An ionic crystalline solid, MX2, has cubic unit cell.?
An ionic crystalline solid, MX2, has cubic unit cell. Which of the following arrangements of the ions is consistent with the stoichiometry of the compound?
I know the answer is M2+ ions at the corners and at each face, 8 X- ions at the body centers. but can someone plane why and how why it is the answer
More Questions From This User See All
Answers & Comments
Verified answer
I don't know what other choices were presented, so I can't eliminate them, but I can explain why the one you describe is consistent with the requirement that there are twice as many X's as M's.
In the cube, there are 8 corners, but each corner is shared by 8 different cells, so only 1/8 of the M ion at a corner is inside the cube. Therefore, the 8 corners together give you only 1 ion of M. The "face" ions are shared between the cube we are looking at and its facing neighbors, so each face ion lies only half inside the cube; the 6 faces together give you only 3 ions of M. That makes 4 ions of M all told. The "body" ions of X are positioned at coordinates (1/4,1/4,1/4), (1/4,1/4,3/4), (1/4,3/4,3/4), (3/4,3/4,3/4), (3/4,3/4,1/4), (3/4,1/4,1/4), (3/4,1/4,3/4), and (1/4,3/4,1/4). Each of them lies entirely inside the cube, so the cube contains 8 ions of X and 4 ions of M, which is the correct ratio for "MX2".