Solid state - crystallisation energy

[kel]

Homework Statement

The ionisation energy of Li is 5.39eV and the electron affinity of F is 3.40eV. Give a lower bound for the modulus of the crystallisation energy of Li+F-

Homework Equations

Not entirely sure of the equation to use, but have this one in my notes:

U = - (q1q2/r(1,2)) . alpha . (1 / 4Pi E)

where E is the permeability of free space (I believe)

The Attempt at a Solution

I haven't really attempted this to any meaningful degree, the problem is that I don't know how I can work this out when I'm not given a value for r (presumably the radius between atoms or ions??) nor am I sure what values I'm supposed to use for the charges q1 and q2.

I don't expect anyone to solve this for me, but if you could clarify what you think I need to do to work this out I'd be greatful. My lecturer is great on giving derivations and formulas, but never seems to explain things very well.

Cheers
Kel

 

[Chi Meson]

The formula you gave is for the electric potential energy of two charged objects near each other. I'm looking through my various books, and I can't find reference to "modulus of crystalization energy." sounds like chemistry to me. I think you might want to repost in "advanced" or "other sciences" if no one helps you soon.

 

[Astronuc]

Perhaps the following link help. Is there any discussion in one's textbook on the modulus of crystallization energy and how is relates to ionization energy and electron affinity, since the problem states, "The ionisation energy of Li is 5.39eV and the electron affinity of F is 3.40eV"?


http://www.almazoptics.com/LiF.htm
lattice constant - 4.03 Å or 403 pm - IIRC, this is the spacing between successive F atoms, so take about half or 202 pm as the space between Li and F. Also, see the bottom link - is which give crystallization energy in kJ/mol rather than on a per atom or bond basis.

The sum of the covalent radii are close to half of the LiF lattice constant.
http://www.webelements.com/webelements/elements/text/Li/radii.html
covalent radius - 134 pm

http://www.webelements.com/webelements/elements/text/F/radii.html
covalent radius - 71 pm

http://www.science.uwaterloo.ca/~cchieh/cact/applychem/lattice.html