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Chapter 12: Problem 20

Amorphous silica, \(\mathrm{SiO}_{2}\), has a density of about $2.2 \mathrm{~g} / \mathrm{cm}^{3}$, whereas the density of crystalline quartz, another form of \(\mathrm{SiO}_{2}\), is \(2.65 \mathrm{~g} / \mathrm{cm}^{3}\). Which of the following statements is the best explanation for the difference in density? (a) Amorphous silica is a network-covalent solid, but quartz is metallic. (b) Amorphous silica crystallizes in a primitive cubic lattice. (c) Quartz is harder than amorphous silica. (d) Quartz must have a larger unit cell than amorphous silica. (e) The atoms in amorphous silica do not pack as efficiently in three dimensions as compared to the atoms in quartz.

Short Answer

Expert verified
The best explanation for the difference in density between amorphous silica and crystalline quartz is (e) - The atoms in amorphous silica do not pack as efficiently in three dimensions compared to the atoms in quartz.

Step by step solution

01

Statement (a)

Amorphous silica is a network-covalent solid, but quartz is metallic. This statement is incorrect because both amorphous silica and crystalline quartz are network covalent solids, not metallic.
02

Statement (b)

Amorphous silica crystallizes in a primitive cubic lattice. This statement is incorrect because amorphous silica does not form a crystalline lattice, as it is amorphous.
03

Statement (c)

Quartz is harder than amorphous silica. This statement is true, but it does not explain the difference in density between the two forms of \(\mathrm{SiO}_{2}\).
04

Statement (d)

Quartz must have a larger unit cell than amorphous silica. This statement is not relevant because amorphous silica does not have a unit cell due to its amorphous structure.
05

Statement (e)

The atoms in amorphous silica do not pack as efficiently in three dimensions compared to the atoms in quartz. This statement is true, and it explains the density difference between amorphous silica and crystalline quartz. In amorphous silica, the atoms are arranged in a less ordered manner, leading to less efficient packing and a lower density compared to the ordered arrangement in crystalline quartz. Therefore, the best explanation for the difference in density between amorphous silica and crystalline quartz is option (e).

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