Re: What do the numbers assigned to represent electronegativity really mean?

Date: Tue Oct 5 12:35:07 1999
Posted By: Raymond Cheong, Undergraduate, Chemical Engineering, University of Maryland
Area of science: Chemistry
ID: 938208452.Ch

Message:

Jennifer,

Electronegativity represents the ability of an atom to attract electrons to itself in a chemical bond. Higher electronegativities mean electrons are attracted more (and hence, a partial negative charge). In the periodic table, electronegativity increases left to right and down to up. So, flourine is the most electronegative atom.

Electronegativity values are unitless. They are simply relative values. However, the most commonly accepted values are based on Linus Pauling's measurements. (Linus Pauling was arguably the best chemist who ever lived and the only person to win two unshared Nobel prizes.) His model measured the difference between the expected versus actual bond energy for an H-X bond, where X is some element. The expected bond energy is:

Expected bond energy = (H-H bond NRG + X-X bond NRG)/2

The actual bond energy differs because in a bond with unequal electron sharing, the atoms develop partial charges that electrostatically attract each other. This attraction increases the bond strength. Based on these kinds of measurements, Pauling was able to develop an electronegativity scale.

Based on chemists' experience with different molecules, it was determined that ionic bonds generally had an electronegativity difference greater than 1.7 (e.g. NaCl, diff = 2.1). Differences less than 1.7 are polar covalent (e.g. O-H, diff = 1.4); differences exactly equal to 0 are pure covalent (e.g. H-H, diff = 0).

Thanks for the good question and I hope you stay interested in chemistry!

Your MAD Scientist,
Raymond Cheong

Got a problem? Ask Chemists! They have all the solutions. -- http://nmr.chem.ohiou.edu/leo/jokes/

References:

Zumdahl, Steven S. Chemistry, Third Edition. Lexington, MA: D.C. Heath and Company, 1993. pp. 345-347

For information on Linus Pauling, see
http://www.nobel.se/lau reates/chemistry-1954.html and
http://www.nobel.se/laureat es/peace-1962.html * The 1999 Nobel prize in literature has been announced (Günter Grass). The remaining 1999 Nobel prizes will be announced between Oct 11 - Oct 15.

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