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.