Question

Edge length of $MX$ (fcc structure) is $7.2\mathring{A}$. Assuming $M^{+}{X}^{-}$ contact along the cell edge, radius of $X^{-}$ ions is: $(r_{M+}=1.6\mathring{A})$

• A. $2.0\mathring{A}$
• B. $5.6\mathring{A}$
• C. $2.8\mathring{A}$
• D. $3.8\mathring{A}$

Answer 1

The correct option is A $2.0\mathring{A}$

For ionic crystal having FCC crystal structure (rock salt type)

Edge length $=2\left[\right(r^{+})+(r^{-}\left)\right]$

$7.2=2\left[\right(1.6)+(r^{-}\left)\right]$

$7.2-3.2=2\left(r^{-}\right)$

$\frac{4}{2}=r^{-}$

$r^{-}=2\mathring{A}$