electronegativity

In chemistry, electronegativity is the force at which an atom attracts electrons into covalent bonds. The larger the atom's electronegativity is, the stronger the attraction the atom has to bonding electrons. As a general rule, electronegativity increases as you move up and to the right across the periodic table of the elements. The most common values for electronegativity were first measured by Linus Pauling.

In the eighteenth century, much attention was paid to how oxygen combined with other elements. Research produced the "Scale of Oxygenicity," created in 1809 by Amedeo Avogadro. Jöns Jacob Berzelius worked further with this initial research, formulating a 'universal scale of electronegativity' of the elements in 1818 based on the fact that oxygen, acid, and oxidized substances accumulated around the positive pole of an electrolytic cell, while metals, bases, and combustible substances accumulated around the negative pole.

Beyond this, however, no one was able to fully describe electronegativity until Linus Pauling's work in 1932. Using differences in bond energies to create his theory of electronegativity, Pauling's understanding was based on the following equation:

Ψ_AB = aΨ_A­B + bΨ_A⁺B¯ + dΨ_A¯B⁺

• A and B are two different elements
• Ψ_AB stands for the energy of a bond. This number is the amount needed to dissociate the bond.
• a, b, and d are all constant terms.
• Ψ_A¯B stands for Pauling's 'normal covalent bond' for A-B.
• Ψ_A+B¯ stands for the energy arising from the "additional ionic character of the bond."

It is this variable that will be central to determining electronegativity. According to Pauling:

"... the energy of an actual bond between unlike atoms is greater than (or equal to) the energy of a normal covalent bond between these atoms. This additional bond energy is due to the additional ionic character of the bond; that is, it is the additional ionic resonance energy that the bond has as compared with a bond between like atoms."**

Further in his proof of this, he arrives at a calculation for the "normal covalent bond" energy and compares it to the measured bond energy.

Δ' = D(A­B) - [ D(A­A) ^. D(B­B) ]^1/2

• Δ' is the difference of the two values on the right side of the equation.
• D(A­B) is the measured bond energy
• [ D(A­A) ^. D(B­B) ]^1/2 stands for the means of calculating normal covalent bond energy, or in his words, the "postulate of the geometric mean."***

Understanding that the difference between the two values must be found, Pauling thinks that this is due to the difference in electronegativities of the two atoms. So, he needs to find the following:

Δ' = f (x_A - x_B)

He calculates 30 to be the correct value for f.

Δ' = 30 (x_A - x_B)²

Therefore, the bond energy is:

D(A­B) = [ D(A­A) ^. D(B­B) ]^1/2 + 30 (x_A - x_B)²

From this, Pauling assigned a value of (4) to fluorine, which is the element with the greatest electronegativity.

Linus Pauling was awarded the Nobel Prize in Chemistry in 1954 for his research in the field. Further research has been done since his pioneering efforts, expanding the linear scale of pure covalent on one end and ionic on the other, making the three bonding types of covalent, ionic and metallic.

*Linus Pauling. The Nature of the Covalent Bond. p. 88.
**Pauling, 80.
***Pauling, 83.
Thanks to Stephen F. Mason's excellent paper, "The Science and Humanism of Linus Pauling (1901 - 1904) at http://www.orst.edu/dept/Special_Collections/subpages/ahp/overview/overview2.htm