saw how many things we use contain carbon. In
fact, we ourselves are made up of carbon compounds. The numbers
of carbon compounds whose formulae are known to chemists was
recently estimated to be in millions! This outnumbers by a large
margin the compounds formed by all the other elements put together.
Why is it that this property is seen in carbon and no other element?
The nature of the covalent bond enables carbon to form a large number
of compounds. Two factors noticed in the case of carbon are –
(i) Carbon has the unique ability to form bonds with other atoms of
carbon, giving rise to large molecules. This property is called
catenation. These compounds may have long chains of carbon,
branched chains of carbon or even carbon atoms arranged in rings.
In addition, carbon atoms may be linked by single, double or triple
bonds. Compounds of carbon, which are linked by only single
bonds between the carbon atoms are called saturated compounds.
Compounds of carbon having double or triple bonds between their
carbon atoms are called unsaturated compounds.
No other element exhibits the property of catenation to the extent
seen in carbon compounds. Silicon forms compounds with
hydrogen which have chains of upto seven or eight atoms, but these
compounds are very reactive. The carbon-carbon bond is very strong
and hence stable. This gives us the large number of compounds
with many carbon atoms linked to each other.
(ii) Since carbon has a valency of four, it is capable of bonding with
four other atoms of carbon or atoms of some other mono-valent
element. Compounds of carbon are formed with oxygen, hydrogen,
nitrogen, sulphur, chlorine and many other elements giving rise to
compounds with specific properties which depend on the elements
other than carbon present in the molecule.
Again the bonds that carbon forms with most other elements are
very strong making these compounds exceptionally stable. One
reason for the formation of strong bonds by carbon is its small size.
This enables the nucleus to hold on to the shared pairs of electrons
strongly. The bonds formed by elements having bigger atoms are
much weaker.
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