In the previous section, we completed the discussion on ionic bond. In this section, we will learn about covalent bond.
But this type of bonding is not always possible. Consider the case of fluorine. It has a configuration 2,7. It has 7 electrons in the outer most shell. So it needs one more electron to attain octet. What if the atom available nearby is also fluorine? Both the atoms will be 'in need of an electron'. Neither one of them will be able to donate an electron. In such cases, sharing of electrons takes place. One pair of electrons is shared between two fluorine atoms. This is shown in the fig.3.11 below:
• Note that, one pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the atoms. So altogether, each atom will have 8 electrons in the outer most shell.
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two fluorine atoms will have to always stick together. Thus a bond is formed between the two fluorine atoms
The chemical bond formed as a result of the sharing of electrons between combining atoms is called covalent bond.
Since one pair of electrons is shared between two fluorine atoms, it is called a single bond. A single bond is represented by a small line between the symbols of the combining atoms. So the single bond between fluorine atoms can be represented as: F-F
Another example: Consider chlorine with atomic number 17. It has a configuration 2,8,7. It has 7 electrons in the outer most shell. So it is in need of an electron to attain octet. It is a case similar to fluorine: ‘need of a single electron’. If we have two chlorine atoms, neither one of them will be able to donate an electron. So the only way to attain octet is to form a covalent bond as shown in fig.3.12 below:
The details about this bond can be written as:
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the chlorine atoms. So altogether, each atom will have 8 electrons in the outer most shell.
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two chlorine atoms will have to always stick together. Thus a bond is formed between the two chlorine atoms
• Since one pair of electrons is shared between two chlorine atoms, it is a single bond. It is represented as: Cl-Cl
Another example: Consider oxygen with atomic number 8. It has a configuration 2,6. It has 6 electrons in the outer most shell. So it is in need of two electrons to attain octet. If we have two oxygen atoms, neither one of them will be able to donate two electrons. So the only way to attain octet is to form a covalent bond as shown in fig.3.13 below:
The details about this bond can be written as:
• Two pairs (that is., four electrons) are shared. The four electrons in the pairs, belongs to both the oxygen atoms. So altogether, each atom will have 8 electrons in the outer most shell.
• As the pairs belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two oxygen atoms will have to always stick together. Thus a bond is formed between the two oxygen atoms
• Since two pairs of electrons are shared between two oxygen atoms, it is called a double bond. A double bond is represented by two small lines between the symbols of the combining atoms. So the double bond between oxygen atoms can be represented as: O=O
Another example: Consider nitrogen with atomic number 7. It has a configuration 2,5. It has 5 electrons in the outer most shell. So it is in need of three electrons to attain octet. If we have two nitrogen atoms, neither one of them will be able to donate three electrons. So the only way to attain octet is to form a covalent bond as shown in fig.3.14 below:
The details about this bond can be written as:
• Three pairs (that is., six electrons) are shared. The six electrons in the pairs, belongs to both the nitrogen atoms. So altogether, each atom will have 8 electrons in the outer most shell.
• As the pairs belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two nitrogen atoms will have to always stick together. Thus a bond is formed between the two nitrogen atoms
• Since three pairs of electrons are shared between two nitrogen atoms, it is called a triple bond. A triple bond is represented by three small lines between the symbols of the combining atoms. So the triple bond between nitrogen atoms can be represented as: N≡N
So we have seen that a covalent bond may be a single, double or triple bond. It is related to the ‘number of pairs’ of electrons shared. This will be more clear from the following table:
The covalent bonds that we have seen so far, are formed between atoms of the same element. In the above cases, two atoms of the same element combined to form a molecule. Thus, two individual atoms, who would be unstable by themselves, attained stability by forming a covalent bond between them. It may be noted that, since two atoms are present, it is a diatomic molecule. We have see the basics about mono, di and poly atomic molecules here.
• Chlorine has an atomic number 17. It’s electronic configuration is 2,8,7. It has 7 electrons in the outer most shell. It also needs one more electron to attain octet.
• We see that both hydrogen and chlorine are in need for a single electron. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.15 below:
The details about this bond can be written as:
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the hydrogen atom and the chlorine atom.
• The hydrogen atom now has the required two electrons for octet.
• The chlorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the hydrogen and chlorine atoms will have to always stick together. Thus a bond is formed between the two atoms
• Since one pair of electrons is shared, it is a single bond. It is represented as: H-Cl
Another example: Consider the formation of carbontetrachloride
• Carbon has an atomic number 6. It’s electronic configuration is 2,4. It has 4 electrons in the outer most shell. It needs four more electrons to attain octet.
• Chlorine has an atomic number 17. It’s electronic configuration is 2,8,7. It has 7 electrons in the outer most shell. It needs one more electron to attain octet.
• We see that both carbon and chlorine are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.16 below:
• The details about this bond can be written as:
• One carbon atom combines with 4 chlorine atoms. Each of the four chlorine atoms share one pair of electrons with a carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the chlorine atom.
• The carbon atom now has the required 8 electrons for octet.
• The chlorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the carbon and chlorine atoms will have to always stick together. Thus a bond is formed between the five atoms
We will now see some solved examples
• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• We see that both carbon and hydrogen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.17 below:
• The details about this bond can be written as:
• One carbon atom combines with 4 hydrogen atoms. Each of the four hydrogen atoms share one pair of electrons with a carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the hydrogen atom.
• The carbon atom now has the required 4 electrons for octet.
• The hydrogen atom now has the required 2 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the carbon and hydrogen atoms will have to always stick together. Thus a bond is formed between the five atoms
• (ii) HF: Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• Fluorine has an atomic number 9. It’s electronic configuration is 2,7. It has 7 electrons in the outer most shell. It also needs one more electron to attain octet.
• We see that both hydrogen and fluorine are in need for a single electron. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.18 below:
The details about this bond can be written as:
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the hydrogen atom and the fluorine atom.
• The hydrogen atom now has the required two electrons for octet.
• The fluorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the hydrogen and fluorine atoms will have to always stick together. Thus a bond is formed between the two atoms
• Since one pair of electrons is shared, it is a single bond. It is represented as: H-F
• (iii) H2O: Oxygen has an atomic number 8. It’s electronic configuration is 2,6. It has 6 electrons in the outer most shell. It needs two more electrons to attain octet.
• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• We see that both oxygen and hydrogen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.19 below:
• The details about this bond can be written as:
• One oxygen atom combines with 2 hydrogen atoms. Each of the 2 hydrogen atoms share one pair of electrons with a oxygen atom
• In total, two pairs (that is., four electrons) are shared. The four electrons in the pairs, belongs to both the oxygen atom and the hydrogen atom.
• The oxygen atom now has the required 8 electrons for octet.
• The hydrogen atom now has the required 2 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the oxygen and hydrogen atoms will have to always stick together. Thus a bond is formed between the three atoms.
In the next section, we will learn about electronegativity.
Covalent bonding
We have seen that the atoms attain octet by donating or accepting electrons. When the donation and acceptance takes place, cations and anions are formed. These ions will then be held together by the electrostatic force of attraction. And thus a molecule is formed. We also learned that, the above type of bonding between atoms is called Ionic bond.But this type of bonding is not always possible. Consider the case of fluorine. It has a configuration 2,7. It has 7 electrons in the outer most shell. So it needs one more electron to attain octet. What if the atom available nearby is also fluorine? Both the atoms will be 'in need of an electron'. Neither one of them will be able to donate an electron. In such cases, sharing of electrons takes place. One pair of electrons is shared between two fluorine atoms. This is shown in the fig.3.11 below:
 |
| Fig.3.11 |
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two fluorine atoms will have to always stick together. Thus a bond is formed between the two fluorine atoms
The chemical bond formed as a result of the sharing of electrons between combining atoms is called covalent bond.
Since one pair of electrons is shared between two fluorine atoms, it is called a single bond. A single bond is represented by a small line between the symbols of the combining atoms. So the single bond between fluorine atoms can be represented as: F-F
Another example: Consider chlorine with atomic number 17. It has a configuration 2,8,7. It has 7 electrons in the outer most shell. So it is in need of an electron to attain octet. It is a case similar to fluorine: ‘need of a single electron’. If we have two chlorine atoms, neither one of them will be able to donate an electron. So the only way to attain octet is to form a covalent bond as shown in fig.3.12 below:
 |
| Fig.3.12 |
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the chlorine atoms. So altogether, each atom will have 8 electrons in the outer most shell.
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two chlorine atoms will have to always stick together. Thus a bond is formed between the two chlorine atoms
• Since one pair of electrons is shared between two chlorine atoms, it is a single bond. It is represented as: Cl-Cl
Another example: Consider oxygen with atomic number 8. It has a configuration 2,6. It has 6 electrons in the outer most shell. So it is in need of two electrons to attain octet. If we have two oxygen atoms, neither one of them will be able to donate two electrons. So the only way to attain octet is to form a covalent bond as shown in fig.3.13 below:
 |
| Fig.3.13 |
• Two pairs (that is., four electrons) are shared. The four electrons in the pairs, belongs to both the oxygen atoms. So altogether, each atom will have 8 electrons in the outer most shell.
• As the pairs belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two oxygen atoms will have to always stick together. Thus a bond is formed between the two oxygen atoms
• Since two pairs of electrons are shared between two oxygen atoms, it is called a double bond. A double bond is represented by two small lines between the symbols of the combining atoms. So the double bond between oxygen atoms can be represented as: O=O
Another example: Consider nitrogen with atomic number 7. It has a configuration 2,5. It has 5 electrons in the outer most shell. So it is in need of three electrons to attain octet. If we have two nitrogen atoms, neither one of them will be able to donate three electrons. So the only way to attain octet is to form a covalent bond as shown in fig.3.14 below:
 |
| Fig.3.14 |
• Three pairs (that is., six electrons) are shared. The six electrons in the pairs, belongs to both the nitrogen atoms. So altogether, each atom will have 8 electrons in the outer most shell.
• As the pairs belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two nitrogen atoms will have to always stick together. Thus a bond is formed between the two nitrogen atoms
• Since three pairs of electrons are shared between two nitrogen atoms, it is called a triple bond. A triple bond is represented by three small lines between the symbols of the combining atoms. So the triple bond between nitrogen atoms can be represented as: N≡N
So we have seen that a covalent bond may be a single, double or triple bond. It is related to the ‘number of pairs’ of electrons shared. This will be more clear from the following table:
The covalent bonds that we have seen so far, are formed between atoms of the same element. In the above cases, two atoms of the same element combined to form a molecule. Thus, two individual atoms, who would be unstable by themselves, attained stability by forming a covalent bond between them. It may be noted that, since two atoms are present, it is a diatomic molecule. We have see the basics about mono, di and poly atomic molecules here.
We will now see a molecule formed from two different atoms. Consider the formation of hydrogen chloride.
• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet. • Chlorine has an atomic number 17. It’s electronic configuration is 2,8,7. It has 7 electrons in the outer most shell. It also needs one more electron to attain octet.
• We see that both hydrogen and chlorine are in need for a single electron. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.15 below:
 |
| Fig.3.15 |
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the hydrogen atom and the chlorine atom.
• The hydrogen atom now has the required two electrons for octet.
• The chlorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the hydrogen and chlorine atoms will have to always stick together. Thus a bond is formed between the two atoms
• Since one pair of electrons is shared, it is a single bond. It is represented as: H-Cl
Another example: Consider the formation of carbontetrachloride
• Carbon has an atomic number 6. It’s electronic configuration is 2,4. It has 4 electrons in the outer most shell. It needs four more electrons to attain octet.
• Chlorine has an atomic number 17. It’s electronic configuration is 2,8,7. It has 7 electrons in the outer most shell. It needs one more electron to attain octet.
• We see that both carbon and chlorine are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.16 below:
 |
| Fig.3.16 |
• One carbon atom combines with 4 chlorine atoms. Each of the four chlorine atoms share one pair of electrons with a carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the chlorine atom.
• The carbon atom now has the required 8 electrons for octet.
• The chlorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the carbon and chlorine atoms will have to always stick together. Thus a bond is formed between the five atoms
Solved example 3.4
Illustrate the chemical bond in the following covalent compounds using electron dot diagram.
(i) CH4, (ii) HF, (iii) H2O
Solution:
• (i) CH4: Carbon has an atomic number 6. It’s electronic configuration is 2,4. It has 4 electrons in the outer most shell. It needs four more electrons to attain octet. • Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• We see that both carbon and hydrogen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.17 below:
 |
| Fig.3.17 |
• One carbon atom combines with 4 hydrogen atoms. Each of the four hydrogen atoms share one pair of electrons with a carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the hydrogen atom.
• The carbon atom now has the required 4 electrons for octet.
• The hydrogen atom now has the required 2 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the carbon and hydrogen atoms will have to always stick together. Thus a bond is formed between the five atoms
• (ii) HF: Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• Fluorine has an atomic number 9. It’s electronic configuration is 2,7. It has 7 electrons in the outer most shell. It also needs one more electron to attain octet.
• We see that both hydrogen and fluorine are in need for a single electron. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.18 below:
 |
| Fig.3.18 |
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the hydrogen atom and the fluorine atom.
• The hydrogen atom now has the required two electrons for octet.
• The fluorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the hydrogen and fluorine atoms will have to always stick together. Thus a bond is formed between the two atoms
• Since one pair of electrons is shared, it is a single bond. It is represented as: H-F
• (iii) H2O: Oxygen has an atomic number 8. It’s electronic configuration is 2,6. It has 6 electrons in the outer most shell. It needs two more electrons to attain octet.
• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• We see that both oxygen and hydrogen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.19 below:
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| Fig.3.19 |
• One oxygen atom combines with 2 hydrogen atoms. Each of the 2 hydrogen atoms share one pair of electrons with a oxygen atom
• In total, two pairs (that is., four electrons) are shared. The four electrons in the pairs, belongs to both the oxygen atom and the hydrogen atom.
• The oxygen atom now has the required 8 electrons for octet.
• The hydrogen atom now has the required 2 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the oxygen and hydrogen atoms will have to always stick together. Thus a bond is formed between the three atoms.
In the next section, we will learn about electronegativity.
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