11.19: Network Covalent Solids

Network covalent solids contain a three-dimensional network of covalently bonded atoms as found in the crystal structures of nonmetals like diamond, graphite, silicon, and some covalent compounds, such as silicon dioxide (sand) and silicon carbide (carborundum, the abrasive on sandpaper). Many minerals have networks of covalent bonds.

To break or to melt a covalent network solid, covalent bonds must be broken. Because covalent bonds are relatively strong, covalent network solids are typically characterized by hardness, strength, and high melting points. For example, diamond is one of the hardest substances known and melts above 3500 °C.

Diamond vs. Graphite

Carbon is an essential element; diamond and graphite are the two most common allotropes of carbon. Allotropes are different structural forms of the same element. Diamond is one of the hardest known substances, whereas graphite is soft enough to be used as a pencil lead. These very different properties stem from the different arrangements of the carbon atoms in the different allotropes.

Diamond is extremely hard because of the strong bonding between carbon atoms in all directions. Graphite is composed of planar sheets of covalent crystals that are held together in layers by noncovalent forces. Unlike typical covalent solids, graphite is very soft and electrically conductive. Graphite (in pencil lead) rubs off onto paper due to the weak attractions between the carbon layers.

Graphene: Material of the Future

A recently discovered form of carbon is graphene. Graphene was first isolated in 2004 by using tape to peel off thinner and thinner layers from graphite. It is essentially a single sheet (one atom thick) of graphite. Graphene is not only strong and lightweight, but it is also an excellent conductor of electricity and heat. These properties may prove very useful in a wide range of applications, such as vastly improved computer chips and circuits, better batteries and solar cells, and stronger and lighter structural materials. The 2010 Nobel Prize in Physics was awarded to Andre Geim and Konstantin Novoselov for their pioneering work with graphene.

This text has been adapted from Openstax, Chemistry 2e, Section: 10.5 The Solid State of Matter.

Network covalent solids are crystalline solids that are made up of a vast three-dimensional network of individual atoms held together by strong covalent bonds.

Examples of network covalent solids include diamond, which has a continuous network of carbon atoms, and quartz, which has a continuous network of silicon and oxygen atoms.

The extremely strong covalent forces between the atoms make these solids hard with very high melting points.

For example, in diamond, each carbon atom is sp³ hybridized and connected tetrahedrally to four neighboring carbon atoms via single covalent bonds.

This strongly interconnected network accounts for the unusual hardness of diamond and its very high melting point. Diamond is a poor electrical conductor, as there are no delocalized electrons.

In quartz, each silicon atom is bonded to four oxygen atoms, and each oxygen atom is shared between a pair of silicon atoms. The strong silicon–oxygen covalent bonding results in the hardness and high melting point of quartz.

Graphite is an unusual network covalent solid because it is soft and conducts electricity. Like diamond, graphite is an allotrope of carbon, meaning that the two materials are composed of carbon atoms in different three-dimensional arrangements.

In graphite, carbon atoms are arranged in layers of interconnected hexagonal rings. Within each layer, each carbon atom is sp² hybridized and is covalently bonded to three neighboring carbon atoms.

The nonbonding electrons are delocalized across the entire layer, making graphite a good electrical conductor. However, these layers are only held together by weak dispersion forces.

Consequently, the layers can slide past each other, making graphite soft and flaky.

This is why graphite is used in pencils: the layers of carbon are easily transferred to the paper.