5. If the radius of an atom is r, what is the length of the side of the body-centered cubic unit cell? (A) 4r/31/2 (B) 2.25r (C) r (D) 2r (E) 81/2r

4. Both ZnS and CaF₂ have a face-centered cubic unit cell where the S²⁻ and Ca²⁺ ions are closest packed in each structure. Which of the following is true? (A) There are 4 tetrahedral holes empty in each structure. (B) In both compounds, one-half of the tetrahedral holes are filled. (C) In both compounds, all the tetrahedral holes are filled. (D) There are 8 Zn²⁺ ions and 4 F⁻ ions in the unit cell. (E) In ZnS, one-half of the tetrahedral holes are filled by Zn²⁺ ions, whereas in CaF₂ all the tetrahedral holes are filled with F⁻ ions.

5. If the radius of an atom is r, what is the length of the side of the body-centered cubic unit cell?
(A) 4r/3^1/2
(B) 2.25r
(C) r
(D) 2r
(E) 81/2r

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