Chapter 3.4 - Valency of Elements

In the previous section, we completed the discussion on Electronegativity and Polar nature. In this section, we will learn about Valency.

Valency

We have seen that elements enter into chemical reactions to attain stability. During chemical reactions, one of the following two may take place:
■ Transfer of electrons (some atoms donate electrons, and some others accept electrons) resulting in ionic bond
■ Sharing of electrons (resulting in covalent bond)

Whether it is transfer or sharing, each atom have a definite 'combining capacity'. This capacity is measured in terms of valency. Valency is defined as the 'number of electrons' lost, gained or shared by an atom during chemical combination.

Based on the above definition, it is easy to calculate the valency of any given element. 
■ Let us take the example of chlorine.
• In NaCl, the chlorine enters into an ionic bonding with sodium. We have seen it's details here. It is shown again below:

• Chlorine has an electronic configuration 2,8,7. So it needs one more electron to attain octet.
• The chlorine atom accepts one electron from sodium
• One electron is accepted. So the valency of chlorine is 1

Another case of chlorine: • In HCl, the same chlorine enters into a covalent bonding with hydrogen. We have seen it's details here. It is shown again below:

• We know that chlorine needs one more electron to attain octet
• It shares one pair of electron with hydrogen
• The two electrons in the shared pair, belongs to both hydrogen and chlorine
• Thus chlorine gets 8 electrons in it's outer most shell
• One pair of electrons is shared. So valency of chlorine is 1

One more case of chlorine: • In CCl₄, the same chlorine enters into a covalent bonding with carbon.We have seen it's details here. It is shown again below:

• We know that chlorine needs one more electron to attain octet
• Even though there are 4 chlorine atoms, from the point of view of chlorine, each of the chlorine atom shares one pair of electron with carbon
• The two electrons in the shared pair, belongs to both carbon and chlorine
• Thus each chlorine atom gets 8 electrons in it's outer most shell
• One pair of electrons is shared in each bond. So valency of chlorine is 1 

In all the above three cases, we find that the valency of chlorine is 1, whether it is ionic bonding or covalent bonding. Based on this discussion, we get an easy method to determine the valency of chlorine: We just subtract the number of electrons in the outer most shell of chlorine from 8. That is:
■ Valency of chlorine = 8 – Number of electrons in it's outer most shell 

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■ Now we will consider the example of oxygen

• In MgO, the oxygen enters into an ionic bonding with magnesium. We have seen it's details here. It is shown again below: 

• Oxygen has an electronic configuration 2,6. So it needs two more electrons to attain octet.
• The oxygen atom accepts two electrons from magnesium
• Two electrons are accepted. So the valency of oxygen is 2

Another case of oxygen:
• In Na₂O, the same oxygen enters into an ionic bonding with sodium atoms. We have seen it's details here. It is shown again below:

• We know that oxygen needs two more electrons to attain octet 
• The oxygen atom accepts two electrons from sodium
• Two electrons are accepted. So the valency of oxygen is 2

One more case of oxygen:
• In H₂O, the same oxygen enters into a covalent bonding with hydrogen. We have seen it's details here. It is shown again below:

• We know that oxygen needs two more electrons to attain octet 
• Even though there are 2 hydrogen atoms, each of the hydrogen atom shares one pair of electron with oxygen
• From the point of view of oxygen, it has to share two pairs of electrons to attain octet 
• Two pair of electrons is shared. So valency of oxygen is 2

In all the above three cases, we find that the valency of oxygen is 2, whether it is ionic bonding or covalent bonding. Based on this discussion, we get an easy method to determine the valency of oxygen: We just subtract the number of electrons in the outer most shell of oxygen from 8. That is:
■ Valency of oxygen = 8 – Number of electrons in it's outer most shell

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■ Now we will consider the example of Sodium

• In NaCl, the sodium enters into an ionic bonding with chlorine. We have seen it's details here.

• sodium has an electronic configuration 2,8,1. So it needs to lose one electron to attain octet.
• The chlorine atom accepts one electron from sodium
• One electron is lost. So the valency of sodium is 1

Another case of sodium:
• In Na₂O, the same sodium enters into an ionic bonding with oxygen atoms. We have seen it's details here. It is shown again below:

• We know that sodium needs to lose one electron to attain octet
• Two sodium atoms lose 1 atom each 
• The oxygen atom accepts two electrons from sodium
• From the point of view of sodium, each of it's atom lose one electron
• One electron is lost. So the valency of sodium is 1 

In both the above cases, we find that the valency of sodium is 1. Based on this discussion, we get an easy method to determine the valency of sodium: We just put the valency equal to the number of electrons in the outer most shell. That is:
■ Valency of sodium = Number of electrons in it's outer most shell

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We just saw the examples of chlorine, oxygen and sodium. We can try any element we like. In all of them, we will get the same result:
• Each element will have the same valency in all reactions. Valency of an element does not depend on the other element or elements with which it enters into reaction
• Each element will have the same valency whether it enters into an ionic bond or a covalent bond
• Valency of any element is related to the number of electrons in it's outer most shell
    ♦ If this number is greater than 4, we subtract it from 8
    ♦ If this number is 4, or less than 4, there is no need for subtraction. The number itself is the valency
Based on the above, we can write:
■ No. of electrons in the outermost shell of an element = 1
• Valency of that element = 1  
■ No. of electrons in the outermost shell of an element = 2
• Valency of that element = 2
■ No. of electrons in the outermost shell of an element = 3
• Valency of that element = 3
■ No. of electrons in the outermost shell of an element = 4
• Valency of that element = 4
■ No. of electrons in the outermost shell of an element = 5
• Valency of that element = 8 - 5 = 3
■ No. of electrons in the outermost shell of an element = 6
• Valency of that element = 8 - 6 = 2
■ No. of electrons in the outermost shell of an element = 7
• Valency of that element = 8 - 7 = 1

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So now we are in a position to determine the valency of any given element. In the next section, we will see an application of this valency.

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Chapter 3.3 - Electronegativity and Polar compounds

In the previous section, we completed the discussion on covalent bond.  In this section, we will learn about Electronegativity.

We have seen that in covalent bonding, pairs of electrons are shared by atoms. The atoms apply an attractive force towards the shared electrons. That is:
■ Each atom in the molecule will try to pull the shared pairs of electrons towards it
• If the molecule has all the atoms same (that is., molecule of an element), the attraction will be equal. 
• Let us see an example: 
    ♦ Consider F₂ molecule. Both the atoms in that molecule are same: that of fluorine
    ♦ So the 'magnitude of the attractive force' on the shared electrons will be equal
    ♦ So the shared electrons will remain exactly midway between the two fluorine atoms
If the molecules has different atoms (that is., molecule of a compound), the attraction will not be equal. 
• Let us consider an example:
    ♦ Consider HCl molecule. It has one atom of hydrogen, and one atom of chlorine. 
    ♦ Out of these two, the chlorine atom has a greater capacity to exert attractive force on the shared electrons
    ♦ So the shared electrons will not remain exactly at midmay between the hydrogen and chlorine atoms. 
    ♦ They will move towards the chlorine atom

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So we want to know the following:

• Which atoms have greater capacity to attract electrons?
• Which atoms have lesser capacity to attract electrons?
■ It is essential to have such a knowledge because, only then will we be able to tell the position of the shared electrons. 
■ The shared electrons will be located near that atom which has the greater attractive capacity, and we want to be able to tell which.

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We can find the attractive capacity of the elements by doing various experiments. Consider the following analogy: We have a large number of magnets in hand. The capacity of each is different. We can find the capacity of each by doing various experiments on them. 

In the same way, the capacity of each of the elements can also be determined. But we do not have to do any experiments. The required experiments have been already done by scientists, and the capacity of each element is now known. The values given by American scientist Linus Pauling is widely accepted. We will also follow it. It is known as Pauling's electronegativity scale. It is in the form of a chart as shown here. A few readings from the chart are given below:

• Electronegativity of fluorine = 3.98
• Electronegativity of manganese = 1.55
• Electronegativity of hydrogen = 2.20  

• Among the above three elements, fluorine has the highest electronegativity. In fact, fluorine has the highest electronegativity among all the elements in the chart
• Manganese has the least electronegativity among the above three
• From the values, we can infer that, in a compound formed between hydrogen and fluorine, the shared electrons will be located nearer to fluorine. This is because, it's electronegativity 3.98 is greater than the electronegativity 2.20 of hydrogen

The application of the chart will become more clear from the following table: 

We will now see each of the compound in the above table in detail:
■ Consider carbon monoxide (CO):
• It's constituent elements are: 
    ♦ Carbon with electronegativity 2.55
    ♦ Oxygen with electronegativity 3.44
• CO is a covalent compound. The shared pairs of electrons will be located nearer to Oxygen
• Difference in electronegativity values = 3.44 - 2.55 = 0.89
■ Consider sodium chloride (NaCl):
• It's constituent elements are: 
    ♦ Sodium with electronegativity 0.93
    ♦ Chlorine with electronegativity 3.16
• NaCl is an ionic compound
• Difference in electronegativity values = 3.16 - 0.93 = 2.23
■ Consider methane (CH₄):
• It's constituent elements are: 
    ♦ Carbon with electronegativity 2.55
    ♦ Hydrogen with electronegativity 2.2
• CH₄ is a covalent compound. The shared pairs of electrons will be located nearer to carbon
• Difference in electronegativity values = 2.55 - 2.2 = 0.35
■ Consider magnesium chloride (MgCl₂):
• It's constituent elements are: 
    ♦ Magnesium with electronegativity 1.31
    ♦ Chlorine with electronegativity 3.16
• MgCl₂ is an ionic compound
• Difference in electronegativity values = 3.16 - 1.31 = 1.85
■ Consider sodium oxide (Na₂O):
• It's constituent elements are: 
    ♦ Sodium with electronegativity 0.93
    ♦ Oxygen with electronegativity 3.44
• Na₂O is an ionic compound
• Difference in electronegativity values = 3.44 - 0.93 = 2.51 

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In the above table, some compounds are ionic, while others are covalent. 
■ If the difference in elecronegativity values of elements in a compound is 1.7, or greater than 1.7, the compound generally shows ionic character. 
■ If the difference is less than 1.7, the compound generally shows covalent character 

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Polar nature

We have seen that, in covalent compounds, the shared pairs of electrons will be located nearer to the atom which has greater electronegativity. Consider the case of Hydrogen chloride molecule (HCl).

• It's constituent elements are: 
    ♦ Hydrogen with electronegativity 2.20
    ♦ Chlorine with electronegativity 3.16

• HCl is a covalent compound. The shared pairs of electrons will be located nearer to the nucleus of chlorine

■ As a result, a small negative charge will develop in the chlorine atom

• This small negative charge is denoted as δ^- (read as Delta negative)

■ A small positive charge will develop in the hydrogen atom

• This small positive charge is denoted as δ⁺ (read as Delta positive)

■ The whole HCl molecule can be represented as:

Such compounds having partial electrical charge separation within the molecule are called polar compounds. HF, HBr, H₂O are examples of polar compounds.

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We will now see a solved example.
Solved example 3.5
Electronegativity values of some elements are given below:
Ca = 1.0, O = 3.5, C = 2.5, S = 2.58, H = 2.2, F = 3.98
Determine whether the following compounds are ionic or covalent:
(i) Sulphur dioxide (SO₂), (ii) Water (H₂O), (iii) Calcium fluoride (CaF₂), (iv) Carbon dioxide (CO₂)
Solution:
(i) Consider SO₂:
• It's constituent elements are: 
    ♦ Sulphur with electronegativity 2.58
    ♦ Oxygen with electronegativity 3.5
• Difference in electronegativity values = 3.5 - 2.58 = 0.92
• Difference is less than 1.7. So SO₂ is a covalent compound
(ii) Consider H₂O:
• It's constituent elements are: 
    ♦ Hydrogen with electronegativity 2.2
    ♦ Oxygen with electronegativity 3.5
• Difference in electronegativity values = 3.5 - 2.2 = 1.3
• Difference is less than 1.7. So H₂O is a covalent compound
(iii) Consider CaF₂:
• It's constituent elements are: 
    ♦ Calcium with electronegativity 1
    ♦ Fluorine with electronegativity 3.98
• Difference in electronegativity values = 3.98 - 1 = 2.98
• Difference is greater than 1.7. So CaF₂ is an ionic compound
(iv) Consider CO₂:
• It's constituent elements are: 
    ♦ Carbon with electronegativity 2.5
    ♦ Oxygen with electronegativity 3.5
• Difference in electronegativity values = 3.5 - 2.5 = 1.0
• Difference is less than 1.7. So CO₂ is a covalent compound.

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In the next section, we will learn about Valency.

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Chapter 3.2 - Covalent bond

In the previous section, we completed the discussion on ionic bond.  In this section, we will learn about covalent bond.

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

• 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

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The chemical bond formed as a result of the sharing of electrons between combining atoms is called covalent bond.

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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

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:

Fig.3.13

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:

Fig.3.14

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

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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:

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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.

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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

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:

Fig.3.16

• 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 

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We will now see some solved examples

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

• 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:

Fig.3.18

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:

Fig.3.19

• 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.

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In the next section, we will learn about electronegativity.

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