Atomic weight of carbon. Carbon - chemical and physical properties. Atomic and molecular mass of carbon

Municipal educational institution "Nikiforovskaya secondary school No. 1"

Carbon and its main inorganic compounds

Essay

Completed by: student of grade 9B

Sidorov Alexander

Teacher: Sakharova L.N.

Dmitrievka 2009

Introduction

Chapter I. All about carbon

1.1. Carbon in nature

1.2. Allotropic modifications of carbon

1.3. Chemical properties of carbon

1.4. Application of carbon

Chapter II. Inorganic carbon compounds

Conclusion

Literature

Introduction

Carbon (lat. Carboneum) C is a chemical element of group IV of the periodic system of Mendeleev: atomic number 6, atomic mass 12.011(1). Let's consider the structure of the carbon atom. The outer energy level of the carbon atom contains four electrons. Let's depict it graphically:

Carbon has been known since ancient times, and the name of the discoverer of this element is unknown.

At the end of the 17th century. Florentine scientists Averani and Tardgioni tried to fuse several small diamonds into one large one and heated them with a burning glass using sunlight. The diamonds disappeared, burning in the air. In 1772, the French chemist A. Lavoisier showed that when diamonds burn, CO 2 is formed. Only in 1797 did the English scientist S. Tennant prove the identity of the nature of graphite and coal. After burning equal amounts of coal and diamond, the volumes of carbon monoxide (IV) turned out to be the same.

The variety of carbon compounds, explained by the ability of its atoms to combine with each other and the atoms of other elements in various ways, determines the special position of carbon among other elements.

ChapterI. All about carbon

1.1. Carbon in nature

Carbon is found in nature, both in a free state and in the form of compounds.

Free carbon occurs in the form of diamond, graphite and carbyne.

Diamonds are very rare. The largest known diamond, the Cullinan, was found in 1905 in South Africa, weighed 621.2 g and measured 10x6.5x5 cm. The Diamond Fund in Moscow houses one of the largest and most beautiful diamonds in world – “Orlov” (37.92 g).

Diamond got its name from the Greek. "adamas" - invincible, indestructible. The most significant diamond deposits are located in South Africa, Brazil, and Yakutia.

Large deposits of graphite are located in Germany, Sri Lanka, Siberia, and Altai.

The main carbon-containing minerals are: magnesite MgCO 3, calcite (lime spar, limestone, marble, chalk) CaCO 3, dolomite CaMg(CO 3) 2, etc.

All fossil fuels - oil, gas, peat, coal and brown coal, shale - are built on a carbon basis. Some fossil coals, containing up to 99% C, are close in composition to carbon.

Carbon accounts for 0.1% of the earth's crust.

In the form of carbon monoxide (IV) CO 2, carbon enters the atmosphere. A large amount of CO 2 is dissolved in the hydrosphere.

1.2. Allotropic modifications of carbon

Elementary carbon forms three allotropic modifications: diamond, graphite, carbine.

1. Diamond is a colorless, transparent crystalline substance that refracts light rays extremely strongly. Carbon atoms in diamond are in a state of sp 3 hybridization. In the excited state, the valence electrons in the carbon atoms are paired and four unpaired electrons are formed. When chemical bonds are formed, the electron clouds acquire the same elongated shape and are located in space so that their axes are directed towards the vertices of the tetrahedron. When the tops of these clouds overlap with clouds of other carbon atoms, covalent bonds occur at an angle of 109°28", and an atomic crystal lattice characteristic of diamond is formed.

Each carbon atom in diamond is surrounded by four others, located from it in directions from the center of the tetrahedrons to the vertices. The distance between atoms in tetrahedra is 0.154 nm. The strength of all connections is the same. Thus, the atoms in diamond are “packed” very tightly. At 20°C, the density of diamond is 3.515 g/cm 3 . This explains its exceptional hardness. Diamond is a poor conductor of electricity.

In 1961, the Soviet Union began industrial production of synthetic diamonds from graphite.

In the industrial synthesis of diamonds, pressures of thousands of MPa and temperatures from 1500 to 3000°C are used. The process is carried out in the presence of catalysts, which can be some metals, for example Ni. The bulk of the diamonds formed are small crystals and diamond dust.

When heated without access to air above 1000°C, diamond turns into graphite. At 1750°C, the transformation of diamond into graphite occurs quickly.

Diamond structure

2. Graphite is a gray-black crystalline substance with a metallic sheen, greasy to the touch, and inferior in hardness even to paper.

Carbon atoms in graphite crystals are in a state of sp 2 hybridization: each of them forms three covalent σ bonds with neighboring atoms. The angles between the bond directions are 120°. The result is a grid made up of regular hexagons. The distance between adjacent nuclei of carbon atoms inside the layer is 0.142 nm. The fourth electron in the outer layer of each carbon atom in graphite occupies a p orbital that does not participate in hybridization.

Non-hybrid electron clouds of carbon atoms are oriented perpendicular to the layer plane and, overlapping each other, form delocalized σ bonds. Adjacent layers in a graphite crystal are located at a distance of 0.335 nm from each other and are weakly connected to each other, mainly by van der Waals forces. Therefore, graphite has low mechanical strength and easily splits into flakes, which themselves are very strong. The bond between layers of carbon atoms in graphite is partially metallic in nature. This explains the fact that graphite conducts electricity well, but not as well as metals.

Graphite structure

Physical properties in graphite vary greatly in directions - perpendicular and parallel to the layers of carbon atoms.

When heated without air access, graphite does not undergo any changes up to 3700°C. At the specified temperature, it sublimes without melting.

Artificial graphite is produced from the best grades of coal at 3000°C in electric furnaces without air access.

Graphite is thermodynamically stable over a wide range of temperatures and pressures, so it is accepted as the standard state of carbon. The density of graphite is 2.265 g/cm3.

3. Carbin is a fine-crystalline black powder. In its crystal structure, carbon atoms are connected by alternating single and triple bonds in linear chains:

−С≡С−С≡С−С≡С−

This substance was first obtained by V.V. Korshak, A.M. Sladkov, V.I. Kasatochkin, Yu.P. Kudryavtsev in the early 60s of the XX century.

It was subsequently shown that carbyne can exist in different forms and contains both polyacetylene and polycumulene chains in which the carbon atoms are linked by double bonds:

C=C=C=C=C=C=

Later, carbyne was found in nature - in meteorite matter.

Carbyne has semiconducting properties; when exposed to light, its conductivity increases greatly. Due to the existence of different types of bonds and different ways of laying chains of carbon atoms in the crystal lattice, the physical properties of carbyne can vary within wide limits. When heated without access to air above 2000°C, carbine is stable; at temperatures around 2300°C, its transition to graphite is observed.

Natural carbon consists of two isotopes

(98.892%) and (1.108%). In addition, minor admixtures of a radioactive isotope, which is produced artificially, were found in the atmosphere.

Previously, it was believed that charcoal, soot and coke are similar in composition to pure carbon and differ in properties from diamond and graphite, representing an independent allotropic modification of carbon (“amorphous carbon”). However, it was found that these substances consist of tiny crystalline particles in which the carbon atoms are bonded in the same way as in graphite.

4. Coal – finely ground graphite. It is formed during the thermal decomposition of carbon-containing compounds without air access. Coals vary significantly in properties depending on the substance from which they are obtained and the method of production. They always contain impurities that affect their properties. The most important types of coal are coke, charcoal, and soot.

Coke is produced by heating coal without access to air.

Charcoal is formed when wood is heated without access to air.

Soot is a very fine graphite crystalline powder. Formed by the combustion of hydrocarbons (natural gas, acetylene, turpentine, etc.) with limited air access.

Activated carbons are porous industrial adsorbents consisting mainly of carbon. Adsorption is the absorption of gases and dissolved substances by the surface of solids. Activated carbons are obtained from solid fuel (peat, brown and hard coal, anthracite), wood and its processed products (charcoal, sawdust, paper waste), leather industry waste, and animal materials, such as bones. Coals, characterized by high mechanical strength, are produced from the shells of coconuts and other nuts, and from fruit seeds. The structure of coals is represented by pores of all sizes, however, the adsorption capacity and adsorption rate are determined by the content of micropores per unit mass or volume of granules. When producing active carbon, the starting material is first subjected to heat treatment without access to air, as a result of which moisture and partially resins are removed from it. In this case, a large-porous structure of coal is formed. To obtain a microporous structure, activation is carried out either by oxidation with gas or steam, or by treatment with chemical reagents.

Carbon is the sixth element of Mendeleev's periodic table. Its atomic weight is 12.

Carbon is in the second period of the Mendeleev system and in the fourth group of this system.

The period number tells us that carbon's six electrons are located in two energy levels.

And the fourth group number says that carbon has four electrons at its outer energy level. Two of them are paired s-electrons, and the other two are not paired R-electrons.

The structure of the outer electron layer of the carbon atom can be expressed by the following schemes:

Each cell in these diagrams means a separate electron orbital, the arrow means an electron located in the orbital. Two arrows inside one cell are two electrons located in the same orbital, but with opposite spins.

When an atom is excited (when energy is imparted to it), one of the paired S-electrons occupied R-orbital.

An excited carbon atom can participate in the formation of four covalent bonds. Therefore, in the vast majority of its compounds, carbon exhibits a valency of four.

Thus, the simplest organic compound, hydrocarbon methane, has the composition CH 4. Its structure can be expressed by structural or electronic formulas:

The electronic formula shows that the carbon atom in the methane molecule has a stable eight-electron outer shell, and the hydrogen atoms have a stable two-electron shell.

All four covalent carbon bonds in methane (and in other similar compounds) are equal and symmetrically directed in space. The carbon atom is located, as it were, in the center of the tetrahedron (regular quadrangular pyramid), and the four atoms connected to it (in the case of methane, four hydrogen atoms) are at the vertices of the tetrahedron.

The angles between the directions of any pair of bonds are the same and amount to 109 degrees 28 minutes.

This is explained by the fact that in a carbon atom, when it forms covalent bonds with four other atoms, from one s- and three p-orbitals as a result sp 3-hybridization produces four hybrids symmetrically located in space sp 3-orbitals elongated towards the vertices of the tetrahedron.

Features of the properties of carbon.

The number of electrons in the outer energy level is the main factor determining the chemical properties of an element.

On the left side of the periodic table there are elements with a low-filled outer electronic level. Elements of the first group have one electron in the outer level, elements of the second group have two.

The elements of these two groups are metals. They are easily oxidized, i.e. lose their outer electrons and become positive ions.

On the right side of the periodic table, on the contrary, there are non-metals (oxidizing agents). Compared to metals, they have a nucleus with a larger number of protons. Such a massive nucleus provides a much stronger pull from its electron cloud.

Such elements lose their electrons with great difficulty, but they are not averse to attaching additional electrons from other atoms, i.e. oxidize them, and at the same time turn into a negative ion.

As the group number in the periodic table increases, the metallic properties of elements weaken, and their ability to oxidize other elements increases.

Carbon is in the fourth group, i.e. just in the middle between metals, which easily give up electrons, and non-metals, which easily gain these electrons.

For this reason carbon does not have a pronounced tendency to donate or gain electrons.

Carbon chains.

An exceptional property of carbon, which determines the variety of organic compounds, is the ability of its atoms to connect with strong covalent bonds to each other, forming carbon circuits of almost unlimited length.

In addition to carbon, chains of identical atoms are formed by its analogue from group IV - silicon. However, such chains contain no more than six Si atoms. Long chains of sulfur atoms are known, but the compounds containing them are fragile.

The valences of carbon atoms that are not used for mutual connection are used for the addition of other atoms or groups (in hydrocarbons - for the addition of hydrogen).

So hydrocarbons ethane ( C 2 H 6) and propane ( C 3 H 8) contain chains of two and three carbon atoms, respectively. Their structure is expressed by the following structural and electronic formulas:

Compounds are known that contain hundreds or more carbon atoms in their chains.

Due to the tetrahedral orientation of carbon bonds, its atoms included in the chain are not located in a straight line, but in a zigzag pattern. Moreover, due to the possibility of rotation of atoms around the bond axis, the chain in space can take on different shapes (conformations):

This structure of the chains makes it possible for terminal or other non-adjacent carbon atoms to come closer together. As a result of the formation of bonds between these atoms, carbon chains can close into rings (cycles), for example:

Thus, the diversity of organic compounds is also determined by the fact that with the same number of carbon atoms in a molecule, compounds with an open, open chain of carbon atoms are possible, as well as substances whose molecules contain cycles.

Simple and multiple connections.

Covalent bonds between carbon atoms formed by one pair of generalized electrons are called simple bonds.

The bond between carbon atoms can be carried out not by one, but by two or three common pairs of electrons. Then we get chains with multiple – double or triple bonds. These connections can be depicted as follows:

The simplest compounds containing multiple bonds are hydrocarbons ethylene(with double bond) and acetylene(with triple bond):

Hydrocarbons with multiple bonds are called unsaturated or unsaturated. Ethylene and acetylene are the first representatives of two homologous series - ethylene and acetylene hydrocarbons.

In this book, the word “carbon” appears quite often: in stories about green leaves and iron, about plastics and crystals, and in many others. Carbon - “giving birth coal” - is one of the most amazing chemical elements. Its history is the history of the emergence and development of life on Earth, because it is part of all living things on Earth.

What does carbon look like?

Let's do some experiments. Let's take sugar and heat it without air. It will first melt, turn brown, and then turn black and turn into coal, releasing water. If you now heat this coal in the presence of , it will burn without a residue and turn into . Therefore, sugar consisted of coal and water (sugar, by the way, is called a carbohydrate), and “sugar” coal is, apparently, pure carbon, because carbon dioxide is a compound of carbon with oxygen. This means carbon is a black, soft powder.

Let's take a gray soft graphite stone, well known to you thanks to pencils. If you heat it in oxygen, it will also burn without a residue, although a little slower than coal, and carbon dioxide will remain in the device where it burned. Does this mean that graphite is also pure carbon? Of course, but that's not all.

If a diamond, a transparent sparkling gemstone and the hardest of all minerals, is heated in oxygen in the same device, it too will burn, turning into carbon dioxide. If you heat a diamond without access to oxygen, it will turn into graphite, and at very high pressures and temperatures you can get a diamond from graphite.

So, coal, graphite and diamond are different forms of existence of the same element - carbon.

Even more amazing is the ability of carbon to “participate” in a huge number of different compounds (which is why the word “carbon” appears so often in this book).

The 104 elements of the periodic table form more than forty thousand studied compounds. And over a million compounds are already known, the basis of which is carbon!

The reason for this diversity is that carbon atoms can be connected to each other and to other atoms by strong bonds, forming complex ones in the form of chains, rings and other shapes. No element in the table except carbon is capable of this.

There is an infinite number of shapes that can be built from carbon atoms, and therefore an infinite number of possible compounds. These can be very simple substances, for example, the illuminating gas methane, in a molecule of which four atoms are bonded to one carbon atom, and so complex that the structure of their molecules has not yet been established. Such substances include

Carbon in the periodic table of elements is located in the second period in group IVA. Electronic configuration of carbon atom ls 2 2s 2 2p 2 . When it is excited, an electronic state is easily achieved in which there are four unpaired electrons in the four outer atomic orbitals:

This explains why carbon in compounds is usually tetravalent. The equality of the number of valence electrons in the carbon atom to the number of valence orbitals, as well as the unique ratio of the charge of the nucleus and the radius of the atom, gives it the ability to equally easily attach and give up electrons, depending on the properties of the partner (Section 9.3.1). As a result, carbon is characterized by various oxidation states from -4 to +4 and the ease of hybridization of its atomic orbitals according to the type sp 3, sp 2 And sp 1 during the formation of chemical bonds (section 2.1.3):

All this gives carbon the opportunity to form single, double and triple bonds not only with each other, but also with atoms of other organogenic elements. The molecules formed in this case can have a linear, branched or cyclic structure.

Due to the mobility of common electrons -MOs formed with the participation of carbon atoms, they are shifted towards the atom of a more electronegative element (inductive effect), which leads to the polarity of not only this bond, but also the molecule as a whole. However, carbon, due to the average electronegativity value (0E0 = 2.5), forms weakly polar bonds with atoms of other organogenic elements (Table 12.1). If there are systems of conjugated bonds in molecules (Section 2.1.3), delocalization of mobile electrons (MO) and lone electron pairs occurs with equalization of the electron density and bond lengths in these systems.

From the point of view of the reactivity of compounds, the polarizability of bonds plays an important role (Section 2.1.3). The greater the polarizability of a bond, the higher its reactivity. The dependence of the polarizability of carbon-containing bonds on their nature is reflected in the following series:

All the considered data on the properties of carbon-containing bonds indicate that carbon in compounds forms, on the one hand, fairly strong covalent bonds with each other and with other organogens, and on the other hand, the common electron pairs of these bonds are quite labile. As a result, both an increase in the reactivity of these bonds and stabilization can occur. It is these features of carbon-containing compounds that make carbon the number one organogen.

Acid-base properties of carbon compounds. Carbon monoxide (4) is an acidic oxide, and its corresponding hydroxide - carbonic acid H2CO3 - is a weak acid. The carbon monoxide(4) molecule is non-polar, and therefore it is poorly soluble in water (0.03 mol/l at 298 K). In this case, first, the hydrate CO2 H2O is formed in the solution, in which CO2 is located in the cavity of the associate of water molecules, and then this hydrate slowly and reversibly turns into H2CO3. Most of the carbon monoxide (4) dissolved in water is in the form of hydrate.

In the body, in red blood cells, under the action of the enzyme carboanhydrase, the equilibrium between CO2 hydrate H2O and H2CO3 is established very quickly. This allows us to neglect the presence of CO2 in the form of hydrate in the erythrocyte, but not in the blood plasma, where there is no carbonic anhydrase. The resulting H2CO3 dissociates under physiological conditions to a hydrocarbonate anion, and in a more alkaline environment to a carbonate anion:

Carbonic acid exists only in solution. It forms two series of salts - hydrocarbonates (NaHCO3, Ca(HC0 3)2) and carbonates (Na2CO3, CaCO3). Hydrocarbonates are more soluble in water than carbonates. In aqueous solutions, carbonic acid salts, especially carbonates, easily hydrolyze at the anion, creating an alkaline environment:

Substances such as baking soda NaHC03; chalk CaCO3, white magnesia 4MgC03 * Mg(OH)2 * H2O, hydrolyzed to form an alkaline environment, are used as antacids (acid neutralizers) to reduce the increased acidity of gastric juice:

The combination of carbonic acid and bicarbonate ion (H2CO3, HCO3(-)) forms a bicarbonate buffer system (section 8.5) - a nice buffer system of the blood plasma, which ensures a constant blood pH at pH = 7.40 ± 0.05.

The presence of calcium and magnesium hydrocarbonates in natural waters causes their temporary hardness. When such water is boiled, its hardness is eliminated. This occurs due to the hydrolysis of the HCO3(-) anion, the thermal decomposition of carbonic acid and the precipitation of calcium and magnesium cations in the form of insoluble compounds CaC03 and Mg(OH)2:

The formation of Mg(OH)2 is caused by complete hydrolysis of the magnesium cation, which occurs under these conditions due to the lower solubility of Mg(0H)2 compared to MgC03.

In medical and biological practice, in addition to carbonic acid, one has to deal with other carbon-containing acids. This is primarily a large variety of different organic acids, as well as hydrocyanic acid HCN. From the standpoint of acidic properties, the strength of these acids is different:

These differences are due to the mutual influence of the atoms in the molecule, the nature of the dissociating bond, and the stability of the anion, i.e., its ability to delocalize the charge.

Hydrocyanic acid, or hydrogen cyanide, HCN - colorless, highly volatile liquid (T kip = 26 °C) with the smell of bitter almonds, miscible with water in any ratio. In aqueous solutions it behaves as a very weak acid, the salts of which are called cyanides. Alkali and alkaline earth metal cyanides are soluble in water, but they hydrolyze at the anion, which is why their aqueous solutions smell like hydrocyanic acid (the smell of bitter almonds) and have a pH > 12:

With prolonged exposure to CO2 contained in the air, cyanide decomposes to release hydrocyanic acid:

As a result of this reaction, potassium cyanide (potassium cyanide) and its solutions lose their toxicity during long-term storage. Cyanide anion is one of the most powerful inorganic poisons, since it is an active ligand and easily forms stable complex compounds with enzymes containing Fe 3+ and Cu2(+) as complexing ions (Sect. 10.4).

Redox properties. Since carbon in compounds can exhibit any oxidation state from -4 to +4, during the reaction free carbon can both donate and gain electrons, acting as a reducing agent or an oxidizing agent, respectively, depending on the properties of the second reagent:

When strong oxidizing agents interact with organic substances, incomplete or complete oxidation of the carbon atoms of these compounds may occur.

Under conditions of anaerobic oxidation with a lack or absence of oxygen, carbon atoms of an organic compound, depending on the content of oxygen atoms in these compounds and external conditions, can turn into C0 2, CO, C and even CH 4, and other organogens turn into H2O, NH3 and H2S .

In the body, the complete oxidation of organic compounds with oxygen in the presence of oxidase enzymes (aerobic oxidation) is described by the equation:

From the given equations of oxidation reactions it is clear that in organic compounds only carbon atoms change the oxidation state, while the atoms of other organogens retain their oxidation state.

During hydrogenation reactions, i.e., the addition of hydrogen (a reducing agent) to a multiple bond, the carbon atoms that form it reduce their oxidation state (act as oxidizing agents):

Organic substitution reactions with the emergence of a new intercarbon bond, for example in the Wurtz reaction, are also redox reactions in which carbon atoms act as oxidizing agents and metal atoms act as reducing agents:

A similar thing is observed in the reactions of the formation of organometallic compounds:

At the same time, in alkylation reactions with the emergence of a new intercarbon bond, the role of oxidizer and reducer is played by the carbon atoms of the substrate and reagent, respectively:

As a result of the reactions of addition of a polar reagent to the substrate via a multiple intercarbon bond, one of the carbon atoms lowers the oxidation state, exhibiting the properties of an oxidizing agent, and the other increases the oxidation degree, acting as a reducing agent:

In these cases, an intramolecular oxidation-reduction reaction of carbon atoms of the substrate takes place, i.e., the process dismutation, under the influence of a reagent that does not exhibit redox properties.

Typical reactions of intramolecular dismutation of organic compounds due to their carbon atoms are the decarboxylation reactions of amino acids or keto acids, as well as the rearrangement and isomerization reactions of organic compounds, which were discussed in section. 9.3. The given examples of organic reactions, as well as reactions from Sect. 9.3 convincingly indicate that carbon atoms in organic compounds can be both oxidizing agents and reducing agents.

Carbon atom in a compound- an oxidizing agent, if as a result of the reaction the number of its bonds with atoms of less electronegative elements (hydrogen, metals) increases, because by attracting the common electrons of these bonds to itself, the carbon atom in question lowers its oxidation state.

Carbon atom in a compound- a reducing agent, if as a result of the reaction the number of its bonds with atoms of more electronegative elements increases(C, O, N, S), because by pushing away the shared electrons of these bonds, the carbon atom in question increases its oxidation state.

Thus, many reactions in organic chemistry, due to the redox duality of carbon atoms, are redox. However, unlike similar reactions in inorganic chemistry, the redistribution of electrons between the oxidizing agent and the reducing agent in organic compounds can only be accompanied by a displacement of the common electron pair of the chemical bond to the atom acting as the oxidizing agent. In this case, this connection can be preserved, but in cases of strong polarization it can be broken.

Complexing properties of carbon compounds. The carbon atom in compounds does not have lone electron pairs, and therefore only carbon compounds containing multiple bonds with its participation can act as ligands. Particularly active in complex formation processes are the electrons of the polar triple bond of carbon monoxide (2) and the hydrocyanic acid anion.

In the carbon monoxide molecule (2), the carbon and oxygen atoms form one and one -bond due to the mutual overlap of their two 2p-atomic orbitals according to the exchange mechanism. The third bond, i.e., another -bond, is formed according to the donor-acceptor mechanism. The acceptor is the free 2p atomic orbital of the carbon atom, and the donor is the oxygen atom, which provides a lone pair of electrons from the 2p orbital:

The increased bond ratio provides this molecule with high stability and inertness under normal conditions in terms of acid-base (CO is a non-salt-forming oxide) and redox properties (CO is a reducing agent at T > 1000 K). At the same time, it makes it an active ligand in complexation reactions with atoms and cations of d-metals, primarily with iron, with which it forms iron pentacarbonyl, a volatile toxic liquid:

The ability to form complex compounds with d-metal cations is the reason for the toxicity of carbon monoxide (H) for living systems (Section. 10.4) due to the occurrence of reversible reactions with hemoglobin and oxyhemoglobin containing the Fe 2+ cation, with the formation of carboxyhemoglobin:

These equilibria are shifted towards the formation of carboxyhemoglobin ННbСО, the stability of which is 210 times greater than that of oxyhemoglobin ННbО2. This leads to the accumulation of carboxyhemoglobin in the blood and, consequently, to a decrease in its ability to carry oxygen.

The hydrocyanic acid anion CN- also contains easily polarizable electrons, which is why it effectively forms complexes with d-metals, including life metals that are part of enzymes. Therefore, cyanides are highly toxic compounds (Section 10.4).

Carbon cycle in nature. The carbon cycle in nature is mainly based on the reactions of oxidation and reduction of carbon (Fig. 12.3).

Plants assimilate (1) carbon monoxide (4) from the atmosphere and hydrosphere. Part of the plant mass is consumed (2) by humans and animals. The respiration of animals and the decay of their remains (3), as well as the respiration of plants, the rotting of dead plants and the combustion of wood (4) return CO2 to the atmosphere and hydrosphere. The process of mineralization of the remains of plants (5) and animals (6) with the formation of peat, fossil coals, oil, gas leads to the transition of carbon into natural resources. Acid-base reactions (7) operate in the same direction, occurring between CO2 and various rocks with the formation of carbonates (medium, acidic and basic):

This inorganic part of the cycle leads to loss of CO2 in the atmosphere and hydrosphere. Human activity in the combustion and processing of coal, oil, gas (8), firewood (4), on the contrary, abundantly enriches the environment with carbon monoxide (4). For a long time there was confidence that thanks to photosynthesis, the concentration of CO2 in the atmosphere remains constant. However, at present, the increase in CO2 content in the atmosphere due to human activity is not compensated by its natural decrease. The total release of CO2 into the atmosphere is growing exponentially by 4-5% per year. According to calculations, in 2000 the CO2 content in the atmosphere will reach approximately 0.04% instead of 0.03% (1990).

After considering the properties and characteristics of carbon-containing compounds, the leading role of carbon should once again be emphasized

Rice. 12.3. Carbon cycle in nature

Organogen No. 1: firstly, carbon atoms form the skeleton of molecules of organic compounds; secondly, carbon atoms play a key role in redox processes, since among the atoms of all organogens, it is carbon that is most characterized by redox duality. For more information about the properties of organic compounds, see module IV "Fundamentals of Bioorganic Chemistry".

General characteristics and biological role of p-elements of group IVA. Electronic analogues of carbon are elements of group IVA: silicon Si, germanium Ge, tin Sn and lead Pb (see Table 1.2). The radii of the atoms of these elements naturally increase with increasing atomic number, and their ionization energy and electronegativity naturally decrease (Section 1.3). Therefore, the first two elements of the group: carbon and silicon are typical non-metals, and germanium, tin, and lead are metals, since they are most characterized by the loss of electrons. In the series Ge - Sn - Pb, metallic properties increase.

From the point of view of redox properties, the elements C, Si, Ge, Sn and Pb under normal conditions are quite stable with respect to air and water (the metals Sn and Pb - due to the formation of an oxide film on the surface). At the same time, lead compounds (4) are strong oxidizing agents:

Complexing properties are most characteristic of lead, since its Pb 2+ cations are strong complexing agents compared to the cations of other p-elements of group IVA. Lead cations form strong complexes with bioligands.

Elements of group IVA differ sharply both in their content in the body and in their biological role. Carbon plays a fundamental role in the life of the body, where its content is about 20%. The content of other group IVA elements in the body is within 10 -6 -10 -3%. At the same time, if silicon and germanium undoubtedly play an important role in the life of the body, then tin and especially lead are toxic. Thus, with increasing atomic mass of group IVA elements, the toxicity of their compounds increases.

Dust consisting of particles of coal or silicon dioxide SiO2, when systematically exposed to the lungs, causes diseases - pneumoconiosis. In the case of coal dust, this is anthracosis, an occupational disease of miners. When dust containing Si02 is inhaled, silicosis occurs. The mechanism of development of pneumoconiosis has not yet been established. It is assumed that with prolonged contact of silicate sand grains with biological fluids, polysilicic acid Si02 yH2O is formed in a gel-like state, the deposition of which in cells leads to their death.

The toxic effect of lead has been known to mankind for a very long time. The use of lead to make dishes and water pipes led to massive poisoning of people. Currently, lead continues to be one of the main environmental pollutants, since the release of lead compounds into the atmosphere amounts to over 400,000 tons annually. Lead accumulates mainly in the skeleton in the form of poorly soluble phosphate Pb3(PO4)2, and when bones are demineralized, it has a regular toxic effect on the body. Therefore, lead is classified as a cumulative poison. The toxicity of lead compounds is associated primarily with its complexing properties and high affinity for bioligands, especially those containing sulfhydryl groups (-SH):

The formation of complex compounds of lead ions with proteins, phospholipids and nucleotides leads to their denaturation. Often lead ions inhibit EM 2+ metalloenzymes, displacing life metal cations from them:

Lead and its compounds are poisons that act primarily on the nervous system, blood vessels and blood. At the same time, lead compounds affect protein synthesis, the energy balance of cells and their genetic apparatus.

In medicine, the following external antiseptics are used as astringents: lead acetate Pb(CH3COO)2 ZH2O (lead lotions) and lead(2) oxide PbO (lead plaster). The lead ions of these compounds react with proteins (albumin) in the cytoplasm of microbial cells and tissues, forming gel-like albuminates. The formation of gels kills microbes and, in addition, makes it difficult for them to penetrate into tissue cells, which reduces the local inflammatory response.

One of the most amazing elements, which is capable of forming a huge variety of compounds of organic and inorganic nature, is carbon. This is an element with such unusual properties that Mendeleev predicted a great future for it, speaking about features that had not yet been revealed.

Later this was practically confirmed. It became known that it is the main biogenic element of our planet, which is part of absolutely all living beings. In addition, it is capable of existing in forms that differ radically in all respects, but at the same time consist only of carbon atoms.

In general, this structure has many features, and we will try to understand them during the course of the article.

Carbon: formula and position in the system of elements

In the periodic table, the element carbon is located in group IV (according to the new model in 14), the main subgroup. Its atomic number is 6 and its atomic weight is 12.011. The designation of an element with the sign C indicates its name in Latin - carboneum. There are several different forms in which carbon exists. Its formula therefore varies and depends on the specific modification.

However, of course, there is a specific notation for writing reaction equations. In general, when talking about a substance in its pure form, the molecular formula of carbon C is accepted, without indexation.

History of element discovery

This element itself has been known since ancient times. After all, one of the most important minerals in nature is coal. Therefore, it was not a secret for the ancient Greeks, Romans and other nations.

In addition to this variety, diamonds and graphite were also used. For a long time there were many confusing situations with the latter, since compounds such as were often mistaken for graphite without analysis of the composition:

• silver lead;
• iron carbide;
• Molybdenum sulfide.

All of them were painted black and were therefore considered graphite. Later this misunderstanding was clarified, and this form of carbon became itself.

Since 1725, diamonds have become of great commercial importance, and in 1970 the technology for producing them artificially was mastered. Since 1779, thanks to the work of Karl Scheele, the chemical properties exhibited by carbon have been studied. This served as the beginning of a number of important discoveries in the field of this element and became the basis for elucidating all its unique features.

Carbon isotopes and distribution in nature

Despite the fact that the element in question is one of the most important biogenic ones, its total content in the mass of the earth’s crust is 0.15%. This happens because it is subject to constant circulation, the natural cycle of nature.

In general, we can name several mineral compounds that contain carbon. These are natural breeds such as:

• dolomites and limestones;
• anthracite;
• oil shale;
• natural gas;
• coal;
• oil;
• brown coal;
• peat;
• bitumens.

In addition, we should not forget about living beings, which are simply a repository of carbon compounds. After all, it forms proteins, fats, carbohydrates, nucleic acids, and therefore the most vital structural molecules. In general, out of 70 kg of dry body mass, 15 are accounted for by the pure element. And so it is for every person, not to mention animals, plants and other creatures.

If we consider water, that is, the hydrosphere as a whole and the atmosphere, then there is a mixture of carbon and oxygen, expressed by the formula CO 2. Dioxide or carbon dioxide is one of the main gases that make up air. It is in this form that the mass fraction of carbon is 0.046%. Even more carbon dioxide is dissolved in the waters of the World Ocean.

The atomic mass of carbon as an element is 12.011. It is known that this value is calculated as the arithmetic mean between the atomic weights of all isotopic varieties existing in nature, taking into account their abundance (as a percentage). This happens with the substance in question. There are three main isotopes in which carbon occurs. This:

• 12 C - its mass fraction is overwhelmingly 98.93%;
• 13 C - 1.07%;
• 14 C - radioactive, half-life 5700 years, stable beta emitter.

In the practice of determining the geochronological age of samples, the radioactive isotope 14 C is widely used, which is an indicator due to its long decay period.

Allotropic modifications of the element

Carbon is an element that, as a simple substance, exists in several forms. That is, it is capable of forming the largest number of allotropic modifications known today.

1. Crystalline variations - exist in the form of strong structures with regular atomic-type lattices. This group includes varieties such as:

• diamonds;
• fullerenes;
• graphites;
• carbines;
• lonsdaleites;
• and tubes.

They all have different lattices, at the nodes of which there is a carbon atom. Hence the completely unique, dissimilar properties, both physical and chemical.

2. Amorphous forms - they are formed by a carbon atom, which is part of some natural compounds. That is, these are not pure varieties, but with admixtures of other elements in small quantities. This group includes:

• Activated carbon;
• stone and wood;
• soot;
• carbon nanofoam;
• anthracite;
• glassy carbon;
• technical variety of a substance.

They are also united by the structural features of the crystal lattice, which explain and exhibit properties.

3. Carbon compounds in the form of clusters. This is a structure in which the atoms are locked into a special conformation that is hollow from the inside, filled with water or the nuclei of other elements. Examples:

• carbon nanocones;
• astralens;
• dicarbon.

Physical properties of amorphous carbon

Due to the wide variety of allotropic modifications, it is difficult to identify any general physical properties for carbon. It's easier to talk about a specific form. For example, amorphous carbon has the following characteristics.

1. All forms are based on fine-crystalline varieties of graphite.
2. High heat capacity.
3. Good conductive properties.
4. Carbon density is about 2 g/cm3.
5. When heated above 1600 0 C, a transition to graphite forms occurs.

Soot and stone varieties are widely used for technical purposes. They are not a manifestation of carbon modification in its pure form, but they contain it in very large quantities.

Crystalline carbon

There are several options in which carbon is a substance that forms regular crystals of various types, where atoms are connected in series. As a result, the following modifications are formed.

1. - cubic, in which four tetrahedrons are connected. As a result, all covalent chemical bonds of each atom are as saturated and strong as possible. This explains the physical properties: carbon density 3300 kg/m3. High hardness, low heat capacity, lack of electrical conductivity - all this is the result of the structure of the crystal lattice. There are technically produced diamonds. They are formed during the transition of graphite to the next modification under the influence of high temperature and a certain pressure. In general, it is as high as the strength - about 3500 0 C.
2. Graphite. The atoms are arranged similar to the structure of the previous substance, however, only three bonds are saturated, and the fourth becomes longer and less strong; it connects the “layers” of hexagonal lattice rings. As a result, it turns out that graphite is a soft, greasy black substance to the touch. It has good electrical conductivity and has a high melting point - 3525 0 C. Capable of sublimation - sublimation from a solid to a gaseous state, bypassing the liquid (at a temperature of 3700 0 C). The density of carbon is 2.26 g/cm3, which is much lower than that of diamond. This explains their different properties. Due to the layered structure of the crystal lattice, graphite can be used to make pencil leads. When passed over paper, the scales peel off and leave a black mark on the paper.
3. Fullerenes. They were discovered only in the 80s of the last century. They are modifications in which carbons are connected to each other into a special convex closed structure with a void in the center. Moreover, the shape of the crystal is a polyhedron, of regular organization. The number of atoms is even. The most famous form of fullerene C 60. Samples of a similar substance were found during research:

• meteorites;
• bottom sediments;
• folgurites;
• shungites;
• outer space, where they were contained in the form of gases.

All varieties of crystalline carbon are of great practical importance because they have a number of useful properties in technology.

Chemical activity

Molecular carbon exhibits low chemical reactivity due to its stable configuration. It can be forced to react only by imparting additional energy to the atom and forcing the electrons of the outer level to vaporize. At this point, the valency becomes 4. Therefore, in compounds it has an oxidation state of + 2, + 4, - 4.

Almost all reactions with simple substances, both metals and non-metals, occur under the influence of high temperatures. The element in question can be either an oxidizing agent or a reducing agent. However, the latter properties are especially pronounced in it, and this is what is based on its use in metallurgical and other industries.

In general, the ability to enter into chemical interactions depends on three factors:

• carbon dispersion;
• allotropic modification;
• reaction temperature.

Thus, in some cases, interaction with the following substances occurs:

• non-metals (hydrogen, oxygen);
• metals (aluminum, iron, calcium and others);
• metal oxides and their salts.

Does not react with acids and alkalis, very rarely with halogens. The most important property of carbon is the ability to form long chains among themselves. They can close in a cycle and form branches. This is how the formation of organic compounds occurs, which today number in the millions. The basis of these compounds are two elements - carbon and hydrogen. The composition may also include other atoms: oxygen, nitrogen, sulfur, halogens, phosphorus, metals and others.

Basic connections and their characteristics

There are many different compounds that contain carbon. The formula of the most famous of them is CO 2 - carbon dioxide. However, in addition to this oxide, there is also CO - monoxide or carbon monoxide, as well as suboxide C 3 O 2.

Among the salts that contain this element, the most common are calcium and magnesium carbonates. Thus, calcium carbonate has several synonyms in its name, since it occurs in nature in the form:

• chalk;
• marble;
• limestone;
• dolomite

The importance of alkaline earth metal carbonates is manifested in the fact that they are active participants in the formation of stalactites and stalagmites, as well as groundwater.

Carbonic acid is another compound that forms carbon. Its formula is H 2 CO 3. However, in its usual form it is extremely unstable and immediately disintegrates into carbon dioxide and water in solution. Therefore, only its salts are known, and not itself as a solution.

Carbon halides are obtained mainly indirectly, since direct syntheses occur only at very high temperatures and with low product yields. One of the most common is CCL 4 - carbon tetrachloride. A toxic compound that can cause poisoning if inhaled. Obtained by radical photochemical substitution reactions in methane.

Metal carbides are carbon compounds in which it exhibits an oxidation state of 4. It is also possible that combinations with boron and silicon exist. The main property of carbides of some metals (aluminum, tungsten, titanium, niobium, tantalum, hafnium) is high strength and excellent electrical conductivity. Boron carbide B 4 C is one of the hardest substances after diamond (9.5 according to Mohs). These compounds are used in technology, as well as the chemical industry, as sources of hydrocarbons (calcium carbide with water leads to the formation of acetylene and calcium hydroxide).

Many metal alloys are made using carbon, thereby significantly increasing their quality and technical characteristics (steel is an alloy of iron and carbon).

Numerous organic carbon compounds deserve special attention, in which it is a fundamental element capable of combining with the same atoms to form long chains of various structures. These include:

• alkanes;
• alkenes;
• arenas;
• proteins;
• carbohydrates;
• nucleic acids;
• alcohols;
• carboxylic acids and many other classes of substances.

Application of carbon

The importance of carbon compounds and its allotropic modifications in human life is very great. You can name several of the most global industries to make it clear that this is indeed the case.

1. This element forms all types of organic fuel from which humans obtain energy.
2. The metallurgical industry uses carbon as a powerful reducing agent to obtain metals from their compounds. Carbonates are also widely used here.
3. Construction and the chemical industry consume huge amounts of carbon compounds to synthesize new substances and produce necessary products.

You can also name such sectors of the economy as:

• nuclear industry;
• jewelry making;
• technical equipment (lubricants, heat-resistant crucibles, pencils, etc.);
• determination of the geological age of rocks - radioactive indicator 14 C;
• Carbon is an excellent adsorbent, which allows it to be used for the manufacture of filters.

Cycle in nature

The mass of carbon found in nature is included in a constant cycle, which cyclically occurs every second around the globe. Thus, the atmospheric source of carbon, CO 2, is absorbed by plants and released by all living beings during respiration. Once it enters the atmosphere, it is absorbed again, and so the cycle continues. In this case, the death of organic remains leads to the release of carbon and its accumulation in the ground, from where it is then again absorbed by living organisms and released into the atmosphere in the form of gas.