Electronegativity

In chemistry, the electronegativity of an element is a size which characterizes its capacity to attract the electrons during the formation of a chemical bond with another element. The difference in electronegativity between these two elements determines the nature of the covalent bond: Not-polar connection when the difference is weak, polar connection when the difference is strong, and ionic when the difference is so strong that one of the elements attracted the electrons completely: the atoms became ions and carry whole electric charges. The concept of electronegativity, which describes the behavior of the electrons in a chemical bond, should not be confused with that of electronic affinity.
The concept of electronegativity was introduced for the first time by Berzelius in 1835. Thereafter, Pauling improved this concept and deduced that electronegativity rests rather on the existence of the ionic and covalent connections, as opposed to what Berzelius had found before.
Electronegativity is noted Χχ where Χ is the symbol of the element considered. The larger Χχ is and the more likely the element is to attract electrons with him in a chemical bond.
Electronegativity and types of chemical bonds
To calculate the variations of electronegativity of the elements, the scale of Pauling generally is used. In the periodic table, electronegativity increases from left to right along a period and upwards along a family. Thus the fluorine, in top on the right of the periodic table, is the most electronegative element with a value of 4,0 while the francium, bellow on the left, is the least electronegative with a value of 0,7. The differences in electronegativity make it possible to identify the covalent bonds nonpolar, the polar covalent bonds and the connections of co-ordination number. Electronegativity makes it possible to identify the loads partial of the atoms of a given molecule. The symbols δ+ and δ- represent the positive and negative loads respectively partial of a connection of which the most electronegative atom carries the negative partial load.
There are three types of covalent bonds:
The first type is the nonpolar covalent bond which has a relatively symmetrical electronic cloud. In this case, the attraction of the electrons towards the cores of the two atoms in question is roughly equal.
The second type is the polar covalent bond. A molecule is polar if it has one dipole moment. One dipole moment is a vectorial representation of the partial burden-sharing at a given distance. This connection has an unequal charge distribution between the two atoms which form it because of the difference in electronegativity of its atoms. The dipole moment is represented by a vector which the arrow leaves the least electronegative atom towards the most electronegative atom. The symbols δ+ and δ- represent the positive and negative loads respectively partial of the connection of which the most electronegative atom carries the negative partial load. There is thus an unequal attraction of the electrons towards the cores of the two atoms. In this type of connection, the electronic cloud is not symmetrical like the first type of connection.
The third type of covalent bond is that of co-ordination number. In this type of covalent bond, there is a division of electrons between two atoms, on the other hand the doublet of electrons forming the bond comes from only one of the two atoms. In other words, the two divided electrons come from the same atom.
Scales of electronegativity
There exist several definitions of electronegativity (Pauling, Mulliken, Parr, Allred and Rochow) what resulted in building several scales.
Definition of Pauling
the difference in electronegativity between the elements has and B has as an expression
\δ\Χ_{AB} = 0,102 * (E_{AB} - (E_{AA} * E_{BB}) ^ {½}) ^ {½}
where
E_{AB}, E_{AA} and E_{BB} are the binding energies of diatomic molecules A-B, A-A and B-B.
Coefficient 0,102 comes from the unit used for the values of energies (initially in eV) which owe, in this formula, being expressed in kJ.mol-1. The average of energies E_{AA} et E_{BB} is often a geometric mean (like here), but certain authors use the arithmetic mean.
This definition gives only the difference between two electronegativities. One thus needs an origin which was fixed arbitrarily by giving the value of 4 to the electronegativity of fluorine, the most electronegative element of classification.
Definition of Mulliken
the electronegativity of an element is the product of the average of its electronic affinity Ae and its energy of ionization that is to say(internal excitation) with a coefficient alpha = 0.317eV-1
\Χ = 0,317 * \ frac {A_e + E_I}{2}
The interest of the scale of Mulliken, compared to that of Pauling, is to use atomic sizes, independent of the chemical environment. It thus makes it possible to determine the electronegativity of rare gases, which Pauling had not been able to do.
Definition of Allred and Rochow: the electronegativity of an element has as an expression
\Χ = \frac\{Z_{eff}e ^ 2}{r_{cov} ^ 2}
where
Z is the effective load of the core, E the elementary charge and r_{cov} the covalent radius of the element
Definition of Parr: electronegativity is opposite derivative of the energy of the atom compared to the number of electrons
\Χ\ = -\left(\frac{\partial E}{\partial n}\right)
The most used scales of electronegativity are the scale of Mulliken, the scale of Allred-Rochow and the scale of Pauling.
Electronegativities of the atoms implied in a connection tend to being equal, principle of equalization of electronegativities of Sanderson, 1951. The equalization of electronegativities is carried out by the electronic transfer of density towards the most electronegative atom.
Electronegativity makes it possible to consider the character ionic of one connection using the relation of Pauling
I_{AB} = 100 (1 - e ^ {-\δ\Χ\} ^ 2
or of that of Haney and Smith
I_{AB} = 16 \δ\Χ\ + 3.5 {\δ\Χ} ^ 2
Electronegativity is also the concept in the beginning of the polarity of certain molecules. Indeed, in a molecule, when the atoms on both sides of the covalent bond have different electronegativities, the most electronegative atom attracts the electrons more. The barycentre of the positive loads is thus not confused with the barycentre of the negative charges. The molecule remains overall neutral but an electric field appears within this one, it is said that the connection is polarized or that the molecule is polar.
The elements whose electronegativity is low are frequently known as electropositive.

Table scale of Pauling

The electronegativity of the chemical elements of the same group of the periodic table, that is to say of the same column of the periodic table tends to decrease when the atomic number grows, because the atomic nucleus then tends to move away from the electrons of valence, which are more écrantés by the electrons of heart. On the other hand, the electronegativity of the elements of the same period of the periodic table tends to grow with the atomic number, because the electric charge of the atomic nucleus, many protons, increases and interacts of advantage with the electrons of valence. The minimum is thus to seek in bottom on the left table while the maximum is in top on the right.
H
2,2
  He
Li
0,98
Be
1,57
  B
2,04
C
2,55
N
3,04
O
3,44
F
3,98
Ne
Na
0,93
Mg
1,31
  Al
1,61
Si
1,9
P
2,19
S
2,58
Cl
3,16
Ar
K
0,82
Ca
1
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
1,9
Zn
1,65
Ga
1,81
Ge
2,01
As
2,18
Se
2,55
Br
2,96
Kr
Rb
0,82
Sr
0,95
Y
1,22
Zr
1,33
Nb
1,6
Mo
2,16
Tc
2,1
Ru
2,2
Rh
2,28
Pd
2,2
Ag
1,93
Cd
1,69
In
1,78
Sn
1,96
Sb
2,05
Te
2,1
I
2,66
Xe
2,6
Cs
0,79
Ba
0,89
  Hf
1,3
Ta
1,5
W
1,7
Re
1,9
Os
2,2
Ir
2,2
Pt
2,2
Au
2,4
Hg
1,9
Tl
1,8
Pb
1,8
Bi
1,9
Po
2
At
2,2
Rn
2,2
Fr
0,7
Ra
0,9
  Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
  La
1,1
Ce
1,12
Pr
1,13
Nd
1,14
Pm
1,13
Sm
1,17
Eu
1,2
Gd
1,2
Tb
1,2
Dy
1,22
Ho
1,23
Er
1,24
Tm
1,25
Yb
1,1
Lu
1,27
   
  Ac
1,1
Th
1,3
Pa
1,5
U
1,7
Np
1,3
Pu
1,3
Am
1,3
Cm
1,3
Bk
1,3
Cf
1,3
Es
1,3
Fm
1,3
Md
1,3
No
1,3
Lr
1,3
   
execution time customer :
runtime server : 0.139 seconds