Question

KF crystallizes in the NaCl type structure. If the radius of $K^{+}$ ions $132$ pm and that of $F^{-}$ ion is $135$ pm, what is the closest KF distance (in pm)?

• A. $428$
• B. $534$
• C. $755$
• D. $378$

Answer 1

The correct option is D $378$

NaCl has a cubic unit cell. It is best thought of as a face-centered cubic array of anions with an interpenetrating fcc cation lattice (or vice-versa). The cell looks the same whether you start with anions or cations on the corners. Each ion is 6-coordinate and has a local octahedral geometry.

Length of unit cell = 2*(R(cation) + R(anion)) a = 534 pm Closest K-K distance = a/√2 = 378 pm