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Electronegativity is "the power of an atom when in a molecule to attract electrons to
itself." An atom which has a greater tendency to pull the electrons in a
covalent bond toward itself is called electronegative, while an
atom which has a lesser tendency to pull the electrons in a covalent bond toward
itself is called electropositive. The electronegativity of an element
depends on its effective nuclear charge and the n-shell. Crudely,
- electronegativity increases across a period as the effective nuclear
charge increases without a change in the n quantum number, and
- electronegativity decreases down a group as the n quantum number increases
and the average distance of the valence electrons from the nucleus increases.
Exceptions occur after the 3d and 4f metals due to large increases in the
effective nuclear charge. The electronegativity of an atom in a molecule
depends on:
- the number of atoms coordinated to it,
- the electronegativity of the atoms coordinated to it and
- the oxidation number for the
atom.
Pauling scale:
The bond energy E(A-B) for a molecule A-B is always greater than the
mean of the bond energies ½[E(AA) + E(BB)] in the homonuclear
molecules AA and BB. Pauling argued that the excess bond energy is due to the
ionic component caused by the partial
charges on the atoms in AB. Pauling proposed a relationship between the ionic
contribution and the electronegativity difference between A and B:
E(AB) = [E(AA) E(BB)]½
+ 96.48(cA - cB)2
where E(AB) is in kJ mol-1 and
cA and
cB are the Pauling electronegativities
of A and B. Originally Pauling
set F arbitrarily at 4.0 although today the value for F is set to 3.98. The
values for all other elements are positive but less than 3.98.
H
2.20 |
|
Li
0.98 |
Be
1.57 |
|
B
2.04 |
C
2.55 |
N
3.04 |
O
3.44 |
F
3.98 |
Na
0.93 |
Mg
1.31 |
|
Al
1.61 |
Si
1.90 |
P
2.19 |
S
2.58 |
Cl
3.16 |
K
0.82 |
Ca
1.00 |
Sc
1.36 |
Ti
1.54 |
V
1.63 |
Cr
1.66 |
Mn
1.55 |
Fe
1.83 |
Co
1.88 |
Ni
1.91 |
Cu
2.00 |
Zn
1.65 |
Ga
1.81 |
Ge
2.01 |
As
2.18 |
Se
2.55 |
Br
2.96 |
Rb
0.82 |
Sr
0.95 |
Y
1.22 |
Zr
1.33 |
Nb
1.60 |
Mo
2.16 |
Te
1.90 |
Ru
2.20 |
Rh
2.28 |
Pd
2.20 |
Ag
1.93 |
Cd
1.69 |
In
1.78 |
Sn
1.96 |
Sb
2.05 |
Te
2.10 |
I
2.66 |
Cs
0.79 |
Ba
0.89 |
La
1.10 |
Hf
1.30 |
Ta
1.50 |
W
2.36 |
Re
1.90 |
Os
2.20 |
Ir
2.20 |
Pt
2.28 |
Au
2.54 |
Hg
2.00 |
Tl
2.04 |
Pb
2.33 |
Bi
2.02 |
Po
2.00 |
At
2.20 |
Ionic character of a bond
The electronegativity difference between two elements can also be used to estimate the percentage ionic character
in a single bond between the elements:
|
cA-
cB |
0.1 |
0.3 |
0.5 |
0.7 |
0.9 |
1.1 |
1.3 |
1.5 |
1.7 |
1.9 |
2.1 |
2.3 |
2.5 |
2.7 |
2.9 |
3.1 |
3.3 |
| % ionic character |
0.5 |
2 |
6 |
12 |
19 |
26 |
34 |
43 |
51 |
59 |
67 |
74 |
79 |
84 |
88 |
91 |
92
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Other electronegativity scales
Other definitions have been proposed for assigning electronegativity values, including:
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