Salt crystal Four ions $\left(\mathrm{Na}^{+}, \mathrm{Cl}^{-}, \mathrm{Na}^{+}, \text { and } \mathrm{Cl}^{-}\right)$ in a row are each separated from their nearest neighbor by $3.0 \times 10^{-10} \mathrm{m} .$ The charge of a sodium ion is $+e$ and that of a chlorine ion is $-e$ . Determine the electric force exerted on the chlorine ion at the right end of the row due to the other three ions.