when they are at their equilibrium spacing, what is the force between the two atoms
Sometimes it is easier to deal with potential energies (E) rather than forces. The equilibrium spacing occurs when the bond energy is a minimum. This is when the net forces between the two atoms is zero, i.e. at the equilibrium spacing, F(r) = 0 when r= r0. As the interatomic distance increases, the force required increases linearly.