4.5
Physical Properties in Giant
Covalent Substances
Diamond
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Structure: Giant covalent macromolecule
Bonding: Each carbon atom has 4 bonding pairs of
electrons and 0 lone pairs of electrons. According to VSEPR
theory in order to minimize the repulsion the 4 pair of
bonding electrons repel each other equally.
Bond angle: 109.5o
Shape: tetrahedral
No intermolecular forces of attraction, only
intramolecular attractions between the carbon atoms.
Physical Properties of Diamond
Hardness
 Hardest substance known
 Each carbon atom is held in the lattice by 4 strong covalent
bonds, making diamond a strong rigid structure.
 The carbon – carbon bonds can only be broken up by
another diamond.
 Because of their hardness diamonds have many applications
in industry. Parts of cars, planes, and other machines are
shaped by diamonds.
Diamond saw
Solubility in water
 The covalent bonds between the carbon
atoms are much stronger than the force
of attraction exerted by polar water
molecules. Diamond is not soluble in
water.
Melting Point
 To break the strong intramolecular
covalent bonds requires a large amount of
energy, therefore they have very high
temperature.
 M.pt of diamond  4000oC
Conductivity of electricity
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All four outer valence electrons in in each C atom are
involved in forming covalent bonds with four other C
atoms. Therefore the valence electrons are not free to
move making diamond a non conductor of electricity.
Graphite
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Structure: Giant Covalent macromolecule
Bonding: Layers of hexagonal rings of carbon atoms.
Within each layer intramolecular covalent bonds attach
each carbon atom to three other carbon atom. The 4th
bonding electron is delocalized (mobile) and moves
between the layers. These electrons form Van der Waal’s
intermolecular forces that hold the layers together.
The Physical Properties of Graphite
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Conducts Electricity
The one electron not used in bonding is free / mobile /
delocalized and can carry a current.
Lubricant
The weak van der Waals forces between the layers allow
them to slide over one another making graphite an excellent
lubricant.
Insoluble in water
The intramolecular covalent bonds between the carbon atoms
are too strong to interact with water.
Low density
Compared to diamond. This is because of the relatively large
amount of space between the layers.
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Soft and slippery
The layers of carbon atoms slide over each other due to
weak Van der Waal’s forces between them. When you
use a pencil, the layers of carbon atoms are rubbed off
and stick to the paper. Graphite is one of the softest
substances.
High melting point and boiling point
Mpt = 3652°C - 3697°C : b.pt = 4200°C.
Similar to diamond. To melt graphite both the
intermolecular van der Waals forces and intramolecular
covalent bonds need to be broken.
Volatility
Not volatile
C60 Fullerenes
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Structure: Giant Covalent macromolecule
Bonding: 60 carbon atoms covalently bonded in a a
polyhedron with 20 hexagonal (6-angled) surfaces and 12
pentagonal (5-angled) surfaces.
Physical Properties of fullerenes
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Reactivity with other compounds
Not very reactive due to the stability of the carbon-carbon
bonds. Chemists have been able to increase the
reactivity by attaching active groups to the surface.
Insoluble in many solvents
Partial conductor of electricity
Due to the one delocalized electron per carbon atom.
Soft
C60 molecules can slide over one another making them
softer than diamond but not as soft as graphite
Volatility
Not volatile
Silicon, Si
Si - semimetal or metalloid element.
Structure:
 Giant Covalent macromolecule
Bonding:
 Tetrahedral structure. Each silicon atom has 4 bonding
pairs of electrons and 0 lone pairs of electrons.
 According to VSEPR theory in order to minimize the
repulsion between the 4 pairs of bonding electrons they
repel each other equally. Bond angle: 109.5o.
 No intermolecular forces of attraction, only
intramolecular attractions between the silicon atoms.
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The Physical Properties of Silicon
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Does not conduct electricity
No delocalized valence elections to carry a current because they are all
involved in bonding. Called an insulator.
Hard
Each silicon atom is held in the lattice by 4 strong covalent bonds,
giving it a strong rigid structure.
Insoluble in water
The intramolecular covalent bonds between the silicon atoms are too
strong to interact with water.
Melting point
Lower than diamond. Si-Si bond is longer than C-C bond in diamond,
because Si has a larger radius. As the bond length increases, the
amount of energy needed to break the attractive force between the pair
of electrons in the covalent bond and the protons in the nucleus
decreases.
Silicon as a semiconductor
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Si can be made into a partial of conductor of electricity.
Si semiconductors are found in the microprocessor chips of all electronic
devices
Silicon can be made into a conductor (but not as good as a metal), by
doping – adding B, P, Ga or As atoms are added to the lattice.
P and As have five valence electrons. The fifth electron does not covalently
bond with the Si and remains delocalized and able to conduct a current.
B and Ga have three outer electrons. When they bond with Si they form a
“hole” (the 4th valence electron that is not involved in bonding). The
absence of an electron on each Si atoms creates the effect of a positive
charge which can conduct a current. Alternatively the non bonded electrons
can move from “hole” to “hole” carrying a current.
Silicon dioxide, SiO2
Naturally occurring in in the earths
crust.
 Each Si atom is linked to four O
atoms, and each O to two Si atoms
in a 1:2 ratio. Bent shape.
Physical Properties:
 High melting point (1650 - 1730 °C)
and boiling point
 insoluble in water
 not volatile
 does not conduct electricity
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Silicone bake ware
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Synthetically manufactured rubber like polymer containing
silicon and oxygen.
Properties that make it useful for baking:
 durable and long lasting
 Soft and flexible
 Doesn’t break like glass, or ceramic (other silicon based
substances)
 Does not dent or rust like metal
 Lightweight
 Does not react with food
 Can withstand high temp
Silicone Breast Implants
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Synthetically manufactured rubber sac filled with silicon gel
like polymer containing silicon and oxygen atoms.
Properties that make is useful for breast
implants:
 Soft, flexible and tactile
 Stable at all temperatures
 Long lasting
 Won’t react with other chemicals in the body
 Despite silicones chemical and physical stability some
women’s bodies have rejected the silicone implants resulting
in many law suits in the US and concerns about their safety.
Bibliography
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http://chemcases.com/silicon/sil15one.htm