Question

If the distance between $N{a}^{+}$ and $C{l}^{-}$ ions in sodium chloride crystal is $ypm$, the length of the edge of the unit cell is:

• A. $4ypm$
• B. $\frac{y}{4}pm$
• C. $\frac{y}{2}pm$
• D. $2ypm$

Answer 1

Answer: (D)

$2ypm$

$NaCl$ is a face centred cubic lattice in which $N{a}^{+}$ is placed in octahedral voids which are present along the edges of the FCC lattice. Therefore length of unit cell will be twice the distance between $N{a}^{+}$ and $C{l}^{-}$ ions.

Given, $(r^{+}+r^{-})=ypm$

$a=2(r^{+}+r^{-})=2ypm$

Option D is correct.