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
• 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.
• 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
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
♦ Hydrogen with electronegativity 2.20
♦ Chlorine with electronegativity 3.16
In the next section, we will learn about Valency.
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
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.
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:
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
■ 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
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)
Such compounds having partial electrical charge separation within the molecule are called polar compounds. HF, HBr, H2O are examples of polar compounds.
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.
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.
In the next section, we will learn about Valency.
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