Electronegativity, symbol
χ (the Greek letter
chi), is a
chemical property that describes the ability of an
atom (or, more rarely, a
functional group) to attract
electrons (or electron density) towards itself."
Electronegativity.",
IUPAC Compendium of Chemical Terminology An atom's electronegativity is affected by both its
atomic weight and the distance that its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an element or compound attracts electrons towards it. First proposed by
Linus Pauling in 1932 as a development of
valence bond theory,{{cite journal |author= Pauling, L.
|authorlink=Linus Pauling
|year= 1932
|journal=
Journal of the American Chemical Society
|volume= 54
|issue= 9
|pages= 3570–3582
|title= The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativity of Atoms
|doi= 10.1021/ja01348a011}} it has been shown to correlate with a number of other chemical properties. Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed and, although there may be small differences in the numerical values of the electronegativity, all methods show the same
periodic trends between
elements.
The most commonly used method of calculation is that originally proposed by Pauling. This gives a
dimensionless quantity, commonly referred to as the
Pauling scale, on a relative scale running from 0.7 to 4.0 (
hydrogen = 2.2). When other methods of calculation are used, it is conventional (although not obligatory) to quote the results on a scale that covers the same range of numerical values: this is known as an electronegativity in
Pauling units.
Electronegativity, as it is usually calculated, is not strictly an atomic property, but rather a property of an atom in a
molecule: the equivalent property of a free atom is its
electron affinity. It is to be expected that the electronegativity of an element will vary with its chemical environment, but it is usually considered to be a
transferable property, that is to say that similar values will be valid in a variety of situations. The opposite of electronegativity is
electropositivity: a measure of an element's ability to donate electrons.
Electronegativities of the elements
Periodic table of electronegativity using the
Pauling scale
See also
Periodic table
Methods of calculation
Pauling electronegativity
Pauling first proposed the concept of electronegativity in 1932 as an explanation of the fact that the covalent bond between two different atoms (A–B) is stronger than would be expected by taking the average of the strengths of the A–A and B–B bonds. According to
valence bond theory, of which Pauling was a notable proponent, this "additional stabilization" of the heteronuclear bond is due to the contribution of ionic
canonical forms to the bonding.
The difference in electronegativity between atoms A and B is given by:
:\chi_{\rm A} - \chi_{\rm B} = ({\rm eV})^{-1/2} \sqrt{E_{\rm d}({\rm AB}) -
d}({\rm AA}) + E_{\rm d}({\rm BB})/2}
where the
dissociation energies,
Ed, of the A–B, A–A and B–B bonds are expressed in
electronvolts, the factor (eV)–½ being included to ensure a dimensionless result. Hence, the difference in Pauling electronegativity between hydrogen and
bromine is 0.73 (dissociation energies: H–Br, 3.79 eV; H–H, 4.52 eV; Br–Br 2.00 eV)
As only differences in electronegativity are defined, it is necessary to choose an arbitrary reference point in order to construct a scale. Hydrogen was chosen as the reference, as it forms covalent bonds with a large variety of elements: its electronegativity was fixed first at 2.1, later revised{{cite journal
|author= Allred, A. L.
|year= 1961
|journal= Journal of Inorganic and Nuclear Chemistry
|volume= 17
|issue= 3–4
|pages= 215–221
|title= Electronegativity values from thermochemical data
|doi= 10.1016/0022-1902(61)80142-5}} to 2.20. It is also necessary to decide which of the two elements is the more electronegative (equivalent to choosing one of the two possible signs for the square root). This is done by "chemical intuition": in the above example,
hydrogen bromide dissolves in water to form H+ and Br– ions, so it may be assumed that bromine is more electronegative than hydrogen.
To calculate Pauling electronegativity for an element, it is necessary to have data on the dissociation energies of at least two types of covalent bond formed by that element. Allred updated Pauling's original values in 1961 to take account of the greater availability of thermodynamic data, and it is these "revised Pauling" values of the electronegativity which are most usually used.
Mulliken electronegativity
Mulliken proposed that the
arithmetic mean of the first
ionization energy and the
electron affinity should be a measure of the tendency of an atom to attract electrons.{{cite journal
|author = Mulliken, R. S.
|year =1934
|journal =
Journal of Chemical Physics
|volume = 2
|title = A New Electroaffinity Scale; Together with Data on Valence States and on Valence Ionization Potentials and Electron Affinities
|doi = 10.1063/1.1749394
|pages = 782–793}}{{cite journal
|author = Mulliken, R. S.
|year =1935
|title = Electronic Structures of Molecules XI. Electroaffinity, Molecular Orbitals and Dipole Moments
|journal =
J. Chem. Phys.
|volume = 3
|doi = 10.1063/1.1749731
|pages = 573–585}} As this definition is not dependent on an arbitrary relative scale, it has also been termed
absolute electronegativity,{{cite journal|author = Pearson, R. G.
|title = Absolute electronegativity and absolute hardness of Lewis acids and bases
|year = 1985
|journal =
J. Am. Chem. Soc.
|volume = 107
|pages = 6801
|doi = 10.1021/ja00310a009}} with the units of
kilojoules per mole or
electronvolts.
However, it is more usual to use a linear transformation to transform these absolute values into values which resemble the more familiar Pauling values. For ionization energies and electron affinities in electronvolts,{{cite book|author = Huheey, J. E.
|year = 1978
|title = Inorganic Chemistry (2nd Edn.)
|publisher = New York: Harper & Row. p. 167}}
:\chi = 0.187(E_{\rm i} + E_{\rm ea}) + 0.17 \,
and for energies in kilojoules per mole,This second relation has been recalculated using the best values of the first ionization energies and electron affinities available in 2006.
:\chi = (1.97\times 10^{-3})(E_{\rm i} + E_{\rm ea}) + 0.19.
The Mulliken electronegativity can only be calculated for an element for which the electron affinity is known, fifty-seven elements as of 2006.
Allred–Rochow electronegativity
Allred and Rochow considered that electronegativity should be related to the charge experienced by an electron on the "surface" of an atom: the higher the charge per unit area of atomic surface, the greater the tendency of that atom to attract electrons. The effective nuclear charge,
Z*experienced by
valence electrons can be estimated using
Slater's rules, while the surface area of an atom in a molecule can be taken to be proportional to the square of the
covalent radius,
rcov. When
rcov is expressed in
ångströms,
:\chi = 0.359} + 0.744.
Sanderson electronegativity
Sanderson has also noted the relationship between electronegativity and atomic size, and has proposed a method of calculation based on the reciprocal of the atomic volume. With a knowledge of bond lengths, Sanderson electronegativities allow the estimation of bond energies in a wide range of compounds. Also Sanderson electronegativities were used to calculate molecular geometry,
s-electrons energy,
NMR spin-spin constants and other parameters for organic compounds. This work underlies the concept of
electronegativity equalization, which suggests that electrons distribute themselves around a molecule to minimize the energy, or to equalize the electronegativity.