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:

 = 8040 ln (14/ A), where: A – nuclei ¹⁴C decay number per minute;  - 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 deposits contain 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:

C O₂ + 2Mg = 2MgO + C

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 Si + 2CO (in electric furnaces, coke)

2. Pure (semiconductive) Si.

SiCl₄ + 2Zn Si + 2ZnCl₂,

or

SiCl₄ + 2Н₂ = Si + НCl,

Crystals of semiconductive Si with required electrophysical and structural properties can be grown (the method of zone melting).

3. Especially pure silicon is produced by silane, SiH₄, decomposition at 600-700C:

SiН₄ Si + 2Н₂,

In a laboratory:

Dry sand with powdered magnesium mixture is ignited by Mg ribbon:

SiO₂ + 2Mg = Si + 2MgO

After cooling, the mixture obtained is sequentially processed by HCl (to eliminate MgO) and HF (to dissolve excess SiO₂).

Germanium. Firstly, GeCl₄ is obtained that hydrolyzes further to GeO₂. The latter is reduced by hydrogen:

GeO₂ + 2H₂ Ge + 2H₂O

Annual production of Ge achieves several hundred tons. Semiconductor technologies are the major applications of Ge, satisfying the requirements for the extremely high purity (impurities should not exceed about 10^-9 at. %).

Tin. Tin ore is initially concentrated by flotation and then reduced by coal (Al or Zn can be also used):

SnO₂ + 2C = Sn + 2CO

Lead. In the extraction of lead, the sulfide ore is first roasted together with quartz in a current of air:

2PbS + 3O₂ = 2PbO + 2SO₂

6e 2e•2

Any lead(II) sulfate formed in this process is converted to lead(II) silicate by reaction with the quartz. The oxide produced is then mixed with limestone and coke and heated in a blast furnace. The following reactions occur:

PbO + C = Pb + CO

PbO + CO = Pb + CO₂

PbSiO₃ + CaO + CO = Pb + CaSiO₃ + CO₂

Crude lead contains traces of a number of metals.

Tin and lead of high purity are produced by electrorefining of these metals (during electrolysis SnSO₄ and Pb[SiF₆] solutions by making the crude metal the anode in an electrolytic bath). A smooth coherent deposit of lead is obtained on the pure lead cathode when the current is passed. The impurities here (i.e. all other metals) form a sludge in the electrolytic bath and are not deposited on the cathode.

The annual production of Sn and Pb in the world achieves hundreds thousands tons.

CHEMICAL PROPERTIES

Carbon

In the ground state, electronic configuration of carbon is 1s²2s²2p²:

p p

^s s

_{401 kJ}

In this state it forms three covalent bonds, two of which are formed according exchange mechanism, and one on donor-acceptor (such bond is in CO). But carbon rarely is bivalent. Excitation of its atoms occurs easily and forms a state with 4 unpaired electrons. Energy required for atom excitation is exceeded by the energy released at formation of two additional covalent bonds. A difference between s- and p-electrons is taken off by sp- sp²- or sp³-hybridization, to which is undergone an atom of carbon depending on a partner in chemical bond. Therefore, mostly carbon behaves like fourvalent element. As d-orbitals in it are absent, its maximal valence is also 4.

Identical number of valence electrons and valence orbitals takes place only at two elements - hydrogen and carbon. For this reason they can form the greatest number of compounds between itself and with other elements.

It must be taken in mind that -bonds between atoms of carbon are most strong in comparison with the homoatomic bonds of other elements, which are able to form similar chains:

kJ·mol^-1

It predetermines extraordinary abundance and variety of derivatives of carbon. From heterobonded atoms of carbon most widespread are C—H as a result of their large strength (441,2 kJ/mol). Carbon is also able to form multiple bonds.

With EN=2,5 carbon is an intermediate between electropositive and electronegative elements, although it is nearer to the last. Therefore even at the maximal polarization of atoms in its compounds there are no free ions C⁴⁺ and C^4-. Effective charges on atoms of carbon in all known compounds are considerably less then 1, that is compounds of carbon are weak polar.

In the simple substances carbon has already used its valence electrons, creating polymeric structures of diamond or graphite. Therefore in its crystalline forms carbon is absolutely chemically inert.

From this, regardless of modification, carbon has neither a taste nor smell, it is extraordinarily heavily melted and evaporates, it does not dissolve in all ordinary solvents. It is well dissolved in many molten metals (Fe, Co, Ni, the Pt metals) only, and at cooling it is again crystallized in the form of graphite.

Its chemical activity rises at transition from the diamond to amorphous carbon, which can be ignited in the atmosphere of oxygen at comparative slight heating:

С + О₂ = СО₂ Н₂₉₈ = –395 kJ/mol

With a fluorine an amorphous carbon interacts already at ordinary conditions:

C + 2F₂ = CF₄

At high temperatures carbon reacts with metals, N₂, O₂, S, Si, B, with oxides of metals and salts, for example:

C + 2S = CS₂

C + CuO = Cu + CO

CHEMICAL PROPERTIES