Electronegativity scales:
1.Pauling scale of electronegativity:
in diatomic molecule (A-B) ,the bond formed between two atoms A and B will be intermediate between pure Covalent (A-B) and pure ionic character, the bond between A and B will be strong than bond energy increased.
if bond (A-B) has been purily covalent than bond energy can be calculated as average bond energy of bond (A-A) and bond (B-B). that means it is equal to bond energy of bond (A-B)
E(A-B) = 1/2[ E(A-A) +E(B-B) ]
however experiemental value of bond (A-B) more than this value because of difference in electronegativity of A and B the difference ∆ is given by simple expression
∆ = E(A-B) - 1/2 [E(A-A) +E(B-B) ]
where E(A-B) is experimental value of bond energy.
if Ҳ(A) and Ҳ(B) are the electronegative of elements A and B than Ҳ(A) - Ҳ(B) = 0.18√∆
for numerical
∆ = E(A-B) -[√(E(A-A) ×E(B-B) )]
Disadvantages of pauling scale of Electronegativity:
main disadvantage of pauling scale is that bond energy are not know with good degree of accurence however the value of relative value.
Question 1
calculate Electronegativity of carbon I C-H bond if bond energy of E(C-H), E(H-H), E(C-C) are 98.8,104 and 83 kcal/mol respectively.
ans
we know
∆ = E(C-H) -[√(E(H-H) ×E(C-C) )]
after putting value we get
∆=98.8-92.9= 2.54
2.Mulliken scale of Electronegativity:
Mulliken principal state that Electronegativity of any elements may be taken as average of Electron affinity and ionisation energy of that element it can be calculated by simple expression
Ҳ(A)=[(I(A) +E(A)) /2]÷62.5
conditions
I. E and E. A are expressed in kcal/mol
the electronegativity value on mulliken scale can be converted into Pauling scale by using the relationship.
pauling scale =( mulliken scale) /2.8
Disadvantages of mulliken scale :
1.electron affinity of few elements are known
2. I. E and E. A changes with change valence state of elements
3.The allred rochow scale:
in this electrostatic approach electronegativity of atom is simple electrostatic force of attraction between the atom in the molecule and the bonding and the bonding electron are situated at covalent radii(r) of that atom.
if e is electronic charge and z* or zeff is effective nuclear charge than Ҳ electronegativity is
Ҳ=( e^2z*) /r^2
the allred rochow electronegativity value in term of pauling scale are obtained by the relation.
Ҳ=[( 0.359×z*) /r^2]+0.744
Disadvantages of allred rochow scale of electronegativity:
1.covalent radii of many elements are not available.
2.the slater's rule for calculating effective nuclear charge are empirical that means does not gives correct value.
Numerical
Question 1
find the Ҳ of arsenic having covalent radii 1.2A° and zeff is 6.4
ans
r=1.24A°
zeff=6.4
we known
Ҳ=[( 0.359×z*) /r^2]+0.744
after value we get
Ҳ= 2.313
# for any atom to find electron density
E. D= Z/(4/3×πr^3)
where z is atomic number and r is radius
1.Pauling scale of electronegativity:
in diatomic molecule (A-B) ,the bond formed between two atoms A and B will be intermediate between pure Covalent (A-B) and pure ionic character, the bond between A and B will be strong than bond energy increased.
if bond (A-B) has been purily covalent than bond energy can be calculated as average bond energy of bond (A-A) and bond (B-B). that means it is equal to bond energy of bond (A-B)
E(A-B) = 1/2[ E(A-A) +E(B-B) ]
however experiemental value of bond (A-B) more than this value because of difference in electronegativity of A and B the difference ∆ is given by simple expression
∆ = E(A-B) - 1/2 [E(A-A) +E(B-B) ]
where E(A-B) is experimental value of bond energy.
if Ҳ(A) and Ҳ(B) are the electronegative of elements A and B than Ҳ(A) - Ҳ(B) = 0.18√∆
for numerical
∆ = E(A-B) -[√(E(A-A) ×E(B-B) )]
Disadvantages of pauling scale of Electronegativity:
main disadvantage of pauling scale is that bond energy are not know with good degree of accurence however the value of relative value.
Question 1
calculate Electronegativity of carbon I C-H bond if bond energy of E(C-H), E(H-H), E(C-C) are 98.8,104 and 83 kcal/mol respectively.
ans
we know
∆ = E(C-H) -[√(E(H-H) ×E(C-C) )]
after putting value we get
∆=98.8-92.9= 2.54
2.Mulliken scale of Electronegativity:
Mulliken principal state that Electronegativity of any elements may be taken as average of Electron affinity and ionisation energy of that element it can be calculated by simple expression
Ҳ(A)=[(I(A) +E(A)) /2]÷62.5
conditions
I. E and E. A are expressed in kcal/mol
the electronegativity value on mulliken scale can be converted into Pauling scale by using the relationship.
pauling scale =( mulliken scale) /2.8
Disadvantages of mulliken scale :
1.electron affinity of few elements are known
2. I. E and E. A changes with change valence state of elements
3.The allred rochow scale:
in this electrostatic approach electronegativity of atom is simple electrostatic force of attraction between the atom in the molecule and the bonding and the bonding electron are situated at covalent radii(r) of that atom.
if e is electronic charge and z* or zeff is effective nuclear charge than Ҳ electronegativity is
Ҳ=( e^2z*) /r^2
the allred rochow electronegativity value in term of pauling scale are obtained by the relation.
Ҳ=[( 0.359×z*) /r^2]+0.744
Disadvantages of allred rochow scale of electronegativity:
1.covalent radii of many elements are not available.
2.the slater's rule for calculating effective nuclear charge are empirical that means does not gives correct value.
Numerical
Question 1
find the Ҳ of arsenic having covalent radii 1.2A° and zeff is 6.4
ans
r=1.24A°
zeff=6.4
we known
Ҳ=[( 0.359×z*) /r^2]+0.744
after value we get
Ҳ= 2.313
# for any atom to find electron density
E. D= Z/(4/3×πr^3)
where z is atomic number and r is radius
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