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

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Solution:

## NaCl has fee structure. In fee lattice $r^+ r^- = \dfrac {a}{2}$ 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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