Re: Why does EN for noble gases exclude helium and neon?

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:  
weibelm@battelle.org

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.