# Coulomb Energy

1. Feb 3, 2013

### mateomy

The ionization of potassium is 4.34 eV; the electron affinity of iodine is 3.06 eV. At what separation distance will the KI molecule gain enough Coulomb energy to overcome the energy needed to form the $K^+$ and $I^-$ ions?

I'm looking at the solution to this problem and I'm not understanding how they get the Coulomb energy. Is it just the
$$E_n=\frac{-Z^2 (13.6eV)}{n^2}$$
equation? I'm not getting the same numbers they're getting so I think I'm confused on the definition of Coulomb energy. Can someone clarify it for me. I understand the Coulomb repulsion/attraction is what I've seen over and over again from electrostatics...but I don't see how to tie that into this problem. Specifically because I don't know the radius of the particular ions.

They're getting values of 3.06 and 4.34 eV