Question

If all the edge centred atom of $N{a}^{+}$ are replaced by $C{l}^{-}$ atom in fcc of $NaCl$, then the chemical formula of the unit cell is:

• A. $NaC{l}_7$
• B. $NaCl$
• C. $N{a}_{7}Cl$
• D. none of these

Answer 1

The correct option is A $NaC{l}_7$

$Na$ atom at the edge centre is shared by 4 neighbouring unit cells. So a total of 3 atoms are located at the edge centres of a unit cell. A single unit cell has 4 atoms of each $Na$ and $Cl$. So we replace the edge centre Na atoms with $Cl$ atoms we will get one $Na$ atom and 7 $Cl$ atoms. So the formula will be $NaC{l}_7$.