Answer
0.34 nm
Work Step by Step
See Figures 29-19 and 29-20. In this cubic-based packing, each Na ion is surrounded by chlorine ions a distance d away. The distance from a sodium ion to the nearest sodium ion is the diagonal of the square whose sides are of length d.
$$D = d\sqrt{2}=(0.24 nm)\sqrt{2}=0.34 nm$$
