|
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
In 1895 the Danish thermochemist Hans Peter Jørgen Julius Thomsen proposed a periodic table that shows electropositive and electronegative elements:
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 the atomic property of 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 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 behavior can be mapped to ("explained in terms of",
or correlated with) Pauling's electronegativity data. This
makes electronegativity an extraordinarily useful concept.
- There is a broad sweep of electronegativity from top-right to bottom-left. (The radioactive elements francium, Fr, & radium Ra, are ignored as are the lighter group 18 elements: helium, He, neon, Ne, and argon, Ar.)
- The electronegative elements, found top-right, present as non-metals. Fluorine, F2, oxygen, O2, & chlorine, Cl2, are strong oxidising agents: they accept electrons and are easily reduced. The electronegative elements all form anions and they may form entities that interact via lone-pairs of electrons. Anions and electron lone pairs are associated with Lewis base behaviour.
- The electropositive elements all present as metals. Metals behave as electron donating reducing agents. Metals form cations that behave as Lewis acids.
- Hydrogen
is shown above and between boron
and carbon. This is because the CH bond is polarised δ-CHδ+ and the BH bond is polarised δ+BHδ.
IUPAC are considering the position of hydrogen on their official periodic
table, here.
In this web book,
electronegative elements are coloured blue
and electropositive elements are coloured
red. The
rational is that:
Electronegative
elements tend to gain electrons to become anionic Lewis bases.
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).
Thus it follows that bond type, material character and chemical reactivity can be predicted from a knowledge of electronegativity.
For example:
- Hydrogen chloride, HCl. Chlorine, 3.16, is more electronegative than hydrogen, 2.20, so the H–Cl bond will be polarised Hδ+–Clδ–. This is pronounced "delta plus" and "delta minus". This tells us that HCl will react as H+ and Cl–, and HCl is a proton donating Brønsted acid.
- Methyl bromide, CH3Br, has a C–Br bond that is polarised Cδ+–Brδ– and the carbon atom in the molecule is susceptible to nucleophilic substitution.
- There are many examples like this in chemistry.
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, 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 both the
geometric and arithmetic mean relationships are given.
For 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:
- The chlorine atom, Cl
- The dichlorine molecule,
Cl2
- Ionic sodium chloride,
NaCl
- Molecular chloromethane,
CH3Cl
- etc.
- 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:


Captured from The
Nature of The Chemical Bond, 3rd Ed, pp99. The experimental values
are from vapour phase dipole moments.
- Electronegativity can be
used to predict metallic, ionic, covalent and intermediate bond type,
and these behaviours can be mapped to the Van Arkel-Ketelaar Triangle
of Bonding, as discussed in detail on the next page of the Chemogenesis
web book, here.
- When valency is included
as an additional parameter, electronegativity can be mapped to the Laing
Tetrahedron of Bonding & Material Type, as discussed on the
next but one page of the Chemogenesis web book, here.
- Electronegativity can be
used to predict chemical reactivity because: "The most stable arrangement
of [polar] covalent bonds connecting a group of atoms is that arrangement
in which the atom with the highest electronegativity be bonded to the
atom with the lowest electronegativity." Jolly, Modern Inorganic Chemistry,
McGraw-Hill (1985) pp 61-62.
It follows that pairs of compounds of the type A-Bm
and X-Yn will react with each other to maximise
and minimise electronegativity difference, as discussed on this page of The Chemogenesis web book: Why
Do Chemical Reactions Happen?, here.
- Electronegativity, along
with bond-length, pKa and other data, is
central to the chemogenesis analysis, as discussed in the sections of
this web book: Quantifying Congeneric Behaviour and Congeneric
Array Interactions, here
and here.
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. Abstract |
Huggins
Scale
1953 |
Alternative
to Pauling's thermochemical procedure. 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, is used to quantify metal-covalent-ionic bonding,
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 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 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.
Thanks to Bruce
Railsback for his helpful comments about this page.
  
Binary Compound Synthlet |
Binary Materials: van Arkel-Ketelaar Triangles
|
© Mark R. Leach 1999-
Queries, Suggestions, Bugs, Errors, Typos...
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please contact Mark R. Leach, the author, using mark@meta-synthesis.com
This free, open
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