Today, February 28, marks the birthday of Linus Pauling. For chemists Pauling likely needs no introduction; he’s famed for his work on the nature of chemical bonds and also on the structures of biological molecules. Here we take a brief look at one aspect of his work to which he lent his name: the Pauling electronegativity scale.
Though Pauling’s name is lent to the scale that measures it, electronegativity wasn’t a concept that he came up with. In fact the term was introduced way back in 1811 by another famous chemist, Jöns Jacob Berzelius. However, Pauling was the first scientist to propose a measurable scale for electronegativity and to fully define the concept as we know it today.
Electronegativity is a measure of an atom’s ability to attract electrons when it is part of a compound. This is a property that varies for different elements as a number of factors can affect it: the number of protons in an element’s nucleus, the distance of bonding electrons from the nucleus, and screening of nuclear charge by an atom’s electrons can all play a part.
Pauling based his scale on bond energy calculations for different elements joined by covalent bonds (a type of chemical bond formed when atoms share electrons). The higher the value on Pauling’s scale, the greater the ability of the atom to attract electrons when it’s found in a compound. The elements with the lowest values are found in the bottom left of the Periodic Table, with Francium having the lowest value (0.7). Fluorine, in the top right of the Periodic Table, has the highest value for electronegativity (3.98).
If we know the values for electronegativity of two adjacent atoms in a chemical compound, we can use it to discern the nature of the chemical bond between them. If there is little or no difference, electrons are evenly shared between the two atoms, and the bond is covalent. An intermediate difference between their electronegativities gives rise to what’s termed a polar covalent bond, where one atom exerts a stronger pull on the electrons between them than the other. Consequently partial charges are seen on either atom. If there is a suitably large difference between the electronegativities, the bond is ionic, and the atoms will have formal positive and negative charges.
If that all seems a little complicated, James Kennedy put together a great graphic to summarise it all. A larger version is available on his site.
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