In the previous section, we completed the discussion on Electronegativity and Polar nature. In this section, we will learn about Valency.
■ 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
• 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
■ Now we will consider the example of Sodium
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
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.
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:
• 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:
• 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:
• 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
■ 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: 
• 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:
• 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
• 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
• 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
• 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
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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