chapter2 - Chapter 2 2,1 Which of the following are...

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Chapter 2 2,1 Which of the following are close-packed? (a) ABCABC…? Any ordering scheme of planes is close-packed if no two adjacent planes have the same position (i.e. no two planes are in register). When two planes are in register, the packing looks like the figure below and to the right, whereas the packing in a close-packed structure allows the atoms of one plane to fit more efficiently into the spaces between the atoms in an adjacent plane, like the figure below and to the left. Notice that the empty spaces between the atoms in the figure to the left are much smaller than the figure to the right. The efficient packing exhibited by close-packed structures is why, for a given type of atom, close-packed structures are denser than any other possible structure. In the case of an ABCABC… structure, no two adjacent planes are in register, so the ordering scheme is close-packed. (b) ABAC…? Once again, no two adjacent planes are in register, so the ordering scheme is close- packed. (c) ABBA…? The packing of planes using this sequence will put two B planes next to each other as well as two A planes next to each other, so the ordering scheme is not close-packed. (d) ABCBC. .? No two adjacent planes are in register, so the ordering scheme is close-packed. (e) ABABC. .? No two adjacent planes are in register, so the ordering scheme is close-packed. (f) ABCCB. .? The packing of planes using the sequence will put two C planes next to each other, so the ordering scheme is not close-packed. 2.4 Describe the structure of WC? A solid-state structure is generally described, when appropriate, as a close-packed array of one type of atom or ion with other types of atoms or ions in the interstitial holes between the packing spheres. Tungsten atoms are considerably larger than carbon atoms, so in this case it is best to envision WC as a close-packed lattice of W atoms with C atoms in interstitial holes. Since the compound has the rock-salt structure, the structure is described as a cubic close-packed array of W atoms with C atoms in all of the octahedral holes.
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2.5 Rubidium chloride? (a) Coordination numbers? The rock-salt polymorph of RbCl is based on a ccp array of Cl - ions in which the Rb + ions occupy all of the octahedral holes. An octahedron has six vertices, so the Rb + ions are six- coordinate. Since RbCI is a 1:1 salt, the C1 ions must be six-coordinate as well. The cesium- chloride polymorph is based on a cubic array of Cl - ions with Rb + ions at the unit cell centers. A cube has eight vertices, so the Rb + ions are eight-coordinate, and therefore the Cl
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chapter2 - Chapter 2 2,1 Which of the following are...

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