5. Principle on which Pauling Scale is
Difference between the measure
energy of AB bond and expected for
purely covalent AB bond .
∆E=E measure - E expected covalent
6. Conditions given by Pauling
If two atom A and B have same electro negativity value
than the molecules AB is bonded by purely covalent bond
.Then energy of AB covalent bond would be the mean of
energy of A2 and B2 molecules.
EA-B =(EA-A +EB-B)1/2
∆E=EA-B -[EA-A * EB-B]1/2
EA-B =[EA-A * EB-B]1/2
7. Second Conditions
If two atoms A and B have different electro negativity .
The bond AB will no longer be purely covalent and
energy would be greater than mean of energy A2 and
EA-B>[EA-A * EB-B ]
This excess bond energy ∆E is known ionic covalent
8. Expression to co-relate ∆E with electro
0.18√∆E =χA – χB ,S.I unit for E=Kcal/mol
0.208√∆E=χA- χB, S.I unit for E=ev/mol
0.1017√∆E=χ A- χB, S.I unit for E=-KJ/mol
9. Advantages And Disadvantages
This method has simple theoretical bases
Different values can be obtained for different
oxidation state of same element.
Only if electro affinities are known.
11. IN 1934 MULLIKEN SUGGESTED AN
APPROACH TO ELECTRONEGATIVITY
BASED ON IONISATION
ENTHALPY & ELECTRON
ACCORDING TO MULLIKEN
ELECTRONEGATIVITY OF AN ELEMENT IS
AIRTHMETIC MEAN OF ITS I.E. & E.A.
E = I(A) - E(B)
E = I(B) - E(A)
If A B are formed then this process
required less energy.
( I(A) –E(B) ) < ( I (B) - E(A) )
13. The correlation between Mulliken electronegativities (x-
axis in KJ / mol) and Pauling electronegativities ( Y axis)
15. Charge On The Atom
Ionization Energy & Electron Affinity
Effective Nuclear Charge
Effect of the Substituent
16. The overall tendency of any bonded atom
to attract electrons is clearly influenced by
its bonding environment. As such
electronegativity is more specifically a
property of the orbital of an atom. It is now
reasonable to expect that electronegativity
should be influenced by such factors like
state of hybridization , oxidation state and
partial charge of the atom.
17. 1. CHARGE ON THE ATOM
Oxidation state may be defined as the charge left
on the central atom when all the other atoms of
the compound have been removed in their usual
An atom which acquires a positive charge would
tend to attract electrons more strongly than a
As the oxidation number of element increases,
18. For example
In PCl5 & PCl3, P (+V) > P (+III)
In SnCl4 & SnCl2, Sn (+ IV) > Sn (+ II)
In TlCl4 & TlCl, Tl (+IV) > Tl (+I)
Also HClO₃ > HCl⁺O
Thus, a cation will be more electronegative than parent
atom ( M+ > M ).
And a parent atom will be more electronegative than an
anion (M+ > M- )
19. So, the decreasing order of electronegativity will be:
M+ > M > M-
Higher the oxidation states, greater will be the force of
attraction for the electrons and higher will be the
The following values of electronegativity(Allred-Rochow) are
20. 2. HYBRIDIZATION
Hybrid orbitals are composed of orbitals of different
character to a varying degree of mixing.
Hybridization affects electronegativity values because
s-orbital due to its higher penetration effect and
thereby higher effective nuclear charge, is more tightly
held than other orbitals. It also has lower energy.
Consequently, it will have greater tendency to attract
electrons and hence show higher electronegativity
21. This conclusion agrees with the observed variations of
acidity among the compounds CH₄ , C₂H₄ & C₂H₂.
Hybridization: sp³ sp² sp
s-character : 25% 33% 50%
Electronegativity of C atom increases as we move
from CH₄ to C₂H₂ . Due to greater Electronegativity
of carbon in C₂H₄ & C₂H₂ , the electron pair of C-H
bond is pulled more towards carbon atom, releasing
H+ ion. The H atoms in CH₄ are neutral while in C₂H₄
is slightly acidic & C₂H₂ is more acidic.
CH₄ C₂H₄ C₂H₂
22. R NH₂
sp³ sp² sp
fairly basic feebly basic No basic
Similarly, the availability of the lone pair of
electrons on the nitrogen atom in a number of
compounds is determined by the state of
hybridization involved. Here, the basic character
increases with decreasing s-character in the
23. 3. IONISATION ENERGY
More is the ionization
energy and electron
affinity, more is the
25. ::IN CALCULATION OF PERCENTAGE OF IONIC CHARACTER ::
If two atoms have similar electronegativity then the bond between them would be
covalent whereas if two atoms have different electronegativities then then bond will
be predominantly ionic.
Percentage ionic character= 18(ӼA -ӼB)1.4
(this is the suggested and empirical equation for calculating % ionic character .)
On the basis of this equation 50% ionic character occurs when electronegativity
This equation was modified by Hanny and Smith
Percentage ionic character = 16 (cA - cB) + 3.5 (cA - cB)2
When electronegativity difference is 2.1 then the bond is 50% ionic.
26. Larger the difference in electronegativity of two combining atoms
more stable will be the bond. It is due to increase in ionic
HF HCl HBr HI
Xa - Xb 1.9 0.9 0.7 0.4
Stability = HF > HCl > HBr > HI
27. The bond angle decreases in the series
NH3 > PH3 > AsH3 > SbH3
Lesser the electronegativity of the central atom in the
poly atomic molecule , the lesser would be the
magnitude of the bond angle.
If electronegativity of central atom is less, it would not
be able to hold the bonded electron towards itself .
Therefore , bonding electron pair will shift more
towards bonding electron . This would result in
decrease in Bond pair-bond pair repulsion and
decrease in bond angle.
• ӼO-H> ӼO -ӼMi.eE.N. difference of
O-H > E.N.difference of O & M
• OH bond will be more polar than O-
• Ionization of molecule in aq sol. Will
take place at O-Hbond
• H+ ions will be released
• ӼO-H < ӼO-ӼH i.e.E.N.difference of O-H > E.N.difference of O
• OH bond will be less polar than O-M bond
• M-O bond will be more polar
• Ionization of molecule in aq. sol. will take place at M-O bond
• OH- ions will bereleased
32. 2 Atoms A & B bonded
through a covalent bond
differ in E.N.
Greater the polarity Shorter the bond length
b/w A & B
Covalent bond acquires ionic
character or polarity
33. Bond length of A & B [d(A-B) ] = rA + rB or r+ + r- or 2rA
[d(A-B) ] = rA + rB - 0.09(ӼA -ӼB)
Where rA & rB are atomic or molecular
rA + rB
Normal length of covalent bond b/w A & B = rA + rB
rA + rB - 0.09(ӼA -ӼB)
Reduction in bond length leads to
34. For ABn type compounds
-Δf H = n x 96.49 (ӼA -ӼB )2
(ӼA -ӼB ) = 0.102
35. Reactants and products have same no. of covalent
For compounds containing N & O
-Δf H = n x 96.49 (ӼA -ӼB )2 – 231 nN – 105 nO
nN = no. of nitrogen atoms
nO = no. of oxygen atoms
36. Electro-negativity and it’s definition.
Ways of calculating electro-negativity
• Pauling’s scale
• Mulliken’s scale
Factors affecting electro-negativity.
Applications of electro-negativity.