CHEM 121 glossary
Covalent-network solid
A solid in which every atom is covalently bonded to its neighbours in one continuous network, so the whole crystal is effectively one giant molecule. Melting it means breaking covalent bonds, so it is very hard and very high-melting; learn the short list: diamond, graphite, Si, , SiC, BN.
No separate molecules: the bonds run through the whole crystal.
Example
In diamond each carbon is bonded tetrahedrally to four others, making it the hardest natural substance. , by contrast, forms separate molecules and dry ice sublimes at −78 °C.