ELECTRONEGATIVITY

 The tendency of an atom in a compound to attract shared electrons towards itself is known as electronegativity of an atom.

Pauling gave an arbitrary scale to measure the electronegativity of different elements. This scale is based upon the calculation of bond energies, In this scale fluorine has the maximum value of electronegativity, i. e. 4.0. whereas noble gases have the minimum value, i. e. 0.0.

On Mulliken scale, the average of ionization potential and electron affinity is known as electronegativity.

In general, the smaller atoms as well  as the atoms with nearly filled shells of electrons, attract electrons more than larger atoms and are, therefore, more electronegative.

Determination of Electronegativity 

  1. Pauling's scale: Pauling's  scale of electronegativity is based upon the calculation of bond energies. He considered the formation of AB covalent molecule as:

 1/2A2+1/2B2 → AB

  1/2(A-A) +1/2 (B-B) → A-B 

According to Pauling, the bond dissociation energy EA-B of covalent molecule AB is always greater than the geometric mean of the bond dissociation energies EA-A and EB-B of A2 and B2 molecules, i. e.

EA-B > [EA-A*EB-B]1/2

The difference between the two values is known as ionic resonance energy of A-B bond and is represented by ∆E.

ΔE = EA-B -[EA-A* EB-B]1/2

∆E is the measure of polarity of bond A-B. It depends upon the electronegativities  xA and xB of atoms A and B as: 

xA-xB = 0.088√ΔE

In the above relation, bond energies are taken in k.j./mole. If bond energies are expressed in k.cals/mole, the relation becomes:

xA-xB = 0.18√ΔE


2. Mulliken scale:         

On this scale, the average of ionization potential and electron affinity is known as electronegativity. Thus, 

Electronegativity = Ionization potential + Electron affinity/2

Mulliken gave the following relation for calculating electronegativity on Pauling scale:
Electronegativity = 0.374[Ionization potential + electron affinity]/2 + 0.17

        = 0.187 [ Ionization potential + Electron affinity]/2+0.17

In the above relation, ionization potential and electron affinity both are measured on electrons volts. The method is not useful as electron affinities of all the elements are not known. 

 

Factors affecting the value of electronegativity

In general, electronegativity depends upon the following factors :

  • Size of the atom :  Smaller is the size of the atom, greater will be its tendency to attract shared pairs of electrons towards itself and hence has greater electronegativity. Thus, electronegavity decreases with the increase in the size of the atom.
  • Effective nuclear charge :  Higher is the effective nuclear charge, greater will be its tendency to attract shared pairs of electrons towards itself and hence has greater electronegativity.
  •  Oxidation state :   Higher is the oxidation state of an atom, greater is the electronegativity. For example, electronegativity of Fe3+ (1.96) is greater than that of Fe2+(1.83).
Pauling electronegativity trend.

Comments

Post a Comment

Popular posts from this blog

THE TYPES OF ELEMENTS

 Using electronic    configuration as the criterion ,we ordinarily recognise four general type of elements ; The inert gas elements  THe representative elements  The transition elements  The inner transition elements. "The classification of these elements into these groups is dependent on the extent to which the s,p,d and f orbitals are filled". (1) Inert Gases (a) s and p _orbital of the most shell of these elements are completely filled. The outermost electronic configuration in ns2np6. (b) HElium is also inert but it's electronic configuration is 1s2. (2) Representative or normal elements:- (a) Outermost shell of these elements is incomplete. The number of electrons is the outermost shell is less than eight. (b) Inner shells are complete. (c) s-  and p-block  elements except inert gases are called normal or representative elements. (3) Transition Elements ; (a) Last two shells of these elements namely outermost and penultimate shells are incomp...
Read more