[noob_]

Hello cf,
my question is what is the relationship between the core charge of an atom and its' electronegativity.
I know the corecharge is the attraction felt by the outershell electrons (keeping in mind the shielding from the inner shell electrons) and i know that the electronegativity is a measure of an atoms ability to attract electrons to itself. So my initial thought is that the higher the electronegativity, the higher the corecharge. However i just needed to get this clarified, thanks!
(Sounds pretty basic hah i feel a bit stupid even asking this but i do so because i see that the Electronegativity of fluorine is 4 and it has a corecharge of +7 and the EN (electroneg...) of Chlroine is 3.2 BUT it also has a core charge of 7)
Thanks again
Edit: Also, i was wondering why are the elements of group 18 omitted from tables that give electronegativity values.
so my two questions are: what is the relationship between the core charge of an atom and its' electronegativity?
AND why are the elements of group 18 omitted from tables that give electronegativity values?
Thanks again!

[cliverlong]

I think the shielding effect of electrons in same and inner orbits is relevant

http://www.chemguide.co.uk/basicorg/bonding/eneg.html

[johnmalkinson]

As you move across a period (left to right), what happens to the number of protons in the nucelus? It gradually increases. The number of electrons increases in the same way, BUT those electrons are in the same shell (similar energy). Consequently, they are "pulled" more and more by the nucleus that has greater and greater nuclear charge. Think about how this influences other properties such as atomic radius; the ability of the atom to draw electron density towards itself (electronegativity) will reflect this trend.
What then happens when you go down to the next period? The nuclear charge still increases, but what about the electrons' energy levels now (compared to the previous group)? You are right that the shielding effect is very important here. The observed trend in electronegativity as you go down a GROUP is a decrease (compare again to atomic radius). Electrons going into the valence (outermost) shell of a bigger atom have less tendency to do so as these electrons feel less electrostatic attraction from the nucleus. You need to be able to explain this in terms of shielding and energy level/distance from nucleus.
John