The radii of `Na^(+)` and `Cl^(-)` ions are 95 pm and 181 pm respectively. The edge length of NaCl unit cell is:

Options

A. 276 pm
B. 138 pm
C. 552 pm
D. 415 pm

Answer

C

Explanation

In a fcc lattice, the distance between the cation and anion is equal to the sum of their radii, which is equal to half of the edge length of unit cell.
i.e. `r^(+) + r^(-) = a/2` (where a = edge length)
`r^(+) = 95 pm, r^(-) = 181 pm`
Edge length `=2r^(+) + 2r^(-) =(2 xx 95 + 2 xx 181)` pm
`=(190 + 362) pm = 552 pm`