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  4. The radii of Na+ and Cl- ions are 95 pm and 181 pm respectively. The edge length of NaCl unit cell is

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

KCETKCET 2006The Solid State

Solution:

NaCl has fcc structure.

In fcc lattice $r^{+}+r^{-}=\frac{a}{2}$

where $a=$ edge length

$r^{+} =95 \,pm r^{-}=181 \,pm $

Edge length $=2 r^{+}+2 r^{-}=(2 \times 95+2 \times 181) pm$

$=190+362=552 \,pm$