# What is the distance r^n in the pauli repulsion force between two ions?

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: "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."

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.