Lecture 29 Group The elements C, Si, Ge, Sn. Pb. 3. Allotropes of C and Si. 4. Contrasting the chemistry of C and Si

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1 2P32 Principles of Inorganic Chemistry Dr. M. Pilkington Lecture 29 Group The elements C, Si, Ge, Sn. Pb 2. Group 14 (4A) and the network 3. Allotropes of C and Si Diamond and graphite Buckminsterfullerine C60 Carbon nanotubes 4. Contrasting the chemistry of C and Si

2 1. The Elements C, Si, Ge, Sn, Pb Carbon, Tin and Lead Carbon has been known in the forms of coal, oil, petroleum, natural gas, and charcoal for thousands of years. The recognition that it was an element in the modern sense, dates back to the late 18 th Century. Lavoisier carried out many experiments on the combustion of diamonds in the 1770 s. It was known that diamonds and graphite were but two forms of the same element. 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. The production and use of bronze (an alloy of copper and tin) is still older. Tin dishes were common in the 1700 s, as were tin plated materials. Tin has more stable isotopes (10) than any other element.

3 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). 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 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.

4 Reasonably pure silicon is prepared by reducing silica with carbon in an electric furnace as shown below: SiO 2 (s) + 2C Si(s) + 2CO(g) electric furnace 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 Carbon is unique in this group. No other element has an entire branch of chemistry built around it. 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.

5 3. Allotropes of C and Si Diamond and Graphite 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 C. 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 15 th Century. Imitation diamonds are yttrium-aluminium garnet, strontium titanate, or cubic zirconia, ZrSiO 4.

6 The diamond structure showing the carbon-carbon bonding. Bonds closer to the viewer are shown thicker. 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

7 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

8 Buckminsterfullerine C 60 In 1985 Harry Kroto, Richard Smalley and Robert Curl produced an amazing third allotrope of carbon. They found that when graphite was vaporized by a laser, a variety of large clusters and even numbers of carbon atoms are formed. One of the most prevalent of these was C 60. The structure remained a mystery and two shapes came into mind: (a) (b) The geodesic domes of the American architect R. Buckminster Fuller and. The shape of the European football or an American soccer ball.

9 Buckminster Fuller's Dome - Expo '67 Montreal The similarity to the geodesic dome has afforded names for C 60 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. C 60 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.

10 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. As a result of these discoveries, Kroto, Smalley and Curl received the Nobel Prize in chemistry. C70, C60's big brother

11 C60-Fullerene at 153 deg.k. C60 crystallizes in a face centered cubic arrangement.

12 Carbon Nanotubes In 1991, Sumio Iijima made still another discovery involving carbon allotropes. He produced long cylindrical (10-30 nm in diameter) multiwalled, tubelike structures with hemispherical end caps. Given their dimensions, they quickly became known as nanotubes. 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. This paved the way to an explosion of research into the physical and chemical properties of carbon nanotubes in laboratories all over the world. 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. While normally nanotubes are straight, ways have been devised to prepare them in a ring form.

13 Nanotube Rings The rings are composed of many layers of single-walled nanotubes, and have a radius of typically 0.7 micron. 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. 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). The nanotubes used to form the rings are extremely small; their diameter is only 1.4 nm. They are 1- dimensional conductors and at low temperatures, SEM (Scanning Electron Microscope) image of nanotube rings on a silicon substrate. The image is magnified 8000 times.

14 Nanotubes Properties and Applications Nanotubes are amazingly flexible, strong and stable. They have a tensile strength (ability to oppose rupture under tension) some times that of steel at one-sixth of the weight. This makes them candidates for a large number of applications and uses. 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. Buckyropes will be used in everything from aircraft frames truck and automobile body panels, bridge supports and rocket nozzles. Tennis rackets, golf clubs or fishing rods could be made out of these materials. 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.

15 In contrast Silicon One allotrope; diamond structure where every Silicon is tetrahedrally bonded to 4 others. Silica cannot undergo pπ-pπ bonding so there are no forms similar to graphite or buckyballs. Silicon hydrides exist, but there are not very many. SiH 4 SiH 6 spontaneously flammable in air availability of d orbitals in Si H O=O can react directly to the SiH 4 H Si H SiO 2 + H 2 O highly flammable H

16 4. Contrasting the chemistry of C and Si 1. SiCl 4 + H 2 O SiO 2 + 4HCl compare CCl 4 + H 2 O no reaction tertiaryamine 6-coordinated d 2 sp 3 hybridized 2. SiCl 4 + 2R 3 N: CCl 4 + 2R 3 N: SiCl 4 (NR 3 ) 2 no reaction compare maximum coordination for C (sp 3 ) no-d orbitals 3. Me 2 CO O acetone CH 3 CCH 3 σ and pπ-pπ bonds Me 2 SiO Si-O-Si-O-Si-O pπ-pπ bonding in Si is weak compared to σ The π bonding is strongest in the 2 nd row elements

17 4. (SiH 3 ) 3 N: Planar Si N Si 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 sp 2 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. (CH 3 ) 3 N: Pyramidal - the N acts as a Lewis base (electron pair donor) that is how the N can react with other molecules H 3 C N CH 3 resembles the ammonia molecule NH 3 CH 3 sp 3 hybridized C does not have d orbitals so cannot bond like Si

18 The pπ-dπ bond is an example of a DATIVE bond both electrons come from the same element. The strength of the bond is determined by the degree of overlapping orbitals. The similar size of orbital, the better overlap occurs. The 3d orbitals decrease in size from left to right (because of effective nuclear charge). The overlap of N (2p orbitals) and Si (3d orbitals) is good enough and favorable to allow for the planar shape.