CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
A materials scientist provides three schematic, two-dimensional ball-and-stick renderings, labelled Diagram I, Diagram II, and Diagram III. Each diagram represents a square array viewed down one crystallographic axis. In every diagram the large spheres are identical atoms of element M.
• Diagram I shows only large spheres arranged at every lattice point with uniform spacing.
• Diagram II shows the same array, but at random lattice points some large spheres are replaced by medium-sized spheres of element N. The medium spheres are drawn approximately the same size as the large spheres.
• Diagram III shows the original array of large spheres, but small spheres of element X are drawn tightly packed in some of the spaces between adjacent large spheres while every large sphere remains in place.
Assume that the diameters of the medium spheres differ from the large spheres by less than 10 %, whereas the small spheres have diameters less than 50 % of the large spheres.
Answer the following questions about the three diagrams.
(a) For each diagram (I – III), identify whether the solid depicted is a pure metal, a substitutional alloy, or an interstitial alloy.
(b) Describe two distinct visual features that led you to assign each diagram to its corresponding category.
(c) Using comparisons of atomic (or ionic) radii, explain why element N is able to occupy the positions shown in Diagram II, whereas element X is unable to substitute for element M in Diagram III.
(d) Rank the three solids in order of expected electrical conductivity at room temperature, from highest to lowest, and justify your ranking in terms of electron mobility within the structures.