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Electronegativity

After atomic number, mass & valency, electronegativity is the most important of all atomic parameters. This page is expanded into a full paper, Electronegativity as a Basic Elemental Property, by Mark Leach available here (PDF).

The History of Electronegativity

It is a common misapprehension that Linus Pauling 'invented' electronegativity in 1932, but the idea pre-dates this time. In 1895 the Danish thermochemist Hans Peter Jørgen Julius Thomsen proposed a periodic table that shows electropositive and electronegative elements:

However, the concept of electronegativity was put on a quantitative footing in 1932 by Linus Pauling in The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativity of Atoms, Journal of the American Chemical Society, 54, p. 3570-3582. The original paper is available in HTML here.

In his textbook The Nature of The Chemical Bond (published 1938, quote from 3rd ed.), Pauling says about electronegativity:

"The power of an atom in a molecule to attract electrons to itself."

In his General Chemistry textbook (pp 183) Pauling writes:

"It has been found possible to assign to the elements numbers representing their power of attraction for the electrons in a covalent bond, by means of which the amount of partial ionic character may be estimated."

The IUPAC Gold Book says:

"Concept introduced by L. Pauling as the power of an atom to attract electrons to itself. There are several definitions. According to Mulliken it is the average of the ionization energy and electron affinity of an atom, but more frequently a relative scale due to Pauling is used where dimensionless relative electronegativity differences are defined on the basis of bond dissociation energies", here.

As discussed here, this author's definition is:

"Electronegativity is a measure, integrated over numerous physical parameters, of the power of a gas phase or bonded atom to attract electrons to itself."

While not too much should be read into absolute values, many trends in structure and reactivity behaviour can be mapped to ("explained in terms of", or correlated with) Pauling's electronegativity data. This makes electronegativity an extraordinarily useful concept.

In this web book, electropositive elements are coloured RED and electronegative elements are coloured BLUE. The rational is that:

Electronegative elements tend to gain electrons to become anionic Lewis bases, and Lewis bases are blue.

Chlorine, eneg 3.16, generally reacts to become chloride ion, Cl–, a lobe-HOMO Lewis base.

During this process, the chlorine is reduced, so chlorine is an oxidising agent.

Electropositive elements, metals, generally react by losing one or more electrons to become cationic Lewis acids.

Lithium, eneg 0.98, generally reacts to become Li+, an s-LUMO Lewis acid.

Lithium loses an electron so it is oxidised, and so it is acting as a reducing agent.

Using this colour representation, the top-right to bottom-left diagonal trend can be clearly seen across the main group elements and across the entire periodic table.

Why Is Electronegativity Important?

The metallic elements are all electropositive, the electronegative elements are all non-metals, the metalloids are found at intermediate electronegativities.

Ionic compounds, like sodium chloride NaCl, or Na+ Cl–, are formed between between electropositive elements (Na, 0.93) and electronegative elements (Cl, 3.16).

Indeed, bond type, material character and chemical reactivity can be predicted from a knowledge of electronegativity.

For example, consider:

Where Do The Numbers Come From?

Pauling's empirical electronegativity scale is derived from thermochemical bond-energy data. Pauling observed that bond enthalpy, EA-B, in kcal/mol between atoms A and B can be predicted using the equation below, where ΧA and ΧB are the electronegativity values of A and B.

In his book The Nature of The Chemical Bond, Pauling comments that it is more accurate to use the geometric mean rather than the arithmetic mean, but then uses the arithmetic mean himself. Other authors note this and then also use the arithmetic mean.

Calculations for the formation of the halogen halides: HF, HCl, HBr & HI from hydrogen, H2, and the halogens, F2, Cl2, Br2& I2 show how the Pauling relationship compares with experimental data:

Download the Excel spreadsheet here. Data is from Pauling's Nature of the Chemical Bond. Note that the equation requires data to be in kcal/mol rather than kJ/mol.

The electronegativity difference between elements A and B is determined from the following relationships:

Note that (a) both the geometric and arithmetic mean relationships are given and (b) or many metals the enthalpy of salt formation data is used as a proxy.

Once a set of electronegativity differences are known, it is a simple matter to assign absolute electronegativity values.

Compounds & Materials, Structure & Reactivity

Chlorine, by way of example, is the third most electronegative element after fluorine and oxygen. This electronegative nature is apparent in the structure and reaction chemistry of:

Electronegativity can be used to predict the dipole moment (bond polarity) of a bond:

Electronegativity can be used to approximately predict the degree of ionic (and therefore covalent) character of a bond between two dissimilar elements:


Electronegativity and Theory

Pauling used bond enthalpy data to construct his electronegativity scale. Other workers have used other starting points.

Eneg Scale
Method
Pauling Scale
1932 		Obtains values by thermochemical methods. Paper
Mulliken Relation
1934 		Defines a relation that depends upon the orbital characteristics of an atom in a molecule. Mulliken electronegativity is the numerical average of the ionisation potential and electron affinity. Wikipedia
Gordy Scale
1946 		Defines electronegativity in terms of the effective nuclear charge and the covalent radius. (Zeff)e/r. Phys. Rev. 69, 604 - 607 (1946)
Gordy developed several scales!
Walsh Scale
1951 		Relates electronegativity to stretching force constants of the bonds of an atom to a hydrogen atom. Paper
Huggins Scale
1953		 Alternative to Pauling's thermochemical procedure. Discussed in this paper
Sanderson Scale
1955 		The ratio of the average electron density of an atom to that of a hypothetical "inert" atom having the same number of electrons. This ratio is a measure of the relative compactness of the atom. J.Chem.Phys. 23, 2467 (1955)
Allred-Rochow Scale
1958 		Defines electronegativity in terms of the effective nuclear charge and covalent radius. Like the Gordy scale but uses (Zeff)e/r2. Wikipedia
Jaffe Scale
1962 		Uses the electronegativity of orbitals rather than atoms to develop group electronegativities for molecular fragments (eg. CH3 vs CF3) that take into account the charge of a group, the effects of substituents, and the hybridization of the bonding orbital. Electronegativity. I. Orbital Electronegativity of Neutral Atoms J. Hinze and H.H.Jaffe, J.Am.Chem. Soc., 1962, 84, 540
Phillips Scale
1968 		Defines electronegativity in terms of the dielectric properties of atoms in a given valence state. Paper
Martynov & Batsanov Scale
1980		 Obtained by averaging the successive ionisation energies of an element's valence electrons. Russ. J. Inorg. Chem., 1980, 25, 1737.
Allen CE Scale
1992 		Configuration energy (CE), the average one-electron valence shell energy of the ground-state free atom, Electronegativity is the average one-electron energy of the valence-shell electrons in ground-state free atoms, Leland C. Allen, J. Am. Chem. Soc. 1989, 111, 25, 9003-9014, is used to quantify metal-covalent-ionic bonding, Extension and completion of the periodic table, Leland C. Allen, J.Am.Chem.Soc., (1992), 114, 1510

Lang & Smith
2015

Peter F. Lang & Barry C. Smith presented a paper: An equation to calculate internuclear distances of covalent, ionic and metallic lattices, Phys. Chem. Chem. Phys., 2015, 17, 3355. Quoted from the paper:

"At the beginning of our work we used different sets of electronegativities, for example the set developed by Allred & Rochow to calculate internuclear distances of inorganic lattices but find that none of them suit the needs of this work. We first considered that electronegativity values are functions of electron affinities and ionisation energies. We produced many sets of electronegativity values based on generally accepted values of electron affinities and ionisation energies but none of them were satisfactory. Finally, we produced a set deduced from the ionisation energies adjusted for pairing and exchange interactions. This set of electronegativity scales as shown in Table 13 improved the agreement between the calculated and the observed internuclear distances. There are some elements such as technetium and polonium, where little observed data on bond lengths or radii or lattice energies are available. In such cases, their electronegativities are estimated by interpolation/extrapolation of electronegativities of neighbouring elements."

More in: H.B. Michaelson, IBM J. Res. Develop. 22 1 (1978). Review article by H. O. Pritchard and H. A. Skinner: The Concept Of Electronegativity, Chem. Rev.; 1955; 55(4) pp 745 - 786.

Electronegativity seems to integrate (average out) a number of arcane atomic electronic parameters. It is a proxy parameter that in a rather simple way maps to chemical structure and reactivity.

In his 1992 paper (J.Am.Chem.Soc., (1992), 114, 1510), Allen argued that configuration energy, CE, is a fundamental atomic property and is the "missing third dimension to the periodic table". He further stated that electronegativity is an 'ad hoc' parameter. More usefully – in this author's judgment– Allen's work shows that configuration energy, CE, correlates with electronegativity.

Indeed, electronegativity is so important that in this author's judgment it should be considered to be a basic atomic property rather than a simple atomic property, here.

In 1960 Pauling defined electronegativity as:

"The power of an atom in a molecule to attract electrons to itself"

However, when considered in the context of semiquantitative tetrahedra of structure of bonding and material type, this statement is literally too narrow because bulk binary compounds can be metallic, ionic or network covalent as well as molecular.

Any definition of electronegativity must not be self-limiting. An updated definition [propsed here] is:

"Electronegativity is a measure, integrated over numerous physical parameters, of the power of a gas phase or bonded atom to attract electrons to itself."

Tables of Electronegativity Data

  Electronegativity  
Pauling
Revised
Pauling
Sanderson
Lang & Smith
1
H
 Hydrogen
2.1
2.20
2.8
2.31
2.20
2.00
2
He
 Helium            
3
Li
 Lithium
1.0
0.98
1.3
0.86
0.97
1.24
4
Be
 Beryllium
1.5
1.57
  
1.61
1.47
2.14
5
B
 Boron
2.0
2.04
1.8
1.88
2.01
1.81
6
C
 Carbon
2.5
2.55
2.5
2.47
2.50
2.30
7
N
 Nitrogen
3.0
3.04
2.9
2.93
3.07
2.82
8
O
 Oxygen
3.5
3.44
3.0
3.46
3.50
3.39
9
F
 Fluorine
4.0
3.98
4.1
3.92
4.10
4.00
10
Ne
 Neon            
11
Na
 Sodium
0.9
0.93
1.2
0.85
1.01
1.18
12
Mg
 Magnesium
1.2
1.31
  
1.42
1.23
1.76
13
Al
 Aluminum
1.5
1.61
1.4
1.54
1.47
1.31
14
Si
 Silicon
1.8
1.90
2.0
1.74
1.74
1.66
15
P
 Phosphorus
2.1
2.19
2.3
2.16
2.06
2.05
16
S
 Sulfur
2.5
2.58
2.5
2.66
2.44
2.49
17
Cl
 Chlorine
3.0
3.16
3.3
3.28
2.83
2.95
18
Ar
 Argon      
3.92
    
19
K
 Potassium
0.8
0.82
1.1
0.74
0.91
1.00
20
Ca
 Calcium
1.0
1.00
  
1.06
1.04
1.40
21
Sc
 Scandium
1.3
1.36
  
1.09
1.20
1.51
22
Ti
 Titanium
1.5
1.54
       1.57
23
V
 Vanadium
1.6
1.63
       1.62
24
Cr
 Chromium
1.6
1.66
       1.65
25
Mn
 Manganese
1.5
1.55
       1.71
26
Fe
 Iron
1.8
1.83
       1.77
27
Co
 Cobalt
1.8
1.88
       1.84
28
Ni
 Nickel
1.8
1.91
       1.92
29
Cu
 Copper
1.9
1.90
       2.02
30
Zn
 Zinc
1.6
1.65
  
1.86
1.66
2.16
31
Ga
 Gallium
1.6
1.81
1.4
2.10
1.82
1.31
32
Ge
 Germanium
1.8
2.01
1.9
2.31
2.02
1.62
33
As
 Arsenic
2.0
2.18
2.2
2.53
2.20
1.95
34
Se
 Selenium
2.4
2.55
2.4
2.76
2.48
2.30
35
Br
 Bromine
2.8
2.96
3.0
2.96
2.74
2.67
36
Kr
 Krypton  
2.90
  
3.17
    
37
Rb
 Rubidium
0.8
0.82
1.0
0.70
0.89
0.96
38
Sr
 Strontium
1.0
0.95
  
0.96
0.99
1.31
39
Y
 Yttrium
1.2
1.22
1.4
0.98
1.11
1.54
40
Zr
 Zirconium
1.4
1.33
       1.57
41
Nb
 Niobium
1.6
1.60
       1.61
42
Mo
 Molybdenum
1.8
2.16
       1.66
43
Tc
 Technetium
1.9
1.90
       1.71
44
Ru
 Ruthenium
2.2
2.20
       1.76
45
Rh
 Rhodium
2.2
2.28
       1.84
46
Pd
 Palladium
2.2
2.20
       1.91
47
Ag
 Silver
1.9
1.93
       1.92
48
Cd
 Cadmium
1.7
1.69
  
1.73
1.46
2.06
49
In
 Indium
1.7
1.78
1.3
1.88
1.49
1.26
50
Sn
 Tin
1.8
1.96
1.8
2.02
1.72
1.49
51
Sb
 Antimony
1.9
2.05
2.0
2.19
1.82
1.73
52
Te
 Tellurium
2.1
2.10
2.2
2.34
2.01
2.01
53
I
 Iodine
2.5
2.66
2.7
2.50
2.21
2.32
54
Xe
 Xenon      
2.63
    
55
Cs
 Cesium
0.7
0.79
1.0
0.69
0.86
0.89
56
Ba
 Barium
0.9
0.89
  
0.93
0.97
1.20
57
La
 Lanthanum
1.1
1.10
  
0.92
1.08
1.28
58
Ce
 Cerium
1.1
1.12
       1.27
59
Pr
 Praseodymium
1.1
1.13
       1.25
60
Nd
 Neodymium
1.1
1.14
       1.27
61
Pm
 Promethium
1.1
1.13
       1.28
62
Sm
 Samarium
1.1
1.17
       1.30
63
Eu
 Europium
1.1
1.20
       1.30
64
Gd
 Gadolinium
1.1
1.20
       1.41
65
Tb
 Terbium
1.1
1.20
       1.35
66
Dy
 Dysprosium
1.1
1.22
       1.36
67
Ho
 Holmium
1.1
1.23
       1.38
68
Er
 Erbium
1.1
1.24
       1.40
69
Tm
 Thulium
1.1
1.25
       1.42
70
Yb
 Ytterbium
1.1
1.10
       1.44
71
Lu
 Lutetium
1.1
1.27
       1.25
72
Hf
 Hafnium
1.3
1.30
       1.57
73
Ta
 Tantalum
1.5
1.50
       1.73
74
W
 Tungsten
1.7
2.36
       1.81
75
Re
 Rhenium
1.9
1.90
       1.80
76
Os
 Osmium
2.2
2.20
       1.94
77
Ir
 Iridium
2.2
2.20
       2.06
78
Pt
 Platinum
2.2
2.28
       2.06
79
Au
 Gold
2.4
2.54
       2.12
80
Hg
 Mercury
1.9
2.00
  
1.92
1.44
2.40
81
Tl
 Thallium
1.8
2.04
  
1.96
1.44
1.34
82
Pb
 Lead
1.8
2.33
  
2.01
1.55
1.51
83
Bi
 Bismuth
1.9
2.02
  
2.06
1.67
1.68
84
Po
 Polonium
2.0
2.00
       1.90
85
At
 Astatine
2.2
2.20
       2.12
86
Rn
 Radon            
87
Fr
 Francium
0.7
0.70
        
88
Ra
 Radium
0.9
0.90
        
89
Ac
 Actinium
1.1
1.10
        
90
Th
 Thorium
1.3
1.30
        
91
Pa
 Protactinium
1.5
1.50
        
92
U
 Uranium
1.7
1.38
        
93
Np
 Neptunium
1.3
1.36
        
94
Pu
 Plutonium
1.3
1.28
        

A plot of the above data shows that, broadly, the various electronegativity systems are numerically equivalent:

This page is expanded into a full paper, Electronegativity as a Basic Elemental Property, by Mark Leach available here.

Some recommended Wikipedia links:

Download an electronegativity & bond character calculator spreadsheet, here.

Many thanks to Bruce Railsback for his helpful comments about this page.

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© Mark R. Leach 1999 –

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