Question

Passage 3
In hexagonal systems of crystals, a frequently encountered
arrangement of atoms is described as a hexagonal prism. Here,
the top and bottom of the cell are regular hexagons and three
atoms are sandwiched in between them. A space-filling model of
this structure, called hexagonal close-packed (hcp), is constituted
of a sphere on a flat surface surrounded in the same plane by six
identical spheres as closely as possible. Three spheres are then
placed over the first layer so that they touch each other and
represent the second layer. Each one of these three spheres
touches three spheres of the bottom layer. Finally, the second
layer is covered with a third layer that is identical to the bottom
layer in relative position. Assume radius of every sphere to be 'r'.
TIIT 2008]
1. The number of atoms in this hcp unit cell is;
(a) 4 (b) 6 (c) 12 (d) 17
2. The volume of this hcp unit cell is:
(a) 24.2 m3 (b) 162r3 (c) 12/2r3 (d)
64
33
3. The empty space in this hcp unit cell is:
(a) 74%
(b) 47.6% (c) 32% (d) 26%
pe 3
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Answer 1

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