2P32 – Principles of Inorganic Chemistry
Dr. M. Pilkington
Lecture 29 – Group 14
1. The elements C, Si, Ge, Sn. Pb
2. Group 14 (4A) and the network
3. Allotropes of C and Si
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Diamond and graphite
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Buckminsterfullerine C60
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Carbon nanotubes
4. Contrasting the chemistry of C and Si
1.
The Elements C, Si, Ge, Sn, Pb
Carbon, Tin and Lead
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Carbon has been known in the forms of coal, oil, petroleum, natural gas, and
charcoal for thousands of years.
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The recognition that it was an element in the modern sense, dates back to the
late 18th Century.
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Lavoisier carried out many experiments on the combustion of diamonds in the
1770’s.
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It was known that diamonds and graphite were but two forms of the same
element.
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The production of tin can be dated back to 3000 B.C. most likely because its
oxide could be readily reduced to the metal by the glowing coals of a wood
fire.
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The production and use of bronze (an alloy of copper and tin) is still older.
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Tin dishes were common in the 1700’s, as were tin plated materials.
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Tin has more stable isotopes (10) than any other element.
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Lead is the oldest metal known. The book of Job, probably written in 400 B.C.
has its author wishing to have his devotion to God be recorded forever “with an
iron pen and lead”.
Lead was easy to hammer into sheets for writing and flooring materials, into
vessels for cooking and storing foods, and into pipes for plumbing.
Lead pipes were the original plumbing materials (some lead pipes still in service
have the insignia of Roman Emperors on them).
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Both words plumbing and plumber are derived from the same Latin word plumbum
for “lead”. This is also the origin of the symbol Pb for the element.
Silicon and Germanium
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Glass of which silicon in the form of silica is a prime component has been known
since about 1500 B.C.
In 1824 Berzelius isolated amorphous Silicon where others before him had
failed.
He named his new compound “silicium” from the Latin silex for “flint” a major
source of silica.
The name silicon was proposed in 1831 with the suffix –on replacing –ium to
establish a parallel with boron and carbon.
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Reasonably pure silicon is prepared by reducing silica with carbon in an electric
furnace as shown below:
SiO2(s) + 2C
Si(s) + 2CO(g)
electric
furnace
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Winkler discovered Germanium in 1817 and named it after his fatherland.
Not much use was found for germanium until 1942, when the transistor was
invented at the Bell Lab.
Now germanium has returned to relative obscurity, having lost out in the
semiconductor transistor market to silicon.
2. Group 14 (4A) and the Network
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Carbon is unique in this group.
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No other element has an entire branch of chemistry built around it.
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The metal/non metal line passed through the heart of the group with carbon
being a nonmetal and lead a metal.
In between are two metalloids (semiconductors) silicon and germanium as well as
the borderline metal, tin.
3. Allotropes of C and Si
Diamond and Graphite
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Diamond and graphite are allotropes of carbon i.e. different forms of the
same element.
Graphite is metallic in appearance, whereas diamond is transparent and one of
the hardest substances known.
The diamond structure is a 3-dimensional covalent network crystal composed of
interconnected C-C single bonds (remember the unit cell is similar to Zinc blende
accept that all the spheres represent carbon instead of alternating between Zn
and S.
Natural diamonds are not easy to come by and formed in rock melts and
temperatures in excess of 14000C.
Synthetic diamonds are made in small grit sizes and used for various grinding
applications.
Gem-quality diamonds can be made but are not as costly as the natural variety.
The custom of giving a diamond engagement ring seems to have been started by
the Venetians toward the end of the 15th Century.
Imitation diamonds are yttrium-aluminium garnet, strontium titanate, or cubic
zirconia, ZrSiO4.
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The diamond structure showing the carbon-carbon bonding. Bonds
closer to the viewer are shown thicker.
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The diamond cubic structure is
a crystal structure wherein the
atoms are arranged on a facecentered cubic (FCC) lattice
with additional atoms.
Every C is bonded to 4 others at
the corners of a tetrahedron
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Graphite is a layered structure characterized by strong pp-pp bonding within
each layer and only London (van der Waals) forces between them.
The relative strength of these two different types of interactions is reflected
in the C-C distances.
The soft and lubricating properties of graphite are due to these layers being
able to easily slide by each other.
Pencil lead is also graphite (mixed with clay). Pressure on the pencil head causes
the layers of graphite to rub off on a piece of paper.
Charcoal and soot are very tiny particles of graphite. The large surface areas of
these materials make them useful for adsorbing various gases and solutes.
π-bonding in graphite
Every C bonded to 3 others
Buckminsterfullerine C60
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In 1985 Harry Kroto, Richard Smalley and Robert Curl produced an amazing
third allotrope of carbon.
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They found that when graphite was vaporized by a laser, a variety of large
clusters and even numbers of carbon atoms are formed.
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One of the most prevalent of these was C60.
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The structure remained a mystery and two shapes came into mind:
(a)
The geodesic domes of the American architect R. Buckminster Fuller and.
(b)
The shape of the European football or an American soccer ball.
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Buckminster Fuller's Dome - Expo '67 Montreal
The similarity to the geodesic dome has
afforded names for C60 such as
buckminsterfullerine, buckyball or even
bucky.
Every C bonded to 3 others pπ-pπ bonds
are involved as well as 5- and 6-membered
rings.
C60 is a truncated icosahedron characterized by 60 vertices,and 32 faces.
The interior diameter of the C60 cluster is about 7 Å and has the potential to
accommodate a variety of small atoms, molecules or ions.
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Since their discovery in the mid 1980’s an incredible variety of fullerenes have
been produced and characterized. Each is best thought of as 12 pentagons and a
varing number of hexagens.
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As a result of these discoveries, Kroto, Smalley and Curl received the Nobel
Prize in chemistry.
C70, C60's big brother
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C60-Fullerene at 153 deg.K. C60 crystallizes in a face centered cubic
arrangement.
Carbon Nanotubes
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In 1991, Sumio Iijima made still another discovery involving carbon allotropes.
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He produced long cylindrical (10-30 nm in diameter) multiwalled, tubelike structures with
hemispherical end caps.
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Given their dimensions, they quickly became known as nanotubes.
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A short time later, Thomas Ebbesen and Pulickel Ajayan, from Iijima's lab, showed how
nanotubes could be produced in bulk quantities by varying the arc-evaporation conditions.
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This paved the way to an explosion of research into the physical and chemical properties of
carbon nanotubes in laboratories all over the world.
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Nanotubes, depending on their structure, can be metals or semiconductors. They are also
extremely strong materials and have good thermal conductivity. The above characteristics
have generated strong interest in their possible use in nano-electronic and nano-mechanical
devices. For example, they can be used as nano-wires or as active components in electronic
devices such as the field-effect transistor.
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While normally nanotubes are straight, ways have been devised to prepare them in a ring
form.
Nanotube Rings
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The rings are composed of many layers of single-walled nanotubes, and have a radius of
typically 0.7 micron.
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Coiling has been observed in proteins and other biomolecules, where hydrogen bonding is
thought to provide the main force for coiling. Carbon nanotubes however present a novel
behavior where coiling involves only van der Waals forces.
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The rings, which we can position on metal electrodes, allow us to study novel electric
transport phenomena. Shown below is an AFM micrograph of a one micron-diameter ring
(the purple circle) placed over gold electrodes (the light blue objects).
SEM (Scanning
Electron Microscope)
image of nanotube
rings on a silicon
substrate. The image
The nanotubes used to form the
rings are extremely small; their
diameter is only 1.4 nm. They are 1dimensional conductors and at low
temperatures,
is magnified 8000
times.
Nanotubes – Properties and Applications
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Nanotubes are amazingly flexible, strong and stable.
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They have a tensile strength (ability to oppose rupture under tension) some 50100 times that of steel at one-sixth of the weight.
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This makes them candidates for a large number of applications and uses.
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The tubes can be fashioned into a variety of fibers, ropes, and cables which will
be put to great use in the next generation of high strength fibers stuffer yet
less brittle than their graphite relatives.
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“Buckyropes” will be used in everything from aircraft frames truck and
automobile body panels, bridge supports and rocket nozzles.
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Tennis rackets, golf clubs or fishing rods could be made out of these materials.
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Single walled nanotubes have unique electronic properties. They can conduct
electricity as easy as a metal so in the future, they could rival copper wires but
be more flexible and lighter.
In contrast – Silicon
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One allotrope; diamond structure where every Silicon is tetrahedrally bonded to 4
others.
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Silica cannot undergo pπ-pπ bonding so there are no forms similar to graphite or
buckyballs.
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Silicon hydrides exist, but there are not very many.
H
SiH4
SiH6
spontaneously flammable in air
availability of d orbitals in Si
H
O=O
Si
H
H
can react directly to the SiH4
SiO2 + H2O highly flammable
4. Contrasting the chemistry of C and Si
1. SiCl4 + H2O
SiO2 + 4HCl
CCl4 + H2O
no reaction
compare
6-coordinated d2sp3 hybridized
tertiaryamine
2. SiCl4 + 2R3N:
SiCl4(NR3)2
CCl4 + 2R3N:
no reaction
compare
maximum coordination for C (sp3) no-d orbitals
3. Me2CO
acetone
Me2SiO
O
CH3CCH3
σ and pπ-pπ bonds
Si-O-Si-O-Si-O
pπ-pπ bonding in Si is weak compared to σ
The π bonding is strongest in the 2nd row elements
4. (SiH3)3N:
Planar
Si
N
the reason why a planar molecule exists is because of
pπ−dπ orbital overlap.
the N dumps electron density to the d orbitals on the
Silicon.
Si
Si
sp2 has a π-orbital with electrons to share
with the Si d orbitalto give a planar pπ-dπ bond.
Shares electrons with three
Si at the same time.
(CH3)3N: Pyramidal - the N acts as a Lewis base (electron pair donor)
that is how the N can react with other molecules
N
CH3
H3C
sp3
CH3
hybridized
C does not
have d orbitals so
cannot bond like Si
resembles the ammonia molecule
NH3
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The pπ-dπ bond is an example of a DATIVE bond – both electrons come from the
same element.
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The strength of the bond is determined by the degree of overlapping orbitals.
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The similar size of orbital, the better overlap occurs.
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The 3d orbitals decrease in size from left to right (because of effective nuclear
charge).
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The overlap of N (2p orbitals) and Si (3d orbitals) is good enough and favorable
to allow for the planar shape.