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Some physical properties of C subgroup elements
Physical properties Carbon
There are four allotropic modifications: diamond, graphite, carbin and fullerene (C60). Diamond.In nature, it is formed at large depths at enormous pressures and high temperatures. In some places on Earth it “walked” up a surface in so called cimberlite tubes (from the name of small town Cimberli in South Africa). The content of diamonds in rocks is very small. So, in the richest Cimberly mine Capa from 2 ton of rocks are obtained 1 g of diamonds. Mass of diamonds is expressed in carats (1 carat = 0,205 g). The greatest from all other diamond named Coullinan (3024 carat) was found in South Africa. Clear transparent diamonds are met rarely (~ 5%). Diamond crystals are processed, forming it into definite facets, and then they are jewels - diamonds. Cloudy, leaden-grey diamonds are named bort. A greater part of all diamonds obtained consists of it. A diamond has an atomic face-centered cubic crystal lattice in which every atom of carbon has tetrahedral configuration of surrounding atoms. Crystal of diamond can be considered as an enormous polymeric molecule C¥, in which every atom of carbon is found in the sp3-hybridization state and is connected with the neighbour atoms by strong two-electronic covalent bonds. Distance of C—C = 0,154 nm. Density - 3,51 g/cm3. Diamond is absolute dielectric. Graphite. It is soft black material, easily scratched by a nail, it is “fat” at touch with a finger. Its density is considerably lower, than of diamond = 2.1-2.3 g/cm3.. It is good conductor of heat and electricity. Graphite is most thermodynamically stable modification of carbon (heat of its combustion is on 0.84 kJ/mol lower than the heat of combustion of diamond). For graphite is characteristic hexagonal crystalline structure. It has a plane-stratified structure:
In the plane of the lattice every atom of carbon is bound to the three neighbours by covalent bonds. The length of bond is less (and also stronger), than in the diamond = 0,142 nm, bond energy is 716 kJ/mol. Distance between parallel grid planes is comparatively large - 0,3345 nm. Bonds between planes are weak; the order of magnitude of intermolecular forces is about 17 kJ/mol. For this reason the hardness of graphite is low (easily slide along the planes).
An atom of carbon in graphite is found in the sp2-hybridized state, and the monoelectronic pz-orbital remains atomic. Like in C6H6 such orbitals are involved in formation of multicentral delocalized p-bond. However, if in C6H6 this bond is six-centered, in graphite it is delocalized within bounds of enormous number of atoms of carbon of one crystal lattice. Such bond reminds metallic, therefore graphite has large conductivity in the direction of planes. Interestingly, that athwart to the planes of cristallic lattices graphite is a dielectric. It is a good example of anisotropy of crystals. Carbine.Prepared by V.V. Corshac at catalytic C2H2 oxidation in 1963. It is a linear polymer. a-carbine –CºC–CºC–Cº (poliine) and b-carbine: =Ñ=Ñ=Ñ= (polycumulene) exist. In this compound sp-hybridization of atoms of carbon is realised. Carbine is black fine-crystalline powder (density 3,23—3,3 g/cm3) with semiconductor properties. Distances between chains in carbine (0,295 nm) are less than between graphite layers. Differences: a-carbine is oxidized by ozone to oxalic acid; b-carbine to coal. Carbine is found in nature as mineral chaoite. Fullerene.At the beginning C60 molecule was discovered in 1990 as a constituent of new carbon allotrope, which was named fullerene. It is produced at evaporation of graphite by voltaic arc in helium atmosphere. Two forms were isolated: yellow-brown fullerene, C60, and red-brownish cluster C70. It has uncharacteristic for allotropes of carbon property to be dissolved in various organic solvents (benzene, hexane etc.). Its density (1.65 g/cm3) is only the half of diamond density, m.p. 360°C. Fullerene is absolutely stable on air. Fullerene has a shape of soccer ball that consists of 20 hexagons and 12 pentagons. Two hexagons and one pentagon are together at one top. The length of bond C—C in a rib, which is common for two neighbouring cycles, makes up 0,1432 nm, and between five- and six-membered cycle - 0,1388 nm. Every atom of carbon has sp2-hybridized state, and one-electron pz-orbital, as well as in graphite, forms nonlocalized p-bond that includes all cluster molecule. The radius of its intramolecular cavity exceeds 0,500 nm. This is enough for placing inside any atom and even small molecules. Such compounds form at evaporation of graphite. Therefore, scientists were disturbed that they could be formed during Chernobyl disaster, where graphite reactor was burned. Long-life radioactive nuclides that have penetrated inside the fullerene sphere could cause the catastrophic consequences because carbon cluster easily passes through biological membranes of biological cells. The modern researches of fullerenes properties are very intensive.
Silicon. Elementary Si is a transparent crystalline substance with face-centred cubic diamond-like lattice. The lattice has the following parameters: density 2.328 g/cm3, m.p. 1415 ° C b.p. 3250 ° C hardness (The Moos scale) - 7. Si orbitals in its crystal are in the state of sp3-hybridization. Compared with carbon, silicon has larger atomic radius, so the length of Si—Si bonds in crystals is larger than C—C, and the strength is lower. Therefore, crystalline Si has lower hardness and melting point than diamond.
There is also a powdered amorphous Si. Investigations of recent decades have shown that it actually consists of small crystallites of a cubic silicon and some impurities. Si is one of the least electronegative elements among non-metals. Its EN is 1.8 and this parameter is close to electronegativity certain metals. Natural Si consists of 3 isotopes: 28Si (92.27%), 29Si (4.68%), 30Si (3.05%).
Germanium, tin, and lead. By its physical properties, Ge is similar to silicon; Sn and Pb are typical metals of white and blue-gray colour. Hardness and brittleness drops fast in the series Ge—Sn—Pb: Ge is extremely hard and brittle, and Pb can be scratched and rolled into thin sheets. Sn occupies an intermediate position. Sn and Pb are low melting elements: m.p. (Sn) = 231.9 oC; m.p. (Pb) = 237.4 oC, while Ge m.p. = 937.1 oC.
Allotropes. Ge has similar to diamond allotropic form. It is greyish-white and brittle substance. Crystalline Ge is a semiconductor (n-type) with energy gap width, D E = 0.785 eV. Pb has only one metallic modification. Sn forms three allotropes. b-Sn differs significantly by its density; it has a close-packed structure of metals, density of which is significantly higher than that in the structure of diamond. At transition to g-Sn density drops, it becomes fragile, and easily grinds into the powder. Transition a-Sn Û b-Sn proceeds with very low rate. Transition b-Sn Þ a-Snaccelerates lowering temperature and the rate becomes maximal at -33 °C. Especially fast is this transition at the presence of a-Sn, which particles are nuclei of crystallisation. This transformation is called “tin plague”. Since the specific volume of a-Sn is larger by 25.6% materials containing metallic Sn are transformed into powder at low temperature.
Carbon was known to the humanity from time immemorial in the form of charcoal which was used for heating, extraction of metals from ores, for medical treatment. It has got the modern name “carboneoum” (from Latine carbo “a coal”) and authentication as a chemical element in 1787.
Silicon.Silicon was first identified by Antoine Lavoisier in 1787 (as a component of the Latin silex, silicis for flint, flints). The first elemental Si samples were extracted in 1811 by Gay-Lussac and Thénard. They passed vapours of SiF4 over heated potassium. However, they didn’t know certainly what was the product formed. Properties of Si were described by J. Berzelius in 1822. He called the new element Si (lat. Silex - Si) and gave him the symbol Si. Germanium.German scientist C. Winkler discovered Ge in 1886 conducting chemical analysis of the new mineral arhyrodite, 4Ag2S•GeS2. The existence of this element was predicted D. Mendeleev in 1871. He called this element ekasilicon and predicted its properties with high accuracy. D. Mendeleev named C. Winkler the strengthener of the Periodic law. Clemens Winkler Dmitry Mendeleev Tin and lead. Metallic Sn and Pb have been known since ancient times. Their articles were found in ancient Egyptian tombs. Generally, Sn alloy with Cu (bronze) played a huge role in history of our culture. Pb had been used in ancient Rome in large amounts to produce water pipes, lead white, minium and other useful substances.
OCCURRENCE Carbon. The natural carbon consists of two isotopes 12C (98,892%) and 13C (1,108%). According to the International convention of 1960 12C isotope is chosen as a standard for computation of relative atomic masses. Composition of natural carbon includes in trace quantities also a slightly radioactive isotope 14C, which is continuously formed in mid air from atoms of N under bombardment with cosmic rays. Calculations show that about 9,8 kg of 14C forms annually in the atmosphere. Its period of half-decay is 5300 years. As for the last 20000 years the intensity of space radiation remains unchanging, formation of radio-active carbon is constant. In air it reacts with O2, transforming into CO2, and at photosynthesis it is consumed by the plants. Through them with a meal it enters in the organisms of living creatures. At dying of plants and living organisms off the isotope not anymore accumulated, but begins to decompose gradually. According the rest content of 14C in the relics of organisms died long ago it is possible to define precisely enough the period of their life. This technique has found application in archaeology, paleontology, oceanography and other fields of knowledge. The term of life is determined then according the formula: t = 8040 ln (14/ A), where: A – nuclei 14C decay number per minute; t - age, in years.
There are a lot of forms of finding of carbon in nature, it is contained in: · tissues of living organisms and products of their destruction (anthracite coal, petroleum, peat); · minerals, mainly carbonates. Most widespread mineral is calcite - CaCO3 (it forms enormous deposits known as limestone, marble, chalk and etc.). Also magnesite MgCO3, dolomite CaCO3•MgCO3 and heavy spars FeCO3, MnCO3, ZnCO3. More rarely are met in nature malachite Cu2CO3(OH)2 and soda Na2CO3. Natural carbonate rock depositscontain in itself a greater part of the Earth natural carbon (more than 99%). · a significant quantity of carbon in a form of CO2 is found in the atmosphere (~ 0,03 vol.% = 2000 billion tons). · in a very small quantities the free carbon is met in the forms of graphite and diamond.
Silicon content in the Earth crust makes up 27.6% by mass. It is found only in compounds. The most common compound of silicon in nature is silicon dioxide, SiO2 (silica). This composition have ordinary river sand, quartz, sometimes occurring semiprecious morion, citrine, amethyst, agate, jasper, which are colourful oxides of different metal. Significantly larger fraction of SiO2 (about 43% by weight of the Earth's crust) is chemically bound as silicates and aluminosilicates (kaolin, clay and a variety of rocks, usually granite). The latter is a natural mixture of quartz, mica and feldspar. As a whole, more than half of the crust consists of SiO2. Germanium, tin, and lead are not common elements, their content in the Earth's crust: Ge - 7∙10-4, Sn - 8∙10-3, Pb - 1,6∙10-3 % wt. Ge is a rare element found in trace quantities in coke obtained from bituminous coal. The predominating part of germanium is extracted from polymetallic copper-zinc and zinc ores. It has no own minerals that are commercially valuable. Minerals that are rich in Ge: germanite Cu2S∙CuS∙GeS2 arhyrodite 4Ag2S∙GeS2 occur very rarely. The most important minerals of Sn and Pb are cassiterite (tinstone), SnO2, and galena, PbS USES Carbon. Diamonds.They are applied for technical purposes - boring drilling, polishing of especially hard materials, cutting of glass, making of bearing, for precise instruments. For all this purposes the diamond is suitable thanks to its extraordinary hardness. Diamonds exceed all other materials on hardness.
Tungsten carbide milling bits Silicon. As the second most abundant element in the earth's crust, silicon is vital to the construction industry as a principal constituent of natural stone, glass, concrete and cement. Silicon's greatest impact on the modern world's economy and lifestyle has resulted from silicon wafers used as substrates in the manufacture of discrete electronic devices such as power transistors, and in the development of integrated circuits such as computer chips. It has photovoltaic applications. Production Carbon.Amorphous carbon, diamond, and graphite are produced for industrial purposes. Diamond.In our time in considerable volumes diamonds are obtained artificially, in particular in Kiev. Artificial diamonds are very small and not transparent. Therefore, they are used in industry. The process of their formation from graphite occurs at temperatures about 3000°, pressures about 100 thousand of atmospheres in the presence of catalysts and in definite reaction mediums. Graphite: coal that contains quartz (SiO2) is heated in electric furnaces by electrical current of a few thousand amperes during 12-24 hours. At these conditions graphite crystals are grown from the molten mixture. Amorphous coal (black carbon): wood or other organic matters are heated at high temperatures without access of air. Coke, wood char-coal, bone coal, animal, blood coal, soot are of great importance for industry types of “amorphous carbon”. Soot is prepared at incomplete combustion of organic substances: CH4 + O2 = C + 2H2O
In laboratory: incineration of magnesium in the atmosphere of carbon dioxide:
Silicon. Si is produced in industry mostly as an alloy with iron (ferrosilicon) by strong annealing the mixture SiO2, iron ore and coal. Content of Si is varied in the range 9-95%. Alloys with less than 20% of Si are produced in blast furnaces, and more than 20% Si in electric furnaces. It is used to remove dissolved oxygen from molten metal, as well as alloying components in the production of acid enamel. In industry: 1. Technical Si (up to 99%) for silicothermy: SiO2 + 2C Date: 2016-01-03; view: 1032
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Si a typical semiconductor with energy gap 1.08 eV. Its valence band is formed by 3p-orbitals, the conduction band — by 3d-orbitals. With temperature growth, some electrons can shift to the conduction band and create an electrical current. Its carriers in the conduction band are electrons in the valence band carriers of charge are holes.
CO2 + 2Mg = 2MgO + C
Si + 2CO (in electric furnaces, coke)