Covalent Properties Main Concept: Covalent network solids have properties that reflect their underlying 2-D or 3-D networks of covalent bonds. Covalent network solids generally have extremely high melting points and are hard.  

Covalent Properties Properties of Covalent Networks Structure Melting Points Other Properties Examples Graphite Silicon

- Covalent network solids consist of atoms covalently bonded into two-dimensional or three-dimensional networks

Covalent network solids are only formed from nonmetals: - elemental (diamond or graphite) - two nonmetals (silicon dioxide and silicon carbide) - properties of covalent network solids reflect their structure - have high melting points because all atoms are covalently bonded

graphite diamond silica - Three-dimensional covalent networks tend to be rigid and hard because covalent bond angles are fixed - Generally, covalent network solids form in carbon group because of ability to form four covalent bonds

Graphite: - an allotrope of carbon that forms sheets of two-dimensional networks - has high melting point because covalent bonds between the carbon atoms making up each layer are relatively strong - is soft because the adjacent layers can slide past each other relatively easily; major forces of attraction between layers  London dispersion forces

Silicon - A covalent network solid and semiconductor - forms a three-dimensional network similar in geometry to a diamond - conductivity increases as temperature increases - Periodicity can be used to understand why doping with an element with one extra valence electron converts silicon into an n-type semiconducting (negative charge carrying) material - basis of modern electronics are junctions between n-doped and p-doped materials used to control electrons flow.