Rubidium ionic crystal radius

Each rubidium halide crystallizing in the NaCl-type lattice has a unit cell length 30 pm greater than that for the corresponding potassium salt of the same halogen. What is the ionic radius of Rb" computed from these data ... 

It should not be inferred that the crystal structures described so far apply to only binary compounds. Either the cation or anion may be a polyatomic species. For example, many ammonium compounds have crystal structures that are identical to those of the corresponding rubidium or potassium compounds because the radius NH4+ ion (148 pm) is similar to that of K+ (133 pm) or Rb+ (148 pm). Both NO j and CO, have ionic radii (189 and 185 pm, respectively) that are very close to that of Cl- (181 pm), so many nitrates and carbonates have structures identical to the corresponding chloride compounds. Keep in mind that the structures shown so far are general types that are not necessarily restricted to binary compounds or the compounds from which they are named. 

---