Crystallography and Crystal structure (50 pts total) Use the crystal structure s

Question

Crystallography and Crystal structure (50 pts total) Use the crystal structure shown to the right to answer the following questions. The ball-and-stick structure shown is called "rocksalt", with the large spheres being Cl and the small spheres being Na. (20 pts total) a. What do you predict the major type of bonding 1. to be for this structure? (5) If one were to draw this structure differently, in terms of polyhedra, what type(s) of polyhedral would the structure consist of? Include the name of each polyhedron, the center atom type and its coordination number. (5) b. In this case, the Na is smaller than the CI. What happens to the structure if the alkali ion gets bigger, such as when Na is replaced by Cs? What are the new coordination numbers? (5) c. d. Comparing the structures from a) and c), which of the monatomic metallic cubic lattices could you use to describe each one of them? (5)

Explanation / Answer

a) Major type of bonding for the given structure is ionic bond. The given structure consists of Na^+ ions and Cl^- ions so there are electrostatic attractions between these two ions, hence ionic bond will be formed.

b) If we draw the structure interms of polyhedra, the type of polyhedra is octahedron. Both the atoms have octahedron. Ecah Cl atom is surrounded by six Na atoms so structure around Cl atom is octahedron and its coordination number is 6. Similarly each Na atom is surrounded by six Cl atoms so it has octahedron and its coordination number is 6.

c) If we replace Na by Cs then formual is CsCl, and it will have BCC structure, in that structure ecah Cs atom is surrounded by eight Cl atoms and ecah Cl atom is surronded by eight Cs atoms. So new coordination numbers are 8 and 8.

d) To describe structure (a) we need FCC crystal structure. So Al has FCC structure so Al metallic cubic lattice can be used describe NaCl structure.

Structure of CsCl is BCC cubic lattic structure, so it needs metal with BCC cubic lattic. Cs itself has BCC cubic lattice structure, so Cs metallic cubic lattice can be used to describe CsCl structure.

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