To know the electrostatic force between two charges or between two ions, you can use the Coulomb's Law. The equation is F = k*q1*q1/r^2, where F is the electrostatic force, q1 and q2 are the charger for Na and Cl, and r is the distance between the centers of both atoms. In literature, the distance is 0.5 nm or 0.5 x 10^-9 meters. The charge for Na+ and Cl- is the same magnitude but different in sign. Since Na+ is a cation, its charge is +1.603x10^-19 C (the charge of an electron). For Cl- being an anion, its charge is -1.603x10^-19 C. The constant k is an empirical value equal to 9x10^9. Using the formula:
F = (9x10^9)(+1.603x10^-19)(-1.603x10^-19)/(0.5 x 10^-9)^2 F = -9.25 x 10^-10 Newtons
The negative denotes that the net force is more towards the Cl- ion.
