What is the magnitude of the electrostatic force between a singly charged sodium ion and an adjacent singly charged chlorine ion?

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.