Lithium iodide crystal has a face-centred cubic unit cell. If the edge length of the unit cell is 620 pm, determine the ionic radius of I^- ion.

Lithium iodide crystal has a face-centred cubic unit cell. If the edge

1.	Lithium iodide crystal has a face-centred cubic unit cell. If the edge length of the unit cell is 620 pm, determine the ionic radius of I^- ion.
Answer» Solution :As LiI has FCC arrangement, `I^-` ions will OCCUPY the CORNERS and face-centres. These ions will touch each other ALONG the face diagonal `therefore` Face diagonal AB=`4r_(1^-)=sqrt(a^2+a^2)=sqrt2a therefore r_(1^-)=(sqrt2a)/4=(1.414xx620"pm")/4` =219.17 pm