Answer
The net electrostatic force on the $CL^{-}$ ion due to the eight $Cs^{+}$ ions is zero.
Work Step by Step
By symmetry, the electrostatic forces on the $CL^{-}$ ion due to two $Cs^{+}$ ions in opposite corners of the cube cancel out.
For every $Cs^{+}$ ion, there is another $Cs^{+}$ ion in the opposite corner.
Therefore, the net electrostatic force on the $CL^{-}$ ion due to the eight $Cs^{+}$ ions is zero.
