KEY2CHEM

Structure and Properties of Covalent Network Solids

Covalent network solids are nonmetals connected in two-dimensional and three-dimensional arrays. Because the atoms are connected by strong covalent bonds, the solids generally have high melting points. Three-dimensional covalent network solids, such as diamond, tend to be rigid and hard because their bond angles are fixed in place. Two-dimensional solids, such as graphite, tend to be softer because adjacent sheets are held together by London dispersion intermolecular forces, and the sheets can slide past one another.

Example 1.

Which statement about covalent network solids is most correct?

A. All covalent network solids are rigid and hard.

B. All covalent network solids are composed of nonmetals.

C. All covalent network solids have hydrogen bonding.

Solution

B. All covalent network solids are composed of nonmetals.

Nonmetals, either elements or covalent compounds, are connected by covalent bonds to form covalent network solids.

Example 2.

Covalent network solids generally form in the _____ group because of their ability to form four covalent bonds.

A. carbon

B. nitrogen

C. oxygen

Solution

A. carbon

Elements such as carbon and silicon (and other elements in this family) can form four covalent bonds.

Example 3.

Which is a property of diamond, which is a covalent network solid?

A. malleable

B. ductile

C. hard

Solution

C. hard

Diamond is a three-dimensional covalent network solids of carbon. Because its covalent bond angles are held in place by its structure, diamond is very hard.