If the distance between Na+ and Cl− ions in sodium chloride crystal is y pm, the length of the edge of the unit cell is:

NaCl is a face centred cubic lattice in which Na+ is placed in octahedral voids which are present along the edges of the FCC lattice. Therefore length of unit cell will be twice the distance between Na+ and Cl− ions. Given, (r+ + r−) = y pm a = 2(r+ + r−) = 2y pm Option D is correct.