|MadSci Network: Chemistry|
Melinda, the quantity "electronegativity" is a general value which incorporates both an atom's ability to gain an electron (termed electron affinity) and to lose an electron (ionization potential). Electronegativities are values which give a general indication of the type of bonding an atom undergoes... for example, a high electronegativity indicates a strong ability to accept an electron (fluorine atoms, for example), while a low electronegativity indicates a low tendency to accept an electron (typically these are electron donors, such as sodium)...then high and low electronegativities are indicative of forming ionic bonds, since one material donates an electron and another accepts it (versus covalent bonding, where the electrons are shared). Noble gases cannot accept an electron, as they become unstable and re- eject it immediately. Also, noble gases cannot donate an electron without a large energy expenditure. While Xe is stable, it can form a stable molecule of XeF2 (very weak bonding, maybe not even covalent or ionic, but a strong Van Der waals interaction), but it takes an energy input to create it. As such, there is no tendency for it to donate or accept an electron spontaneously. Probably, this is why electronegativities for Noble gases are not generally reported (they are not zero, but undefined) See also any general chem textbook, and the CRC Handbook. Please feel free to email me if you have further questions: email@example.com Best Regards, Mike Dan Berger adds: Covalent bonds are strengthened by electronegativity difference, for reasons I won't go into here. For example, an O-H bond (electronegativity difference is about 1.5 Pauling units) is stronger than a C-H bond (electronegativity difference less than 0.5 Pauling units). Therefore one of the inputs for calculating the electronegativity of an element X is the strength of the X-X bond versus the strength of the X-F bond (F is fluorine, with electronegativity defined as 4.0). Noble gases don't form bonds to themselves, and so again their electronegativities are not defined.
Try the links in the MadSci Library for more information on Chemistry.