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 of unit cell,
i.e., `r^(+)+r^(-)=(a)/(2) " "` (where a = edge length)
`r^(+)=95 p m, r^(-)=181` pm
Edge length `= 2r^(+)+2r^(-)=(2xx95+2xx181)` pm
`=(190+362)` pm = 552 pm.