Macromolecules (Network
Covalent)
Last part of Topic 4.3
• characteristics
– some elements from group 14 (carbon and
silicon form giant
macromolecules
• the entire piece is one molecule
– insoluble in almost all solvents
2
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”
1. graphite
2. graphene
3. diamond
– molecular and have a definite formula
4. fullerenes
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
• can be rolled up to form a carbon nanotube or
folded into a sphere (fullerene)
Graphite
• multiple layers of graphene
• between the layers are weak bonds (called
London forces)
– electricity can easily move between the sheets since it
has freely moving charged particles (electrons)
– layers can slide past one another therefore a good
lubricant
– brittle and high melting point
Fullerenes
• 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
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
see all the
tetrahedrals put
together
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
silicon is in group 14 just like carbon, so notice
the same covalent network solid shape
– 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