Question 12.3.1: Analyze an ionic cubic unit cell. Solid strontium chloride h......
Analyze an ionic cubic unit cell.
Solid strontium chloride has a crystal structure with the cubic unit cell pictured here, where the Cl^{-} ions are represented as yellow spheres. Describe the positions of the anions in the lattice. What is the coordination number of the strontium ions in the crystal? How many SrCl_{2} formula units are there per unit cell?
You are asked to describe the position of anions in a crystal lattice, to identify the coordination number of the cations in the lattice, and to determine the number of formula units per unit cell.
You are given a picture of the unit cell for the solid.
The lattice can be viewed as a cubic close-packed array of strontium ions with chloride ions in tetrahedral holes. This gives four strontium ions and eight chloride ions per unit cell. The number of formula units per unit cell is therefore four.
To visualize the coordination number of the strontium ions, focus on the strontium ion on the top face of the cell. This ion has four chloride ions in the unit cell directly beneath it as nearest neighbors. But because of the translational symmetry, the strontium ion on the bottom face of the cell is identical to the one on the top face of the cell. This strontium ion is closest to the four chloride ions directly above it in the cell. Therefore, in the extended structure, each strontium ion lies at the center of a cube of chloride ions and has a coordination number of 8.