The edge length of face centred cubic unit cell is `5.8` pm. if the radius of the caiton is `110` pm. The radius of the anion is

Options

A. `288` pm
B. `398` pm
C. `144` pm
D. `618` pm

Answer

3

Explanation

Imagine `NaCl` crystal which has fcc unit cell.The chloride ions are at the corners while sodium ion is at the edge centre thus.
edge length =`2r_(+)+2r_(-)=2(r_(+)+r_(-))`
`r_(+)+r_(-)=("Edge length")/(2)=(508p m)/(2)`
`=254p m`
Hence
`r_(-)=254p m-r_(+)`
`=(254p m)-(110p m)`
`=144p m`