Electronegativity Explained

Electronegativity is a measure of the attraction of an atom for bonding electrons in molecules compared to that of other atoms. For example – For molecule HCL, it is well established that electrons in a covalent bond between hydrogen and chlorine atom are more attracted towards chlorine as compared to hydrogen. This phenomenon is called Electronegativity.

Linus Pauling’s Definition of Electronegativity

As Electronegativity is a measure of attraction of electrons by an atom in a molecule there’s a lot of disagreement about how to measure it? or what exactly is Electronegativity? That’s why during 20th century many chemists have came to different conclusions about Electronegativity.

First definition of Electrochemistry was proposed by Linus Pauling in 1932 in his paper titled “The Nature Of The Chemical Bond, The Energy Of Single Bonds And The Relative Electronegativity Of Atoms”. In this paper, Linus defined “Electronegativity as power of an atom in a molecule to attract electrons towards itself”. He also concluded a mathematically formula for calculating Relative Electronegativity of an atom with respect to another atom in a molecule.

Δ = (X-Y)measured – (X-Y)expected
Here Δ = Bond Energy
(X-Y)expected = Average of X-X bond energy and Y-Y bond energy
(X-Y)measured = X-Y bond energy calculated using experiment

Then Pauling used Δ = Bond Energy calculated from above equation and through experiments came to conclusion that Relative Electronegativity of X with respect to Y minus Relative Electronegativity of Y with respect to X will be 0.102(under root of Δ).

\left|\chi_{X}-\chi_{Y}\right|=0 \cdot 208 \sqrt{\Delta}
Here ΧX and ΧY are Electronegativities of element X and Y

As these are Relative Electronegativies, in order to calculate Absolute Electronegativies of different elements. Pauling assigned a value of 4.0 to Electronegativity of Fluorine and based upon this assumption he calculated electronegativities for other elements.

Below is a table containing Electronegativity predictions which were made by Linus Pauling assuming Electronegativity of Fluorine to be 4.0

ElementPredicted Electronegivity by Linus Pauling
Hydrogen2.1
Lithium1.0
Sodium0.9 (Minimum Electronegativity)
Beryllium1.5
Magnesium1.2
Boron2.0
Aluminum1.5
Carbon2.5
Silicon1.8
Nitrogen3.0
Phosphorus2.1
Oxygen3.5
Sulfur2.5
Flourine4.0 (Maximum Electronegativity)
Chlorine3.0
Bromine2.8
Iodine2.5

Later in 1961, A. L. ALLRED from Department of Chemistry at Northwestern University took newly available bond energies as calculated by experimentations by other chemists and then re-evaluated Electronegativities of elements using methodology as was proposed by Linus Pauling in 1932.

Below is a plot on periodic table mentioning Electronegativities of elements as was calculated by A. L. ALLRED.

Plot containing Average Electronegativities of Elements calculated by AL Allred in 1961 using Linus Pauling Methology

Mulliken’s Definition of Electronegativity

In 1934, Mulliken proposed Absolute Electronegativity in terms of ionization energies and electron affinities of atoms in a molecule. This means that for an atom X, it’s Absolute Electronegativity should be 0.5(Ionization Energy of X + Electron Affinity of X).

Mulliken stipulated that the ionization energy and electron affinity should correspond to the atom in its valence state, which he defined as having “as nearly as possible the same condition of the interaction of the atom’s electrons with one another as when the atom is part of a molecule”. Thus he came to following conclusion regarding Electronegativity of an atom.

\chi=0.5\left(I_{\mathrm{vs}}+A_{\mathrm{vs}}\right)
Here \chi is Absolute Electronegativity
I_{\mathrm{vs}} is ionization energy of atom in valence state
A_{\mathrm{vs}} is electron affinity of atom in valence state


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