Network covalent solids are materials in which atoms are joined by covalent bonds in a continuous lattice rather than as separate molecules. This bonding pattern makes many of them extremely hard, chemically stable, and difficult to melt. They are important in gemstones, cutting tools, electronics, glass, ceramics, and geological minerals.
Diamond, graphite, and silicon dioxide show how different network shapes can produce very different properties.
Key Facts
- Network covalent solids do not contain separate molecules, so there is no simple molecular formula for the whole solid.
- High melting points occur because many strong covalent bonds must be broken to change the solid into a liquid.
- Diamond: each carbon atom forms 4 covalent bonds in a tetrahedral 3D network.
- Graphite: each carbon atom forms 3 covalent bonds in flat hexagonal sheets, with weak forces between sheets.
- Silicon dioxide: each Si atom bonds to 4 O atoms and each O atom bonds to 2 Si atoms, giving an extended SiO2 network.
- Typical bond energy comparison: covalent network bonds are much stronger than intermolecular forces, so network solids usually melt at far higher temperatures than molecular solids.
Vocabulary
- Network covalent solid
- A solid made of atoms connected by covalent bonds in a continuous repeating structure.
- Covalent bond
- A chemical bond in which atoms share pairs of electrons.
- Lattice
- An ordered repeating arrangement of particles in a solid.
- Tetrahedral
- A shape in which one central atom is bonded toward the corners of a four-sided pyramid.
- Allotrope
- A different structural form of the same element, such as diamond and graphite for carbon.
Common Mistakes to Avoid
- Calling diamond a molecule is wrong because diamond is one continuous covalent network, not a collection of separate carbon molecules.
- Assuming all carbon solids have the same properties is wrong because diamond and graphite have different bonding arrangements and therefore different hardness and conductivity.
- Explaining graphite's slipperiness by weak covalent bonds within the sheets is wrong because the covalent bonds in each sheet are strong, while the forces between sheets are weak.
- Writing silicon dioxide as a small SiO2 molecule is misleading because SiO2 represents the atom ratio in an extended network, not an isolated molecule under normal solid conditions.
Practice Questions
- 1 Diamond has each carbon atom bonded to 4 other carbon atoms. How many covalent bonds are connected to 25 carbon atoms if each bond is shared by 2 atoms?
- 2 A sample of silicon dioxide contains 120 oxygen atoms. Using the formula ratio SiO2, how many silicon atoms are present?
- 3 Diamond and graphite are both made only of carbon. Explain why diamond is very hard while graphite is soft and slippery, using their bonding structures.