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.