A different student constructs a particulate model for magnesium oxide, MgO
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
A different student constructs a particulate model for magnesium oxide, MgO. The student states that the model consists of an alternating array of Mg²⁺ and O²⁻ ions in a simple cubic lattice with the following feature:
• All cations are placed exactly at the centers of the unit cells, while anions occupy the eight corners. However, the student also records that the Mg²⁺—Mg²⁺ nearest-neighbor distance is exactly the same as the Mg²⁺—O²⁻ nearest-neighbor distance.
Assume ionic radii are such that the two ion types cannot physically occupy the same point in space.
(a) Describe how the reported equality of Mg²⁺—Mg²⁺ and Mg²⁺—O²⁻ distances violates the electrostatic principles that govern a stable ionic lattice.
(b) Explain why a realistic MgO lattice must exhibit different nearest-neighbor distances for like-charged and opposite-charged ion pairs.
(c) Propose one modification to the student’s model that would make the particulate representation more consistent with Coulomb’s law and the typical coordination environment in MgO.