[antimatter101]

I copied down the whole list of element electronegativities (Pauling units) from hydrogen to lawrencium in my science book, but I can't calculate the electronegativities of molecules. What is the method?

My second question is how come some atoms like fluorine and chlorine are able to strongly possess an extra electron (giving them a charge of - 1), so strongly possess that other atoms do not remove that electron from the atom?

Please answer.

[SapereAude1490]

I don't think molecules have electronegativity - look at the definition:

"Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself." - wikipedia

The difference in electronegativity can tell you if a molecule is polar, non-polar or ionic. Also, some groups can "hog" electrons, as I remember from organic chemistry and you can call them electronegative.

I think I can answer the second question, but please take this with a pinch of salt.

The closer the electron is to the nucleus, the stronger the electrostatic force. Fluorine is a very small atom, and the p orbitals are closer to the nucleus than in other atoms; the electrostatic attraction between the nucleus and an e^- is very strong. That's why you never see F⁺ except if you bombard it with some kind of radiation, but that's another story. (this story (ionization energy) is actually very important depending on which electronegativity you are talking about)

Now, when fluorine forms an ionic bond with say Na, that new electron finds it's place in a p orbital.
You may think that other electrons (-) push it away, and you would be right. However, the effective charge (Z_eff)the new electron is experiencing is so strong (compared to other atoms in the PTE) that no other atom can pull it away. Fluorine just won't let go of it.

(unless a ₂⁴He core flies nearby  )

[antimatter101]

I mean, how to calculate the EXACT polarity of a molecule, like EXACTLY how much positive or negative charge each atom in the molecule has. I know how to determine whether a molecule is polar, but how polar?
And why does chlorine in HCl chemical reactions take other's atom's electrons instead of hydrogen's?

[antimatter101]

seriously can anyone reply?

[Borek]

I doubt it can be done. Reality is not that simple.

[Babcock_Hall]

One can use quantum mechanics to calculate electron density.  Then one has to do some sort of population analysis.  This is far away from the sort of chemistry I normally do, so take it with generous grains of salt.
http://www.huntresearchgroup.org.uk/teaching/teaching_comp_chem_year4/L7_bonding.pdf

[curiouscat]

One can use quantum mechanics to calculate electron density.  Then one has to do some sort of population analysis.  This is far away from the sort of chemistry I normally do, so take it with generous grains of salt.
http://www.huntresearchgroup.org.uk/teaching/teaching_comp_chem_year4/L7_bonding.pdf

I used to do this but it has been a while. Basically first you solve for the electronic density. Then there are ways to project this density (or charge distribution) onto atoms. We called it "Projected Charge". And then you could calculate this polarity.

[Borek]

Just remember QM calculations have nothing to do with the original question about using electronegativity to calculate molecule properties.

[curiouscat]

I mean, how to calculate the EXACT polarity of a molecule, like EXACTLY how much positive or negative charge each atom in the molecule has. I know how to determine whether a molecule is polar, but how polar?
And why does chlorine in HCl chemical reactions take other's atom's electrons instead of hydrogen's?

What if you treat it as a bunch of dipoles and calculate a vector resultant net dipole?

[Babcock_Hall]

Just remember QM calculations have nothing to do with the original question about using electronegativity to calculate molecule properties.

Good point.  I was responding more to the third comment in the thread, although I may have misunderstood it.