Carbon and silicon. Elements of the IVA group. General characteristics of the IVA group of the Periodic system Among the elements of the iva group, the most electronegative is

The IVA group contains the most important elements, without which there would be neither us nor the Earth on which we live. This is carbon - the basis of all organic life, and silicon - the "monarch" of the mineral kingdom.

If carbon and silicon are typical non-metals, and tin and lead are metals, then germanium occupies an intermediate position. Some textbooks classify it as a non-metal, while others classify it as a metal. It is silvery white in color and looks like a metal, but has a diamond-like crystal lattice and is a semiconductor, like silicon.

From carbon to lead (with decreasing non-metallic properties):

w the stability of the negative oxidation state decreases (-4)

w the stability of the highest positive oxidation state decreases (+4)

w increases the stability of a low positive oxidation state (+2)

Carbon is the main constituent of all organisms. In nature, there are both simple substances formed by carbon (diamond, graphite) and compounds (carbon dioxide, various carbonates, methane and other hydrocarbons in the composition of natural gas and oil). The mass fraction of carbon in hard coal reaches 97%.
The carbon atom in the ground state can form two covalent bonds by the exchange mechanism, but such compounds are not formed under normal conditions. A carbon atom, going into an excited state, uses all four valence electrons.
Carbon forms quite a few allotropic modifications (see Fig. 16.2). These are diamond, graphite, carbine, various fullerenes.

In inorganic substances, the oxidation state of carbon is + II and + IV. There are two oxides with these oxidation states of carbon.
Carbon monoxide (II) is a colorless toxic gas, odorless. The trivial name is carbon monoxide. It is formed during the incomplete combustion of carbon-containing fuel. Electronic structure see its molecules on page 121. In terms of chemical properties, CO is a non-salt-forming oxide; when heated, it exhibits reducing properties (reduces many oxides of not very active metals to metal).
Carbon monoxide(IV) is a colorless, odorless gas. The trivial name is carbon dioxide. Acid oxide. It is slightly soluble in water (physically), partially reacts with it, forming carbonic acid H2CO3 (the molecules of this substance exist only in very dilute aqueous solutions).
Carbonic acid is a very weak dibasic acid that forms two series of salts (carbonates and bicarbonates). Most carbonates are insoluble in water. Of the bicarbonates, only bicarbonates exist as individual substances. alkali metals and ammonium. Both the carbonate ion and the bicarbonate ion are particles of the base; therefore, both carbonates and bicarbonates in aqueous solutions undergo anion hydrolysis.
From carbonates highest value have sodium carbonate Na2CO3 (soda, soda ash, washing soda), sodium bicarbonate NaHCO3 (baking soda, baking soda), potassium carbonate K2CO3 (potash) and calcium carbonate CaCO3 (chalk, marble, limestone).
Qualitative reaction for presence in gas mixture carbon dioxide: the formation of a precipitate of calcium carbonate when the test gas is passed through lime water (saturated solution of calcium hydroxide) and the subsequent dissolution of the precipitate upon further passing of the gas. Reactions taking place:

Ca2 + 2OH + CO2 = CaCO3 + H2O;
CaCO3 + CO2 + H2O = Ca2 + 2HCO3 .

In pharmacology and medicine, various carbon compounds are widely used - derivatives of carbonic acid and carboxylic acids, various heterocycles, polymers and other compounds. So, carbolene (activated carbon) is used to absorb and remove various toxins from the body; graphite (in the form of ointments) - for the treatment of skin diseases; radioactive isotopes of carbon - for scientific research(radiocarbon analysis).

Carbon is the basis of all organic substances. Every living organism is made up largely of carbon. Carbon is the basis of life. The source of carbon for living organisms is usually CO 2 from the atmosphere or water. As a result of photosynthesis, it enters biological food chains in which living things eat each other or each other's remains and thereby extract carbon to build their own body. The biological cycle of carbon ends either with oxidation and return to the atmosphere, or with disposal in the form of coal or oil.

Analytical reactions carbonate - ion CO 3 2-

Carbonates are salts of an unstable, very weak carbonic acid H 2 CO 3, which in the free state in aqueous solutions is unstable and decomposes with the release of CO 2: H 2 CO 3 - CO 2 + H 2 O

Ammonium, sodium, rubidium, cesium carbonates are soluble in water. Lithium carbonate is slightly soluble in water. Other metal carbonates are slightly soluble in water. Hydrocarbons dissolve in water. Carbonate - ions in aqueous solutions are colorless, undergo hydrolysis. Aqueous solutions of alkali metal bicarbonates do not stain when a drop of phenolphthalein solution is added to them, which makes it possible to distinguish carbonate solutions from bicarbonate solutions (pharmacopoeia test).

1. Reaction with barium chloride.

Ba 2+ + COz 2 - -> BaCO 3 (white fine crystalline)

Similar precipitates of carbonates give calcium cations (CaCO 3) and strontium (SrCO 3). The precipitate is soluble in mineral acids and in acetic acid. In a solution of H 2 SO 4 a white precipitate BaSO 4 is formed.

A solution of HC1 is slowly added dropwise to the precipitate until the precipitate is completely dissolved: BaCO3 + 2 HC1 -> BaC1 2 + CO 2 + H 2 O

2. Reaction with magnesium sulfate (pharmacopoeia).

Mg 2+ + CO3 2 - -> MgCO 3 (white)

Bicarbonate - HCO 3 ion - forms a precipitate of MgCO 3 with magnesium sulfate only when boiling: Mg 2+ + 2 HCO3- -> MgCO 3 + CO 2 + H 2 O

The precipitate of MgCO 3 dissolves in acids.

3. Reaction with mineral acids (pharmacopoeia).

CO 3 2- + 2 H 3 O \u003d H 2 CO 3 + 2H 2 O

HCO 3 - + H 3 O + = H 2 CO 3 + 2H 2 O

H 2 CO 3 -- CO 2 + H 2 O

Evolved gaseous CO 2 is detected by turbidity of baritone or lime water in a device for detecting gases, gas bubbles (CO 2), in a test tube - receiver - turbidity of the solution.

4. Reaction with uranyl hexacyanoferrate (II).

2CO 3 2 - + (UO 2) 2 (brown) -> 2 UO 2 CO 3 (colorless) + 4 -

A brown solution of uranyl hexacyanoferrate (II) is obtained by mixing a solution of uranyl acetate (CH 3 COO) 2 UO 2 with a solution of potassium hexacyanoferrate (II):

2(CH 3 COO) 2 GO 2 + K 4 -> (UO 2) 2 + 4 CH 3 COOK

To the resulting solution is added dropwise a solution of Na 2 CO 3 or K 2 CO 3 with stirring until the brown color disappears.

5. Separate discovery of carbonate - ions and bicarbonate - ions by reactions with calcium cations and ammonia.

If the solution simultaneously contains carbonate - ions and bicarbonate - ions, then each of them can be opened separately.

To do this, first, an excess of CaCl 2 solution is added to the analyzed solution. In this case, CO3 2 - is precipitated in the form of CaCO 3:

COz 2 - + Ca 2+ \u003d CaCO 3

Bicarbonate - ions remain in solution, since Ca (HCO 3) 2 solutions in water. The precipitate is separated from the solution and ammonia solution is added to the latter. HCO 2 - -anions with ammonia and calcium cations again precipitate CaCO 3: HCO s - + Ca 2+ + NH 3 -> CaCO3 + NH 4 +

6. Other reactions of the carbonate - ion.

Carbonate - ions when reacting with iron (III) chloride FeCl 3 form a brown precipitate Fe (OH) CO 3, with silver nitrate - a white precipitate of silver carbonate Ag 2 CO3, soluble in HbTO3 and decomposing when boiling in water to a dark precipitate Ag 2 O ISO 2: Ag 2 CO 3 -> Ag 2 O + CO 2

Analytical reactions of acetate - ion CH 3 COO "

Acetate - ion CH 3 COO- - anion of a weak monobasic acetic acid CH 3 COOH: colorless in aqueous solutions, undergoes hydrolysis, does not have redox properties; a fairly effective ligand and forms stable acetate complexes with many metal cations. When reacting with alcohols acidic environment gives esters.

Ammonium, alkali and most other metal acetates are highly soluble in water. Silver acetates CH 3 COOAg and mercury (I) are less soluble in water than acetates of other metals.

1. Reaction with iron (III) chloride (pharmacopoeia).

At pH = 5-8, the acetate - ion with Fe (III) cations forms a soluble dark red (strong tea color) acetate or iron (III) hydroxyacetate.

In aqueous solution, it is partially hydrolyzed; acidification of the solution with mineral acids inhibits hydrolysis and leads to the disappearance of the red color of the solution.

3 CH3COOH + Fe --> (CH 3 COO) 3 Fe + 3 H +

When boiling, a red-brown precipitate of basic iron acetate (III) precipitates from the solution:

(CH 3 COO) 3 Fe + 2 H 2 O<- Fe(OH) 2 CH 3 COO + 2 СН 3 СООН

Depending on the ratio of the concentrations of iron (III) and acetate ions, the composition of the precipitate may change and correspond, for example, to the formulas: Fe OH (CH 3 COO) 2, Fe 3 (OH) 2 O 3 (CH 3 COO), Fe 3 O (OH) (CH 3 COO) 6 or Fe 3 (OH) 2 (CH 3 COO) 7.

The reaction is interfered with by anions CO 3 2 -, SO 3 "-, PO 4 3 -, 4, which form precipitates with iron (III), as well as SCN- anions (giving red complexes with Fe 3+ cations), iodide - ion G, oxidizing to iodine 1 2, giving the solution a yellow color.

2. Reaction with sulfuric acid.

Acetate - an ion in a strongly acidic environment turns into weak acetic acid, the vapors of which have a characteristic smell of vinegar:

CH 3 COO- + H +<- СН 3 СООН

The reaction is hindered by anions NO 2 \ S 2 -, SO 3 2 -, S 2 O 3 2 -, which also emit gaseous products with a characteristic odor in a concentrated H 2 SO4 medium.

3. The reaction of the formation of acetic ethyl ether (pharmacopoeia).

The reaction is carried out in a sulfuric acid medium. With ethanol:

CH 3 COO- + H + -- CH 3 COOH CH 3 COOH + C 2 H 5 OH \u003d CH 3 COOS 2 H 4 + H 2 O

The released ethyl acetate is detected by a characteristic pleasant smell. Silver salts catalyze this reaction, so it is recommended to add a small amount of AgNO 3 during the reaction.

Similarly, when reacting with amyl alcohol C 5 HcOH, a pleasant-smelling amyl acetate CH 3 COOS 5 Ni (-pear-) is also formed. A characteristic smell of ethyl acetate is felt, which increases with careful heating of the mixture.

Analytical reactions tartrate - ROS ion - CH(OH) - CH(OH) - COMP. Tartrate ion - anion of a weak dibasic tartaric acid:

HO-CH-COOH

HO-CH-COOH

Tartrate - an ion is highly soluble in water. In aqueous solutions, tartrate ions are colorless, undergo hydrolysis, and are prone to complex formation, giving stable tartrate complexes with cations of many metals. Tartaric acid forms two rows of salts - medium tartrates containing a double charge tartrate - COCH (OH) CH (OH) COO - ion, and acid tartrates - hydro tartrates containing a singly charged hydro tartrate - HOOOCH (OH) CH (OH) COO - ion. Potassium hydrotartrate (-tartar-) KNS 4 H 4 O 6 is practically insoluble in water, which is used to open potassium cations. The average calcium salt is also slightly soluble in water. The average potassium salt K 2 C 4 H 4 O 6 is highly soluble in water.

I. Reaction with potassium chloride (pharmacopoeia).

C 4 H 4 O 6 2 - + K + + H + -> KNS 4 H 4 O 6 1 (white)

2. Reaction with resorcinol in an acidic medium (pharmacopoeia).

Tartrates, when heated with resorcinol meta - C 6 H 4 (OH) 2 in a medium of concentrated sulfuric acid, form cherry red reaction products.

14) Reactions with the ammonia complex of silver. A black precipitate of metallic silver falls out.

15) Reaction with iron (II) sulfate and hydrogen peroxide.

Addition of a dilute aqueous solution of FeSO 4 and H 2 O 2 to a solution containing tartrates. leads to the formation of an unstable iron complex of a crushed color. Subsequent treatment with an alkali solution of NaOH leads to a blue coloration of the complex.

Analytical reactions of the oxalate ion C 2 O 4 2-

Oxalate ion C 2 O 4 2- - anion of dibasic oxalic acid H 2 C 2 O 4 of medium strength, relatively well soluble in water. Oxalate ion in aqueous solutions is colorless, partially hydrolyzed, strong reducing agent, effective ligand - forms stable oxalate complexes with cations of many metals. Oxalates of alkali metals, magnesium and ammonium are soluble in water, while other metals are slightly soluble in water.

1 Reaction with barium chloride Ba 2+ + C 2 O 4 2- \u003d BaC 2 O 4 (white) The precipitate dissolves in mineral acids and in acetic acid (when boiling). 2. Reaction with calcium chloride (pharmacopoeia): Ca 2+ + C 2 O 4 2 - = CaC 2 O 4 (white)

The precipitate is soluble in mineral acids but insoluble in acetic acid.

3. Reaction with silver nitrate.

2 Ag + + C 2 O 4 2 - -> Ag2C2O 4 .|. (curdled) Solubility test. The sediment is divided into 3 parts:

but). Add HNO 3 solution dropwise to the first test tube with the precipitate with stirring until the precipitate dissolves;

b). Add a concentrated solution of ammonia dropwise to the second test tube with a precipitate with stirring until the precipitate dissolves; in). Add 4-5 drops of HCl solution to the third test tube with sediment; a white precipitate of silver chloride remains in the test tube:

Ag 2 C 2 O 4 + 2 HC1 -> 2 AC1 (white) + H 2 C 2 O 4

4. Reaction with potassium permanganate. Oxalate ions with KMPO 4 in an acidic environment are oxidized with the release of CO 2; the KMnO 4 solution becomes colorless due to the reduction of manganese (VII) to manganese (II):

5 C 2 O 4 2 - + 2 MnO 4 "+ 16 H + -> 10 CO 2 + 2 Mp 2+ + 8 H 2 O

Dilute solution of KMPO 4 . The latter is discolored; there is a release of gas bubbles - CO 2 .

38 Elements of the VA group

general characteristics VA group of the Periodic Table. in the form s x p y the electronic configuration of the external energy level of the elements of the VA group.

Arsenic and antimony have different allotropic modifications: both with molecular and metallic crystal lattices. However, based on a comparison of the stability of cationic forms (As 3+ , Sb 3+), arsenic is classified as a non-metal, and antimony as a metal.

oxidation states stable for elements of the VA group

From nitrogen to bismuth (with decreasing non-metallic properties):

w decreases the stability of the negative oxidation state (-3) (m. properties of hydrogen compounds)

w the stability of the highest positive oxidation state decreases (+5)

w increases the stability of a low positive oxidation state (+3)

Elements carbon C, silicon Si, germanium Ge, tin Sn and lead Pb make up the IVA group of the Periodic Table of D.I. Mendeleev. General electronic formula valence level of atoms of these elements - n s 2n p 2 , the predominant oxidation states of elements in +2 and +4 compounds. By electronegativity, the elements C and Si are classified as non-metals, and Ge, Sn and Pb are referred to as amphoteric elements, metallic properties which increase as the serial number increases. Therefore, in tin(IV) and lead(IV) compounds chemical bonds covalent, for lead(II) and to a lesser extent for tin(II) ionic crystals are known. In the series of elements from C to Pb, the stability of the +4 oxidation state decreases, and the +2 oxidation state increases. Lead(IV) compounds are strong oxidizing agents, compounds of other elements in the +2 oxidation state are strong reducing agents.

Simple substances carbon, silicon and germanium are chemically quite inert and do not react with water and non-oxidizing acids. Tin and lead also do not react with water, but under the action of non-oxidizing acids they pass into solution in the form of tin(II) and lead(II) aquacations. Alkalis do not transfer carbon into solution, silicon is transferred with difficulty, and germanium reacts with alkalis only in the presence of oxidizing agents. Tin and lead react with water in an alkaline medium, turning into hydroxo complexes of tin(II) and lead(II). Reactivity simple substances IVA-group-py increases with increasing temperature. So, when heated, they all react with metals and non-metals, as well as with oxidizing acids (HNO 3, H 2 SO 4 (conc.), etc.). In particular, concentrated nitric acid, when heated, oxidizes carbon to CO 2 ; silicon chemically dissolves in a mixture of HNO 3 and HF, turning into hydrogen hexafluorosilicate H 2 . Dilute nitric acid converts tin to tin(II) nitrate, and concentrated nitric acid to hydrated tin(IV) oxide SnO 2 n H 2 O, called β - tin acid. Lead under the influence of hot nitric acid forms lead(II) nitrate, while cold nitric acid passivates the surface of this metal (an oxide film is formed).

Carbon in the form of coke is used in metallurgy as a strong reducing agent that forms CO and CO 2 in air. This makes it possible to obtain free Sn and Pb from their oxides - natural SnO 2 and PbO, obtained by roasting ores containing lead sulfide. Silicon can be obtained by the magnesium thermal method from SiO 2 (with an excess of magnesium, Mg 2 Si silicide is also formed).

Chemistry carbon- it's mostly chemistry organic compounds. Of the inorganic derivatives of carbon, carbides are characteristic: salt-like (such as CaC 2 or Al 4 C 3), covalent (SiC) and metal-like (for example, Fe 3 C and WC). Many salt-like carbides are completely hydrolyzed with the release of hydrocarbons (methane, acetylene, etc.).

Carbon forms two oxides: CO and CO 2 . Carbon monoxide is used in pyrometallurgy as a strong reducing agent (it converts metal oxides into metals). CO is also characterized by addition reactions with the formation of carbonyl complexes, for example. Carbon monoxide is a non-salt-forming oxide; it is poisonous ("carbon monoxide"). Carbon dioxide is an acid oxide, in aqueous solution it exists in the form of CO 2 · H 2 O monohydrate and weak dibasic carbonic acid H 2 CO 3. Soluble salts of carbonic acid - carbonates and bicarbonates - due to hydrolysis have pH > 7.

Silicon forms several hydrogen compounds (silanes), which are highly volatile and reactive (self-ignite in air). To obtain silanes, the interaction of silicides (for example, magnesium silicide Mg 2 Si) with water or acids is used.

Silicon in the +4 oxidation state is included in SiO 2 and very numerous and often very complex in structure and composition of silicate ions (SiO 4 4–; Si 2 O 7 6–; Si 3 O 9 6–; Si 4 O 11 6– ; Si 4 O 12 8–, etc.), the elementary fragment of which is a tetrahedral group. Silicon dioxide is an acidic oxide; it reacts with alkalis during fusion (forming polymetasilicates) and in solution (forming orthosilicate ions). From solutions of alkali metal silicates, under the action of acids or carbon dioxide, a precipitate of silicon dioxide hydrate SiO 2 n H 2 O, in equilibrium with which a weak ortho-silicic acid H 4 SiO 4 is always in solution in a small concentration. Aqueous solutions of alkali metal silicates have pH > 7 due to hydrolysis.

Tin And lead in the +2 oxidation state they form the oxides SnO and PbO. Tin(II) oxide is thermally unstable and decomposes into SnO 2 and Sn. Lead(II) oxide, on the other hand, is very stable. It is formed during the combustion of lead in air and is found in nature. Tin(II) and lead(II) hydroxides are amphoteric.

Tin(II) aquacation exhibits strong acidic properties and is therefore stable only at pH< 1 в среде хлорной или азотной кислот, анионы которых не обладают заметной склонностью вхо­дить в состав комплексов олова(II) в качестве лигандов. При раз­бавлении таких растворов выпадают осадки основных солей раз­личного состава. Галогениды олова(II) – ковалентные соединения, поэтому при растворении в воде, например, SnCl 2 протекает внача­ле гидратация с образованием , а затем гидролиз до выпадения осадка вещества условного состава SnCl(OH). При наличии избытка хлороводородной кислоты, SnCl 2 нахо­дится в растворе в виде комплекса – . Большинство солей свинца(II) (например, иодид, хлорид, сульфат, хромат, карбонат, сульфид) малорастворимы в воде.

Tin(IV) and lead(IV) oxides are amphoteric with a predominance of acidic properties. They are answered by EO 2 polyhydrates n H 2 O, passing into solution in the form of hydroxo complexes under the action of an excess of alkalis. Tin(IV) oxide is formed during the combustion of tin in air, and lead(IV) oxide can only be obtained by the action of strong oxidizing agents (for example, calcium hypochlorite) on lead(II) compounds.

Covalent tin(IV) chloride is completely hydrolyzed by water with the release of SnO 2, and lead(IV) chloride decomposes under the action of water, releasing chlorine and being reduced to lead(II) chloride.

Tin(II) compounds exhibit reducing properties, especially strong in an alkaline environment, and lead(IV) compounds exhibit oxidizing properties, especially strong in an acidic environment. A common lead compound is its double oxide (Рb 2 II Рb IV)О 4 . This compound decomposes under the action of nitric acid, and lead (II) passes into solution in the form of a cation, and lead (IV) oxide precipitates. The lead(IV) present in the double oxide is responsible for the strong oxidizing properties of this compound.

Germanium(IV) and tin(IV) sulfides, due to the amphoteric nature of these elements, when an excess of sodium sulfide is added, form soluble thiosalts, for example, Na 2 GeS 3 or Na 2 SnS 3 . The same tin(IV) thiosalt can be obtained from tin(II) sulfide SnS by its oxidation with sodium polysulfide. Thiosalts are destroyed under the action of strong acids with the release of gaseous H 2 S and a deposit of GeS 2 or SnS 2 . Lead(II) sulfide does not react with polysulfides, and lead(IV) sulfide is unknown.

IVA group chemical elements periodic system D.I. Mendeleev includes non-metals (carbon and silicon), as well as metals (germanium, tin, lead). The atoms of these elements contain on the outer energy level four electrons (ns 2 np 2), two of which are unpaired. Therefore, the atoms of these elements in compounds can exhibit valency II. Atoms of group IVA elements can go into an excited state and increase the number of unpaired electrons up to 4 and, accordingly, in compounds exhibit a higher valence equal to the number of group IV. Carbon in compounds exhibits oxidation states from –4 to +4, for the rest, oxidation states stabilize: –4, 0, +2, +4.

In a carbon atom, unlike all other elements, the number of valence electrons is equal to the number of valence orbitals. This is one of the main reasons for the stability of the C–C bond and the exceptional tendency of carbon to form homochains, as well as the existence of a large number of carbon compounds.

Changes in the properties of atoms and compounds in the C–Si–Ge–Sn–Pb series show secondary periodicity (Table 5).

Table 5 - Characteristics of atoms of elements of group IV

	6C	1 4 Si	3 2 Ge	50 sn	82Pb
Atomic mass	12,01115	28,086	72,59	118,69	207,19
Valence electrons	2s 2 2p 2	3s 2 3p 2	4s 2 4p 2	5s 2 5p 2	6s 2 6p 2
Covalent radius of an atom, Ǻ	0,077	0,117	0,122	0,140	–
Metallic atomic radius, Ǻ	–	0,134	0,139	0,158	0,175
Conditional ion radius, E 2+ , nm	–	–	0,065	0,102	0,126
Conditional ion radius E 4+ , ​​nm	–	0,034	0,044	0,067	0,076
Ionization energy E 0 - E +, ev	11,26	8,15	7,90	7,34	7,42
Content in earth's crust, at. %	0,15	20,0	2∙10 –4	7∙10 – 4	1,6∙10 – 4

Secondary periodicity (nonmonotonic change in the properties of elements in groups) is due to the nature of the penetration of external electrons to the nucleus. Thus, the nonmonotonicity of the change in atomic radii upon passing from silicon to germanium and from tin to lead is due to the penetration of s-electrons, respectively, under the screen of 3d 10 electrons in germanium and the double screen of 4f 14 and 5d 10 electrons in lead. Since the penetrating power decreases in the series s>p>d, the internal periodicity in the change in properties is most clearly manifested in the properties of elements determined by s-electrons. Therefore, it is most typical for compounds of elements of the A-groups of the periodic system, corresponding to the highest degree element oxidation.

Carbon differs significantly from other p-elements of the group by its high ionization energy.

Carbon and silicon have polymorphic modifications with different structures of crystal lattices. Germanium belongs to the metals, silvery-white in color with a yellowish tinge, but has a diamond-like atomic crystal lattice with strong covalent bonds. Tin has two polymorphic modifications: a metallic modification with a metallic crystal lattice and a metallic bond; non-metallic modification with an atomic crystal lattice, which is stable at temperatures below 13.8 C. Lead is a dark gray metal with a metallic face-centered cubic crystal lattice. A change in the structure of simple substances in the series germanium-tin-lead corresponds to a change in their physical properties. So germanium and non-metallic tin are semiconductors, metallic tin and lead are conductors. The change in the type of chemical bond from predominantly covalent to metallic is accompanied by a decrease in the hardness of simple substances. So, germanium is quite hard, while lead is easily rolled into thin sheets.

Compounds of elements with hydrogen have the formula EN 4: CH 4 - methane, SiH 4 - silane, GeH 4 - german, SnH 4 - stannan, PbH 4 - plumbane. Insoluble in water. From top to bottom, in the series of hydrogen compounds, their stability decreases (the plumbane is so unstable that its existence can only be judged by indirect signs).

Compounds of elements with oxygen have the general formulas: EO and EO 2. The oxides CO and SiO are non-salt-forming; GeO, SnO, PbO are amphoteric oxides; CO 2, SiO 2 GeO 2 - acidic, SnO 2, PbO 2 - amphoteric. With an increase in the degree of oxidation, the acidic properties of oxides increase, while the basic properties weaken. The properties of the corresponding hydroxides change similarly.

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know

• position of carbon and silicon in the periodic table, presence in nature and practical application;
• atomic structure, valency, oxidation states of carbon and silicon;
• methods of obtaining and properties of simple substances - graphite, diamond and silicon; new allotropic forms of carbon;
• main types of carbon and silicon compounds;
• features of the elements of the germanium subgroup;

be able to

• draw up equations for the reactions of obtaining simple substances of carbon and silicon and reactions characterizing the chemical properties of these substances;
• compare the properties of elements in the carbon group;
• characterize practically important compounds of carbon and silicon;
• carry out calculations according to the equations of reactions in which carbon and silicon participate;

own

Skills for predicting the course of reactions involving carbon, silicon and their compounds.

The structure of atoms. Prevalence in nature

Group IVA of the periodic table consists of five elements with even atomic numbers: carbon C, silicon Si, germanium Ge, tin Sn and lead Pb (Table 21.1). In nature, all elements of the group are mixtures of stable isotopes. Carbon has two isogones - *|С (98.9%) and *§С (1.1%). In addition, in nature there are traces of the radioactive isotope "|C with t t= 5730 years. It is constantly formed during collisions of cosmic radiation neutrons with nitrogen nuclei in the earth's atmosphere:

Table 21.1

Characteristics of the elements of the IVA group

* Biogenic element.

The main isotope of carbon is special meaning in chemistry and physics, since on its basis the atomic mass unit is adopted, namely { /2 part of the mass of an atom ‘ICO Yes).

Silicon has three isotopes in nature; among them, the most common is ^)Si (92.23%). Germanium has five isotopes (j^Ge - 36.5%). Tin - 10 isotopes. This is a record among chemical elements. The most common is 12 5 gSn (32.59%). Lead has four isotopes: 2 SgPb (1.4%), 2 S|Pb (24.1%), 2S2βL (22.1%), and 2S2βL (52.4%). The last three isotopes of lead are the end products of the decay of natural radioactive isotopes of uranium and thorium, and therefore their content in the earth's crust has increased throughout the entire existence of the Earth.

In terms of prevalence in the earth's crust, carbon is among the top ten chemical elements. It occurs in the form of graphite, many varieties of coal, as part of oil, natural combustible gas, limestone layers (CaCO e), dolomite (CaCO 3 -MgC0 3) and other carbonates. Natural diamond, although it makes up an insignificant part of the available carbon, is extremely valuable as a beautiful and hardest mineral. But, of course, the highest value of carbon lies in the fact that it is the structural basis of bioorganic substances that form the bodies of all living organisms. Carbon is rightly considered the first among many chemical elements necessary for the existence of life.

Silicon is the second most abundant element in the earth's crust. Sand, clay, and many rocks that you see are made up of silicon minerals. With the exception of crystalline varieties of silicon oxide, all of its natural compounds are silicates, i.e. salts of various silicic acids. These acids themselves have not been obtained as individual substances. Orthosilicates contain SiOj ~ ions, metasilicates consist of polymer chains (Si0 3 ") w. Most silicates are built on a framework of silicon and oxygen atoms, between which atoms of any metals and some non-metals (fluorine) can be located. Widely known silicon minerals include quartz Si0 2, feldspars (orthoclase KAlSi 3 0 8), micas (muscovite KAl 3 H 2 Si 3 0 12). In total, more than 400 silicon minerals are known. Silicon compounds are more than half of jewelry and ornamental stones. Oxygen-silicon framework causes low solubility silicon minerals in water.Only from hot underground springs, over thousands of years, growths and crusts of silicon compounds can be deposited.Jasper belongs to rocks of this type.

There is no need to talk about the time of discovery of carbon, silicon, tin and lead, since they have been known in the form of simple substances or compounds since ancient times. Germanium was discovered by K. Winkler (Germany) in 1886 in the rare mineral argyrodite. It soon became clear that the existence of an element with such properties was predicted by D. I. Mendeleev. The naming of the new element caused controversy. Mendeleev, in a letter to Winkler, strongly supported the name germanium.

Group IVA elements have four valence electrons on the outer s- and p-sublevels:

Electronic formulas of atoms:

In the ground state, these elements are divalent, and in the excited state they become tetravalent:

Carbon and silicon form very little chemical compounds in the divalent state; in almost all stable compounds they are tetravalent. Further down the group, for germanium, tin, and lead, the stability of the divalent state increases and the stability of the tetravalent state decreases. Therefore, lead(IV) compounds behave as strong oxidizers. This pattern is also manifested in the VA group. An important difference carbon from the remaining elements of the group is the ability to form chemical bonds in three different states of hybridization - sp, sp2 And sp3. Silicon has practically only one hybrid state left. sp3. This is clearly manifested when comparing the properties of carbon and silicon compounds. For example, carbon monoxide CO 2 is a gas (carbon dioxide), and silicon oxide Si0 2 is a refractory substance (quartz). The first substance is gaseous because at sp- carbon hybridization all covalent bonds are closed in the CO 2 molecule:

The attraction between molecules is weak, and this determines the state of matter. In silicon oxide, four hybrid 5p 3 silicon orbitals cannot be closed on two oxygen atoms. A silicon atom is bonded to four oxygen atoms, each of which is in turn bonded to another silicon atom. It turns out a frame structure with the same strength of bonds between all atoms (see diagram, vol. 1, p. 40).

Compounds of carbon and silicon with the same hybridization, for example, methane CH 4 and silane SiH 4, are similar in structure and physical properties. Both substances are gases.

The electronegativity of the IVA elements is lower compared to the elements of the VA group, and this is especially noticeable in the elements of the 2nd and 3rd periods. The metallicity of elements in the IVA group is more pronounced than in the VA group. Carbon in the form of graphite is a conductor. Silicon and germanium are semiconductors, while tin and lead are true metals.

Element	C	Si	Ge	sn	Pb
Serial number	6	14	32	50	82
Atomic mass (relative)	12,011	28,0855	72,59	118,69	207,2
Density (n.o.), g/cm 3	2,25	2,33	5,323	7,31	11,34
t pl, °C	3550	1412	273	231	327,5
t bale, °C	4827	2355	2830	2600	1749
Ionization energy, kJ/mol	1085,7	786,5	762,1	708,6	715,2
Electronic formula	2s 2 2p 2	3s 2 3p 2	3d 10 4s 2 4p 2	4d 10 5s 2 5p 2	4f 14 5d 10 6s 2 6p 2
Electronegativity (according to Pauling)	2,55	1,9	2,01	1,96	2,33

Electronic formulas of inert gases:

• He - 1s 2 ;
• Ne - 1s 2 2s 2 2p 6 ;
• Ar - 1s 2 2s 2 2p 6 3s 2 3p 6 ;
• Kr - 3d 10 4s 2 4p 6 ;
• Xe - 4d 10 5s 2 5p 6 ;

Rice. The structure of the carbon atom.

Group 14 (IVa group according to the old classification) of the periodic table of chemical elements of D. I. Mendeleev includes 5 elements: carbon, silicon, germanium, tin, lead (see table above). Carbon and silicon are non-metals, germanium is a substance that exhibits metallic properties, tin and lead are typical metals.

The most common element of the 14 (IVa) group in the earth's crust is silicon (the second most abundant element on Earth after oxygen) (27.6% by weight), followed by: carbon (0.1%), lead (0.0014%) , tin (0.00022%), germanium (0.00018%).

Silicon, unlike carbon, does not occur in nature in free form, it can only be found in bound form:

• SiO 2 - silica, found in the form of quartz (part of many rocks, sand, clay) and its varieties (agate, amethyst, rock crystal, jasper, etc.);
• silicates are rich in silicon: talc, asbestos;
• aluminosilicates: feldspar, mica, kaolin.

Germanium, tin and lead are also not found in free form in nature, but are part of some minerals:

• germanium: (Cu 3 (Fe, Ge)S 4) - germanite mineral;
• tin: SnO 2 - cassiterite;
• lead: PbS - galena; PbSO 4 - anglesite; PbCO 3 - cerussite.

All elements of the 14(IVa) group in the unexcited state at the external energy level have two unpaired p-electrons (the valency is 2, for example, CO). Upon transition to an excited state (the process requires energy costs), one paired s-electron of the outer level "jumps" to a free p-orbital, thus forming 4 "lonely" electrons (one at the s-sublevel and three at the p-sublevel) , which expands the valency of the elements (valency is 4: for example, CO 2).

Rice. The transition of a carbon atom to an excited state.

For the above reason, elements of group 14(IVa) can exhibit oxidation states: +4; +2; 0; -4.

Since it takes more and more energy to “jump” an electron from the s-sublevel to the p-sublevel in the series from carbon to lead (it takes much less energy to excite a carbon atom than to excite a lead atom), carbon “willingly” enters into compounds in which it exhibits valence four; and lead - two.

The same can be said about oxidation states: in the series from carbon to lead, the manifestation of oxidation states +4 and -4 decreases, and the oxidation state +2 increases.

Since carbon and silicon are non-metals, they can exhibit both positive and negative oxidation states, depending on the compound (in compounds with more electronegative elements, C and Si donate electrons, and gain in compounds with less electronegative elements):

C +2 O, C +4 O 2, Si +4 Cl 4 C -4 H 4, Mg 2 Si -4

Ge, Sn, Pb, like metals in compounds, always donate their electrons:

Ge +4 Cl 4 , Sn +4 Br 4 , Pb +2 Cl 2

The elements of the carbon group form the following compounds:

• unstable volatile hydrogen compounds (general formula EH 4), of which only methane CH 4 is a stable compound.
• non-salt-forming oxides- lower oxides CO and SiO;
• acid oxides - higher oxides CO 2 and SiO 2 - they correspond to hydroxides, which are weak acids: H 2 CO 3 (carbonic acid), H 2 SiO 3 (silicic acid);
• amphoteric oxides- GeO, SnO, PbO and GeO 2, SnO 2, PbO 2 - the latter correspond to the hydroxides (IV) of germanium Ge (OH) 4, strontium Sn (OH) 4, lead Pb (OH) 4;