excess charge on each ion is equivalent to the charge of two electrons
now we have
[tex]q = 2e = 3.2 * 10^{-19}C[/tex]
now the electrostatic force between them is given by
[tex]F = \frac{kq_1q_2}{r^2}[/tex]
[tex]F = \frac{9*10^9 * 3.2* 10^{-19}* 3.2 * 10^{-19}}{1.25 * 10^{-9}^2}[/tex]
so from above the force between them is
[tex]F = 5.89 * 10^{-10} N[/tex]
