Electronegativity - Practice Questions & MCQ

Electronegativity

The tendency of an atom to attract the shared pair of electrons towards itself is called electronegativity. It is a relative quantity. This concept was introduced in 1932 by Pauling. It has no units. Fluorine is the most electronegative element known so far and its value is arbitrarily assigned as 4.0. In moving from left to right in a period, the electronegativity increases while in moving to the top to bottom in a group, the electronegativity decreases.

Factors affecting Electronegativity

There are various following factors which affect the electronegativity.

• Atomic Size: As the atomic size increases, the electronegativity decreases.

• Effective Nuclear Charge: With the increase of effective nuclear charge, the electronegativity of the atom increases.

• Oxidation State: As the oxidation state of an atom increases, the electronegativity also increases. For example, Fe³⁺ is more electronegative than Fe²⁺.

Variation of Electronegativity

• In moving from top to bottom in a group the atomic size increases thus the force of attraction decreases and hence the electronegativity decreases.

• In moving from left to right in a period, the atomic size decreases and effective nuclear charge increases, thus the electronegativity increases.

• Halogens are the most electronegative elements and fluorine has the highest electronegativity. 

• For transition elements, the electronegativity values vary between 1.1 to 1.3.

• Metals have lower electronegativity values while non-metals have higher electronegativity values.

Importance of Electronegativity

The following predictions can be made out of the information of electronegativities of atoms.

• Nature of Element: The elements with lower electronegativity values are metals while the elements with higher electronegativity values are non-metals. The elements with intermediate electronegativity values are metalloids. Fluorine has the highest electronegativity value, thus it is the most non-metallic element. Similarly, cesium has the lowest electronegativity value, thus it is the most metallic element.

• Nature of Oxides: The nature of the oxides formed by the elements can also be predicted by electronegativity. When M_O - M_A difference is lower, then the oxide is acidic in nature but when this difference M_O - M_A is large, then the oxide is basic in nature. M_O here is the electronegativity of oxygen.

Scales for measuring electronegativity

There are various following scales to measure the electronegativity of elements.

• Pauling Scale: The electronegativity of the elements in Pauling scale is given by the following formula:

M_A - M_B = 0.208[E_A-B - (E_A-A x E_B-B)^½]^½

Where, E_A-B is the bond energy of A-B

            E_A-A is the bond energy of A-A

             E_B-B is the bond energy of B-B

This formula is used only when the energy is taken in kcal/mol.

When bond energy is taken in kJ/mol, then:

 M_A - M_B = 0.102[E_A-B - (E_A-A x E_B-B)^½]^½

• Mulliken Scale: Mulliken considered the electronegativity as the average of the ionisation potential and electron gain enthalpy of an atom.

(i) When ionisation potential and electron gain enthalpy are in taken in electron-volts:

    Electronegativity = (IE + EA)/2

(ii) When ionisation potential and electron gain enthalpy are in taken in kJ/mol.

   Electronegativity =(IE + EA)/(2 x 96.48)

• Allred and Rochow Scale: In this scale, the electronegativity is given by the following formula:

Electronegativity = 0.744 + (3590 Z/r²)    Where Z is the effective nuclear charge and r is the covalent radius in pm.

Applications of Electronegativity

The following predictions can be made out of the information of electronegativities of atoms.

• Nomenclature: The nomenclature of binary compounds can be done using the electronegativity value of the atoms. The atom with higher electronegativity is written with ide as suffix. For example, in HCl, chlorine atom has higher electronegativity, thus it is written as chloride and the complete name is hydrogen chloride.

• Nature of Bond: From the electronegativity values of respective atoms, the nature of bond can be estimated.

(i) When the electronegativity difference between two atoms i.e, M_A- M_B = 0, then the bond is purely covalent.

(ii) When M_A- M_B is small, then the bond is polar but covalent.

(iii) When M_A- M_B is 2.1, then the bond is 50% ionic and 50% covalent.

(iv) When M_A- M_B is more than 2.1, the bond is very much ionic and less covalent.

      The percentage of ionic character is given by the following formula:

      Percentage of ionic character = 16(M_A - M_B) + 3.5(M_A - M_B)²

      Where M_A and M_B are the electronegativities of two bonded atoms i.e, A and B.

• Nature of Element: The elements with lower electronegativity values are metals while the elements with higher electronegativity values are non-metals. The elements with intermediate electronegativity values are metalloids. Fluorine has the highest electronegativity value, thus it is the most non-metallic element. Similarly, caesium has the lowest electronegativity value, thus it is the most metallic element.

• Nature of Oxides: The nature of the oxides formed by the elements can also be predicted by the electronegativity. When M_O - M_A difference is lower, then the oxide is acidic in nature but when this difference M_O - M_A is large, then the oxide is basic in nature. M_O here is the electronegativity of oxygen.

• Bond Strength: The more the electronegativity difference between the atoms, the stronger is the bond. Thus,

H-F > H-Cl > H-Br > H-I

• Acidic Nature: When the electronegativity difference is less, then bond between the atoms is weaker and thus it is easier to lose the proton. Thus,

H-I > H-Br > H-Cl > H-F