Question

Which one of the following statements about unit cells and packing in solids is incorrect?

a. In any unit cell of a solid crystal, each face of the cell must have an opposite face that is equal and parallel to it. b.The faces of a unit cell must all be at angles of 90' to each other.
c. The coordination number of atoms in a close-packed metal is 12.
d. The packing efficiency in fcc structures is higher than in bcc structures.
e.The packing efficiency in fcc and hcp structures is the same.
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Answer 1

The statement that is incorrect is:

a. In any unit cell of a solid crystal, each face of the cell must have an opposite face that is equal and parallel to it.

This statement is incorrect because, while it is true that the faces of a unit cell must be equal and parallel, they do not necessarily need to have an opposite face that is equal and parallel to them. In fact, some unit cells, such as the tetragonal and rhombohedral unit cells, have faces that are not parallel to each other or to their opposite faces. Therefore, the correct answer is a.