Question

One ionic unit cell of $NaCl$ contains $X$ atoms of $N{a}^{+}$ and $Y$ atoms of $C{l}^{-}$. Then $X$ and $Y$ are respectively:

• A. $3,3$
• B. $3,4$
• C. $4,4$
• D. $4,5$

Answer 1

Answer: (C)

$4,4$

$NaCl$ has FCC crystal structure.

In $NaCl$, sodium occupies edges and body center

So, number of sodium ions $=12\times \frac{1}{4}+1\times 1=4\downarrow \downarrow edgebodycenter$

and chlorine occupiees corners and face centers

so, numbers of chlorine ions $=8\times \frac{1}{8}+6\times \frac{1}{2}=4\downarrow \downarrow cornersfacecenter$

Hence, there are $4N{a}^{+}$ and $4{Cl}^{-}$ atoms in one ionic limit cell of $NaCl$.