Edge length of MX (fcc structure) is 7.2 A˚. Assuming M+X− contact along the cell edge, radius of X− ions is: (rM+=1.6 A˚)

Edge length of MX (fcc structure) is 7.2 A˚. Assuming M+X− contact along the cell edge, the radius of X− ions is: (rM+=1.6 A˚)
(A) 2A
(B) 5.6A
(c) 2.8A
(D) 3.8A

solution=
For ionic crystal

Edge length =2[(r+)+(r−)]

7.2=2[(1.6)+(r−)]

7.2−3.2=2(r−)

42=r−

r−=2A˚