Electronegativity - Features, Scales, Applications, Practice Problems and FAQ

Consider a sumo wrestler versus a frail young man in a tug of war match.

The game's outcome is self-evident. Because the sumo wrestler is stronger, he can simply drag the weaker man towards himself. 

Similarly, the electronegativity of an element is the tendency of an atom to attract the shared pair of electrons more towards itself in a molecule. So, the sumo wrestler is like a more electronegative atom capable of pulling the shared electron (the rope for this case).

TABLE OF CONTENTS

• What is Electronegativity?
• Key Features of Electronegativity
• Different Scales of Electronegativity
• Applications of Electronegativity
• Practice problems
• Frequently Asked Questions-FAQs

What is Electronegativity?

Electronegativity is the tendency of an atom to attract the shared pair of electrons more towards itself in a covalently bonded molecule.

For a non-polar covalent bond, i.e., the bond between two identical atoms, the bonded pair of electrons is shared equally by the two atoms. 

For example: In HCl, the electron pair is attracted more towards chlorine that makes Cl attain a partial negative charge (represented by ) and H, a partial positive charge (represented by ).

From the given image above we can see that the electron cloud is distributed symmetrically in the case of, while there is an asymmetrical distribution of electron cloud in case of .

Key Feature of Electronegativity

• In the case of a polar covalent bond, i.e., the bond between two different atoms, the bonded pair of electrons is not shared equally by the two atoms. 
• The atom that has a greater tendency to attract the shared pair of electrons towards itself acquires a partial negative charge, while the other atom acquires a partial positive charge. 
• This relative tendency of an atom to attract the shared pair of electrons towards itself in a covalently bonded molecule was termed the electronegativity of the atom by Pauling. 
• Unlike ionisation energy and the electron gain enthalpy, which have fixed values for an element, the electronegativity values are not constant. They change depending on the element to which it is bound.

Different Scales of Electronegativity

Though electronegativity is not a measurable property, a number of numerical scales of electronegativity of elements have been developed, to understand it in a better way. It is similar to the latitudes and longitudes present on the globe for understanding. However, in reality, these are not tangible lines. The different scales of electronegativity are therefore devised as follows: 

Pauling Scale of Electronegativity

Pauling scale is the most commonly used scale for electronegativity. American scientist Linus Pauling (1922) arbitrarily assigned a value of 4.0 to Fluorine, the element considered to have the highest electronegativity.

He concluded that the bonds formed between the two atoms A and B must be stronger than the average of single bond energies of and molecules. According to him, the electronegativity difference between two atoms and is as follows:

= → Eq. 1

Where, Δ = Actual bond energy - Energy for 100% covalent bond energy. 

It also represents the polarity of the bond. According to Pauling, 100% covalent bond energy can be given by the geometric mean of the covalent bond energies of and molecules, i.e.,

i.e., → Eq. 2

Substituting Eq. 2 in Eq.1,

Where, is the bond energy of bond ()

is the bond energy of bond ()

is the bond energy of bond ()

is the electronegativity of and is the electronegativity of

Mulliken Scale of Electronegativity

According to the Mulliken scale, the electronegativity 𝜒 (chi) is taken as the arithmetic mean of the ionisation energy (I.E.) and the electron affinity (E.A.) of an atom (both expressed in )

Mulliken’s values are 2.8 times greater than Pauling’s values.

Where, = Electronegativity in Pauling scale and = Electronegativity in Mulliken scale

Higher the ionisation energy and the electron affinity of an atom, the higher will be its electronegativity. This scale fails in the case of noble gases, which have higher IE values and generally lower electronegativity values.

Alfred-Rochow Scale of Electronegativity

Allred-Rochow electronegativity is a measure that determines the values of the electrostatic force exerted by the effective nuclear charge on the valence electrons. 

In his table of values also, the electronegativities increase from left to right just like Pauling's scale because the Z is increasing. As we go down the group, it decreases because of the larger atomic size that increases the distance between the electrons and nucleus.

They came up with an equation: 

Application of Electronegativity

There are multiple applications of electronegativity.

• It helps in predicting the nature of the bond between two atoms, i.e., the percentage of ionic character in a covalent bond. 

According to Hannay and Smith's equation:  

Where Δ𝜒 = Difference in the electronegativity between two atoms.

If Δ𝜒 = 1.7 %, then Ionic character = 50%

If Δ𝜒 > 1.7, Predominantly ionic bond.

If Δ𝜒 < 1.7, Predominantly covalent bond.

• Values of electronegativity can be used to predict the acidic and basic properties of metal oxides.

() difference predicts the nature of the oxides, formed by the element A. 

is the electronegativity of oxygen and is the electronegativity of element A. 

As the difference in electronegativity of oxygen and the element in oxide increases, the basic strength of oxides of different elements increases.

Nature of Oxides:

Oxides of metals are generally basic and non-metallic oxides are acidic. Amphoteric oxides (,) can react with both acids and bases. Neutral oxides (, ) do not react with acids and bases.

As we move along the period, the non-metallic character increases due to the increase in the ionisation energy, and the metallic character decreases. Hence, across the period from left to right, the basic character of oxides decreases and the acidic character increases.

As we move down a group, the metallic character increases and the non-metallic character decreases, and hence the basic character of oxides increases and the acidic character decreases.

• Electronegativity can also predict the acidic or basic nature of the molecule. molecules can act as both base and acid. When the electronegativity difference of bond () is greater than the electronegativity difference of ( ) bond, then  bond will be preferentially dissociated in water to give and hence it will act as a base.

For example, acts as a base. 

• When the electronegativity difference of bond () is smaller than the electronegativity difference of ( ) bond, then  bond will be preferentially dissociated in water to give and hence it will act as an acid.

For example acts as an acid.

• In general, the higher the electronegativity difference, the stronger and shorter will be the covalent bond. As the electronegativity difference increases between the two covalently bonded atoms, the partial positive and partial negative charge increases. So, the electrostatic force of attraction increases, and hence the bond length decreases and the bond strength increases. 

• Electronegativity and non-metallic nature: Non-metallic elements have a strong tendency to gain electrons. Therefore, electronegativity is directly related to the non-metallic character of the elements. More is the electronegativity, more is the non-metallic nature.

Practice problems

Q 1. Which of the following statement is incorrect? 

a. An element that has a high electronegativity always has a low electron gain enthalpy.
b. The electron gain enthalpy is the property of an isolated atom.
c. Electronegativity is the property of the bonded atoms.
d. Both B and C

Answer: According to Mulliken,

Higher the ionisation energy and the electron affinity of an atom, the higher the electronegativity. So, the statement given in option A) is incorrect.

Electron gain enthalpy is the energy change associated when a neutral gaseous isolated atom accepts an electron. Hence it is the property of an isolated atom. So, the statement given in option B) is correct. 

Electronegativity is the tendency of an atom to attract the shared pair of electrons towards itself, Hence, it is not the property of an isolated atom, rather it is the property exhibited by an atom in a molecule (or) it is the property of the bonded atoms. So, the statement given in option (C) is correct.

Q 2. Is it justified that metallic character increases with an increase in electronegativity?

Answer: No, it is not so. In fact, the non-metallic character increases with an increase in electronegativity. As the metallic character increases, the electronegativity decreases.

Q 3. What is the outermost electronic configuration of the most electronegative element?

a.  
b.   
c.  
d.  

Answer: On moving from left to right across any period, electronegativity increases and the halogens have the highest electronegativity values. So, from the electronic configurations, it can be seen that option C has a configuration () belonging to group 17 and it should be the most electronegative element. Hence, option C) is the correct answer.

Frequently Asked Questions - FAQ

Q 1. Why fluorine is the most electronegative element?

Answer: Fluorine has an electronic configuration of . Hence, it is just one electron away from achieving the most sought after octet configuration - the most stable state for any atom. Hence, it tends to attract the shared pair of electrons in any bond towards itself. So, fluorine is the most electronegative element.

Q 2. What is the difference between electronegativity and electron affinity?

Answer: Electronegativity is a chemical property of a bonded atom, that shows how well an atom can attract the bonded electron pair towards itself, whereas electron affinity is the amount of energy released when an electron is added to a neutral atom. It is opposite in sign to that of electron gain enthalpy.

Q 3. What are electropositive and electronegative elements?

Answer: Electropositive elements are those which have low ionisation energy and readily tend to lose electrons to form a cation. Basically, they are metallic elements like sodium, potassium etc.  Electronegativity is greater for non-metallic elements which are found more towards the right side of the periodic table.

Q 4. What is the importance of electronegativity?

Answer: Electronegativity helps us in ascertaining the nature of a bond (ionic or covalent), bond strength, bond length, the nature of an oxide whether it is acidic or basic and so much more. Hence, it is of great importance.

Q 5. Though fluorine is the most electronegative element, it is less oxidising then chlorine. Why?

Answer: The atomic size of fluorine is smaller than that of chlorine as it has only 2 shells (). Hence, it faces inter-electronic repulsion towards the incoming electron owing to greater charge density. Unlike fluorine, chlorine has vacant d-orbitals () that can readily accept electrons and therefore it is more oxidising.

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