Electronegativity is a measure of the attraction that an atom has for the bonding pair of electrons in a covalent bond.
Two scales of electronegativity are in common use: the Pauling scale (proposed in 1932) and the Mulliken scale (proposed in 1934). There is also a Allred-Rochow scale.
Pauling Scale
The Pauling scale was devised in 1932. On this scale, the most electronegative element (fluorine) is given an electronegativity value of 4.0; the least electronegative element (francium) has a value of 0.7, and the remaining elements have values in between. The elements in the second period of the periodic table are sometimes given rounded values (to make them easier to remember):
Each element has a characteristic electronegativity ranging from 0 to 4 on the Pauling scale. A strongly electronegative element, like fluorine, has an electronegativity of 4 while weakly electronegative elements, such as lithium, have values close to 1. Electronegativity trends are to higher values for elements in the top right of the periodic table. Bonds between atoms with a large electronegativity difference (greater than or equal to 2.0 on the Pauling scale) are usually considered to be ionic, while values between 2.0 and 0.4 are considered polar covalent. Values below 0.4 are considered non-polar covalent bonds.
Electronegativity decreases down the periodic table and increases across, as shown below. Additionally, atomic radius decreases across but ionization energy increase.
Pauling was a pioneer in the application of quantum mechanics to chemistry, and in 1954 was awarded the Nobel Prize in chemistry for his work describing the nature of chemical bonds.
Pauling had formulated a model for the structure of hemoglobin in which atoms were arranged in a helical pattern, and applied this idea to proteins in general.
Pauling also studied enzyme reactions and was among the first ones to point out that enzymes bring about reactions by stabilizing the transition state of the reaction, a view which is central to understanding their mechanism of action.
On this scale, the most electronegative element (fluorine) is given an electronegativity value of 4.0; the least electronegative element (francium) has a value of 0.7, and the remaining elements have values in between.
On the Paulingscale, hydrogen is arbitrarily assigned a value of 2.1 or 2.2.
Bonds between atoms with a large electronegativity difference (greater than or equal to 2.0 on the Paulingscale) are usually considered to be ionic, while values between 2.0 and 0.4 are considered polar covalent.
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