Lattice energy

by Soilwork
Tags: energy, lattice
 P: 87 I've just come across a problem and the equation within this problem isn't entirely explained in the text. I've searched on the net for this same 'lattice energy' equation, but there have been no results. Here is the equation: $$E = Aexp^ \frac{-r} { \alpha} - \frac{Me^2} {4 \pi \epsilon_0 r}$$ Here is the question that the equation came from: The lattice energy, E, per ion of LiF may be written in the form $$E = Aexp^ \frac{-r} { \alpha} - \frac{Me^2} {4 \pi \epsilon_0 r}$$, where M is the Madelung constant, e is the electronic charge, r is the separation of neighbouring ions and A and $$\alpha$$ are constants. Calculate E for LiF. (M=1.75, $$\alpha$$ = 0.02 nm; lattice parameter of LiF = 0.4 nm. Anyway I was wondering if anyone knew what the first exponential equation was and if they could tell me what the constant A is?? Is it calculated using any of the given values above or something?? EDIT: Well I know that I have to calculate it now, but I have no idea what this constant is called or where it comes from. Thanks anyway guys :)