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Our teacher gave us the following equation for the Fnet between two ions: F=Fcoul+Frepul= -Aa^2/r^2 + Ba^10/r^10 (where a is the equilibrium point,that is Fcoul=Frepul) Ok, I get that r^2 in Fcoul coming from the Fc=K Q1xQ2/r^2 but where does that r^10 come from? I've read that it's actually r^n and n is usually from 7 to 10. What is n and how can I calculate it?
Here is what I found: enter image description here "where r is the center to center, spacing between the atoms and A, B, M and N are constants characteristic of the molecule. The first term represents the attractive force and the second term the repulsive force. Near the equilibrium position the second term must increase more rapidly for diminishing values of r than does the first, and N is necessarily greater than M. Since the attractive forces in interatomic bonds are largely electrostatic, M is usually 2 as per Coulomb's law and the value of N is usually 7 to 10."

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The Coulombic attraction is simply for the two ions as they approach since they have different charges.

The rub here is that two ions like $^{23}_{11}\rm{Na}^+$ and $^{37}_{17}\rm{Cl}^-$ can't collapse together to form an atom of $^{60}_{28}\rm{Ni}$ by a chemical reaction.

Rather as the atoms approach the electrons shells of one atom starts to interact with the nucleus of the other canceling the charge. Also the two nuclei are both positive so they start to repel each other. Thus all in all the exact expression for the repulsion force is very messy. To calculate this accurately you would have to do quantum mechanical modeling.

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