Question

$Na$ and $Mg$ crystallize in $BCC$ and $FCC$ type of crystals respectively, then the number of atoms of $Na$ and $Mg$ present in the unit cell of their respective crystals is:

• A. $4,2$
• B. $9,14$
• C. $14,9$
• D. $2,4$

Answer 1

The correct option is D $2,4$

$\to$ In BCC structure $8$ atoms of $Na$ are present at body corner and one at body centre.

$Z_{Na}=8\times \frac{1}{8}+1=2$

$\to$ In FCC structure $8$ atoms of $Mg$ are present at body corner and six at face centre.

$Z_{Mg}=8\times \frac{1}{8}+\frac{1}{2}\times 6=4$