# Applying Coulomb's law to HCl and LiF

## Homework Statement

During a chem lesson my professor was demonstrating Coulomb's law using HCl.

Fe = kq1q2/r^2

so what he did was he replaced q1 and q2 with 0.17 and -0.17.

Then he showed an example with LiF and then replaced q1 and a2 with 1 and -1.

I think he was trying to show us how strong ionic bonds were. Can someone please explain why and how the numbers 0.17 and -0.17 were chosen for HCl and 1 and -1 were chosen for LiF?

mfb
Mentor
LiF has a large difference in electronegativity between the different atoms, the bond is ionic (one electron moves from Li to F, so +1 and -1 are good approximations). For HCl, the electronegativity is more similar, so you just get some polarization. 0.17 is probably the result of a measurement or a simulation, there is no easy way to get that number.

So are you saying that +1 and -1 and 0.17 and -0.17 are determined experimentally. Secondly are they charges so does that mean that the lithium atom has a charge of 1 C and the fluorine atom has a charge of -1 C. If so what is the explanation for why the one with the ionic bond had such higher charges? Sorry this entire concept of quantitatively calculating the electrostatic forces involved in a bond is new to me. Thanks again!

mfb
Mentor
So are you saying that +1 and -1 and 0.17 and -0.17 are determined experimentally.
I don't say they are, I say it is possible.

Secondly are they charges so does that mean that the lithium atom has a charge of 1 C and the fluorine atom has a charge of -1 C.
Yes. Look up electronegativity. A single fluorine atom has one free spot in the second shell, which has a lower energy level than the single electron in the second shell of lithium (because the lithium nucleus has just 3 positive charges, while fluorine has 9).

So electronegativity is the tendency of an atom to attract electrons in a bond. We know that fluorine has a greater electronegativity than lithium and therefore attracts more electrons. In HCl chlorine is more electronegative than hydrogen. For LiF the electronegativity difference is greater making it an ionic bond and HCl a polar attraction. Knowing this why are the charges (q1 and q2) for lithium and fluorine so much greater than the charges for hydrogen and chlorine (q1 and q2)? Is it because the fluorine atom takes more electrons from the lithium (due to its electronegativity) which increases the overall charge of fluorine to 1 C and the overall charge of lithium to -1 C, both of which are numbers that may be determined experimentally? Is my reasoning correct?

mfb
Mentor
Knowing this why are the charges (q1 and q2) for lithium and fluorine so much greater than the charges for hydrogen and chlorine (q1 and q2)?
Still due to the different differences in electronegativity (3.0 versus 1.0).
Fluorine takes "exactly" one electron from lithium.
Hydrogen and chlorine share two electrons (one from each atom), both a bit more on the chlorine side.