Q. The radii of $Na^+$ and $Cl^{-1}$ ions are 95 pm and 181 pm respectively. The edge length of NaCl unit cell is

Solution:

NaCl has fee structure. In fee lattice
$r^+ r^- = \frac {a}{2}$
where a = edge length
$r^+ 95 \,\, pm \,\,\,\,r ^- = 181 \,\, pm$
Edge length =$2r^+ + 2r^- $
$= (2 \times 95 + 2 \times 181 ) $pm
$= 190 + 362 = 552 \,\,$pm

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