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If the distance between $\mathrm{Na}^{+}$and $\mathrm{Cl}^{-}$ions in sodium chloride crystal is $x \mathrm{pm}$, the length of the edge of the unit cell is
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The correct answer is:
$2 x \mathrm{pm}$
In fcc unit cell of $\mathrm{NaCl}, \mathrm{Na}^{+}$ion is present at edge centre and $\mathrm{Cl}^{-}$ion is present at corner.
$\therefore$ Edge length, $a=2 r^{+}+2 r^{-}$
$\therefore \quad a=2\left(r^{+}+r^{-}\right)$
$=2 x$ $\left[\because x=r^{+}+r^{-}\right]$
$\therefore$ Edge length, $a=2 r^{+}+2 r^{-}$
$\therefore \quad a=2\left(r^{+}+r^{-}\right)$
$=2 x$ $\left[\because x=r^{+}+r^{-}\right]$
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