Calculate Binding Energy & Equilibrium Sep. of Na+/Cl- in NaCl

[ptar]

Homework Statement

The question is;
The interaction energy between Na+ and Cl- ions in a NaCl crystal can be written as:
E(r)= -A/r + (B/r^8)
where A = 4.03 x 10^-28 J.m and B = 6.97×10^-96 J.m^8. The energy is given in Joules per
ion pair.

(a) Calculate the binding energy (E0) and equilibrium separation (r0)
between the ions. (b) Estimate the Bulk modulus.

Homework Equations

I'm fairly sure that the equilibrium separation is calculated by setting dE/dr=0 and solving for r, so that should be easily done.

What I'm really confused about is how to find the binding energy? Also can't seem to find the relevant equation for estimating the bulk modulus.

Thanks in advance for any help.

 

[turin]

What is binding energy? Hint: What is the potential energy when the two ions are "very far apart"?

For bulk modulus, you have dE/dr. What kind of quantity is that? Then, all you need is the density, which you should be able to intuite from the r0. Think in terms of "small perturbations".