1. Macromolecules (Covalent Network Solids) Last part of Topic 4.3

2. characteristics
some elements from group 14 (carbon and silicon) form giant covalent substances
have localized electrons (shared electrons) arranged in fixed geometries
insoluble in almost all solvents

3. Carbon allotropes
the same element can be covalently bonded in four of different forms (allotropes)
covalent network solids- atoms are held together by covalent bonds in a 3-D lattice structure that “go on forever”
graphite
graphene
diamond
molecular and have a definite formula
fullerenes

4. Graphene- The super material!
strongly bonded carbon in a trigonal planar structure (120°)
one of the thinnest and strongest known materials
only one atom thick, a two dimensional crystal
superb conductor of electricity (300 times more conductive than copper)
can be rolled up to form a carbon nanotube or folded into a sphere (fullerene)

6. Graphite multiple layers of graphene
between the layers are weak bonds (London dispersion forces)
electricity can easily move between the sheets since it has freely moving charged particles (electrons) in between the sheets
layers can slide past one another therefore a good lubricant
brittle and high melting point

9. Diamond repeating tetrahedral structure (109.5°)
one of the hardest materials known
very high melting point
no free moving electrons since they are being shared in covalent bonds
cannot conduct electricity

10. see all the tetrahedrals put together

12. Silicon and silicon dioxide
both (Si and SiO2) are very “strong” structures
very high melting point
very high boiling point
silicon
giant (macro) molecules made up of tetrahedrals covalently bonded together
this structure is repeated over and over

13. silicon is in group 14 just like carbon, so notice the same covalent network solid shape

14. silicon dioxide (sand)
same structure as silicon but each silicon atom is bridged to its neighbors by an oxygen atom
most common crystalline form is quartz
high melting and boiling point
does not conduct electricity

16. Fullerenes (not actually a covalent network solid since it is a molecule with a definite formula)
has some delocalized (moving) electrons but cannot conduct electricity since electrons can’t move from one molecule to the next
most famous is the C60 fullerene often referred to as buckeyballs
60 carbons in 12 pentagons and 20 hexagons
can form a hollow, cage-like structure or tubes