1. The problem statement, all variables and given/known data
Calculate the attractive force between a pair of Cu^{2+} and O^{2-} ions in the ceramic CuO that has an interatomic separation of 200pm.

2. Relevant equations
[itex]E_A= -\frac{(z_1\cdot e)(z_2\cdot e)}{4\pi\cdot\epsilon_o\cdot r}[/itex]
Where z_1 and z_2 are the valences of the two ion types, e is the charge of an electron (1.602 * 10^-19 C), epsilon_o is the permittivity of a vacuum (8.85*10^-12 F/m), and r is the distance between the two ions.

[itex]E_n=\frac{m\cdot e^4 \cdot z^2}{2n^2 \cdot \hbar^2}[/itex]
Where m= mass of electron, z= atomic number, e= charge of an electron, n is the energy level.

3. The attempt at a solution
The problem is that I don't know how to find z_1 and z_2. Do I use [itex]E_n=\frac{m\cdot e^4 \cdot z^2}{2n^2 \cdot \hbar^2}[/itex] to find the energy in the valence electrons? The problem is that I don't know how to use that equation because when I plug in what I think I should for the variables it gives me an answer with units all wrong... Here's an example from another problem where I tried to use that equation...

Well I don't know the force equation, my teacher only gave us the equation for bonding energy...
Perhaps since energy=force*distance we can find force by dividing our energy equation by some distance?

I'm still stuck but I see now that z_1= 2 and z_2=-2.

The equation F= k_{e}(|q_{1}q_{2}|)/r^{2} looks good.

So if I plug in 3.204 × 10^-19 coulombs for q_{1} and -3.204 × 10^-19 coulombs for q_{2} (because O2- has a net charge equal to -2 times the charge of an electron and Cu2+ has a net charge equal to twice the charge of an electron), then I get