A metallic bond is a bond formed between atoms under conditions of strong delocalization (distribution of valence electrons over several chemical bonds in a compound) and electron deficiency in the atom (crystal). It is unsaturated and spatially non-directional.
Delocalization of valence electrons in metals is a consequence of the multicenter nature of the metal bond. The multicenter nature of the metal bond ensures high electrical conductivity and thermal conductivity of metals.
Saturability determined by the number of valence orbitals involved in the formation of a chemical. communications. Quantitative characteristic – valency. Valence is the number of bonds that one atom can form with others; - is determined by the number of valence orbitals involved in the formation of bonds according to the exchange and donor-acceptor mechanisms.
Focus – the connection is formed in the direction of maximum overlap of electron clouds; - determines the chemical and crystal chemical structure of a substance (how atoms are connected in a crystal lattice).
When a covalent bond is formed, the electron density is concentrated between the interacting atoms (drawing from notebook). In the case of a metallic bond, the electron density is delocalized throughout the crystal. (drawing from notebook)
(example from notebook)
Due to the unsaturated and non-directional nature of the metallic bond, metallic bodies (crystals) are highly symmetrical and highly coordinated. The vast majority of metal crystal structures correspond to 3 types of atomic packings in crystals:
1. GCC– grain-centered cubic close-packed structure. Packing density – 74.05%, coordination number = 12.
2. GPU– hexagonal close-packed structure, packing density = 74.05%, c.h. = 12.
3. BCC– volume is centered, packing density = 68.1%, c.h. = 8.
A metallic bond does not exclude some degree of covalency. Metallic bonding in its pure form is characteristic only of alkali and alkaline earth metals.
A pure metallic bond is characterized by an energy of the order of 100/150/200 kJ/mol, 4 times weaker than a covalent bond.
36. Chlorine and its properties. В=1(III, IV, V and VII) oxidation state=7, 6, 5, 4, 3, 1, −1
yellow-green gas with a pungent irritating odor. Chlorine occurs in nature only in the form of compounds. In nature, in the form of potassium chloride, magnesium, nitrium, formed as a result of the evaporation of former seas and lakes. Receipt.prom:2NaCl+2H2O=2NaOH+H2+Cl2, by electrolysis of waters of chlorides Me.\2KMnO4+16HCl=2MnCl2+2KCl+8H2O+5Cl2/Chemically, chlorine is very active, directly combines with almost all Me, and with non-metals (except carbon, nitrogen, oxygen, inert gases), replaces hydrogen in hydrocarbons and joins unsaturated compounds, displaces bromine and iodine from their compounds. Phosphorus ignites in an atmosphere of chlorine PCl3, and with further chlorination - PCl5; sulfur with chlorine = S2Сl2, SСl2 and other SnClm. A mixture of chlorine and hydrogen burns. With oxygen, chlorine forms oxides: Cl2O, ClO2, Cl2O6, Cl2O7, Cl2O8, as well as hypochlorites (salts of hypochlorous acid), chlorites, chlorates and perchlorates. All oxygen compounds of chlorine form explosive mixtures with easily oxidized substances. Chlorine oxides are unstable and can spontaneously explode; hypochlorites slowly decompose during storage; chlorates and perchlorates can explode under the influence of initiators. in water - hypochlorous and salty: Cl2 + H2O = HClO + HCl. When aqueous solutions of alkalis are chlorinated in the cold, hypochlorites and chlorides are formed: 2NaOH + Cl2 = NaClO + NaCl + H2O, and when heated, chlorates are formed. When ammonia reacts with chlorine, nitrogen trichloride is formed. interhalogen compounds with other halogens. Fluorides ClF, ClF3, ClF5 are very reactive; for example, in a ClF3 atmosphere, glass wool spontaneously ignites. Known compounds of chlorine with oxygen and fluorine are chlorine oxyfluorides: ClO3F, ClO2F3, ClOF, ClOF3 and fluorine perchlorate FClO4. Application: production of chemical compounds, water purification, food synthesis, pharmaceutical industry-bactericide, antiseptic, bleaching of papers, fabrics, pyrotechnics, matches, destroys weeds in agriculture.
Biological role: biogenic, component of plant and animal tissues. 100g is the main osmotically active substance in blood plasma, lymph, cerebrospinal fluid and some tissues. Daily sodium chloride requirement = 6-9g - bread, meat and dairy products. Plays a role in water-salt metabolism, promoting tissue retention of water. The regulation of acid-base balance in tissues is carried out along with other processes by changing the distribution of chlorine between the blood and other tissues; chlorine is involved in energy metabolism in plants, activating both oxidative phosphorylation and photophosphorylation. Chlorine has a positive effect on the absorption of oxygen by roots, a component of iron sap.
37. Hydrogen, water. B = 1; st. oxide = + 1-1 The hydrogen ion is completely devoid of electron shells and can approach very close distances and penetrate into electron shells.
The most common element in the Universe. It makes up the bulk of the Sun, stars and other cosmic bodies. In a free state on Earth, it is found relatively rarely - it is found in oil and combustible gases, present in the form of inclusions in some minerals, and most of it in water. Receipt: 1. Laboratory Zn+2HCl=ZnCl2+H 2 ; 2.Si+2NaOH+H 2 O=Na 2 SiO 3 +2H 2; 3. Al+NaOH+H 2 O=Na(AlOH) 4 +H 2. 4. In industry: conversion, electrolysis: СH4+H2O=CO+3H2\CO+H2O=CO+ H2/Him St. In no.:H 2 +F 2 =2HF. When irradiated, illuminated, catalysts: H 2 + O 2 , S, N, P = H 2 O, H 2 S, NH 3 , Ca + H2 = CaH2\F2 + H2 = 2HF\N2 + 3H2 → 2NH3\Cl2 + H2 → 2HCl, 2NO+2H2=N2+2H2O,CuO+H2=Cu+H2O,CO+H2=CH3OH. Hydrogen forms hydrides: ionic, covalent and metallic. To ionic –NaH -& ,CaH 2 -& +H 2 O=Ca(OH) 2 ;NaH+H 2 O=NaOH+H 2 . Covalent –B 2 H 6 , AlH 3 , SiH 4 . Metal – with d-elements; variable composition: MeH ≤1, MeH ≤2 – are introduced into the voids between atoms. Conducts heat, current, solids. WATER.sp3-hybrid highly polar molecule at an angle of 104.5 , dipoles, the most common solvent. Water reacts at room temperature with active halogens (F, Cl) and interhalogen compounds with salts, weak forms and weak bases, causing their complete hydrolysis ; with carbonic and inorganic anhydrides and acid halides. acid; with active metal organ compounds; with carbides, nitrides, phosphides, silicides, hydrides of active Me; with many salts, forming hydrates; with boranes, silanes; with ketenes, carbon dioxide; with fluorides of noble gases. Water reacts when heated: with Fe, Mg, coal, methane; with some alkyl halides. Application:hydrogen -synthesis of ammonia, methanol, hydrogen chloride, TV fats, hydrogen flame - for welding, melting, in metallurgy for the reduction of Me from oxide, fuel for rockets, in pharmacy - water, peroxide-antiseptic, bactericide, washing, hair bleaching, sterilization.
Biological role: hydrogen-7kg, The main function of hydrogen is the structuring of biological space (water and hydrogen bonds) and the formation of a variety of organic molecules (included in the structure of proteins, carbohydrates, fats, enzymes) Thanks to hydrogen bonds,
copying a DNA molecule. Water takes part in a huge
number of biochemical reactions, in all physiological and biological
processes, ensures metabolism between the body and the external environment, between
cells and within cells. Water is the structural basis of cells and is necessary for
maintaining their optimal volume, it determines the spatial structure and
functions of biomolecules.
All metals have the following characteristics:
A small number of electrons at the outer energy level (except for some exceptions, which may have 6,7 and 8);
Large atomic radius;
Low ionization energy.
All this contributes to the easy separation of outer unpaired electrons from the nucleus. At the same time, the atom has a lot of free orbitals. The diagram of the formation of a metallic bond will precisely show the overlap of numerous orbital cells of different atoms with each other, which as a result form a common intracrystalline space. Electrons are fed into it from each atom, which begin to wander freely through different parts of the lattice. Periodically, each of them attaches to an ion at a site in the crystal and turns it into an atom, then detaches again to form an ion.
Thus, A metallic bond is the bond between atoms, ions, and free electrons in a common metal crystal. An electron cloud moving freely within a structure is called an “electron gas.” It explains most of the physical properties of metals and their alloys.
How exactly does metal realize itself? chemical bond? Various examples can be given. Let's try to look at it on a piece of lithium. Even if you take it the size of a pea, there will be thousands of atoms. So let’s imagine that each of these thousands of atoms gives up its single valence electron to the common crystalline space. At the same time, knowing the electronic structure of a given element, you can see the number of empty orbitals. Lithium will have 3 of them (p-orbitals of the second energy level). Three for each atom out of tens of thousands - this is the common space inside the crystal in which the “electron gas” moves freely.
A substance with a metal bond is always strong. After all, electron gas does not allow the crystal to collapse, but only displaces the layers and immediately restores them. It shines, has a certain density (most often high), fusibility, malleability and plasticity.
Where else is metal bonding sold? Examples of substances:
Metals in the form of simple structures;
All metals alloy with each other;
All metals and their alloys in liquid and solid states.
Specific examples You can cite just an incredible number, because there are more than 80 metals in the periodic table!
The mechanism of formation is generally expressed by the following notation: Me 0 - e - ↔ Me n+. From the diagram it is obvious what particles are present in the metal crystal.
Any metal can give up electrons, becoming a positively charged ion.
Using iron as an example: Fe 0 -2e - = Fe 2+
Where do the separated negatively charged particles - electrons - go? A minus is always attracted to a plus. The electrons are attracted to another (positively charged) iron ion in the crystal lattice: Fe 2+ +2e - = Fe 0
The ion becomes a neutral atom. And this process is repeated many times.
It turns out that free electrons of iron are in constant motion throughout the entire volume of the crystal, breaking off and joining ions at lattice sites. Another name for this phenomenon is delocalized electron cloud. The term "delocalized" means free, not tied.
Purpose of the lesson
- Give an idea of metal chemical bonding.
- Learn to write down patterns of metal bond formation.
- Get acquainted with the physical properties of metals.
- Learn to clearly distinguish between species chemical bonds .
Lesson Objectives
- Find out how they interact with each other metal atoms
- Determine how a metal bond affects the properties of the substances formed by it
Key terms:
- Electronegativity - chemical property atom, which is quantitative characteristics the ability of an atom in a molecule to attract shared electron pairs.
- Chemical bond -the phenomenon of interaction of atoms, due to the overlap of electron clouds of interacting atoms.
- Metal connection is a bond in metals between atoms and ions, formed through the sharing of electrons.
- Covalent bond - a chemical bond formed by overlapping a pair of valence electrons. The electrons that provide the connection are called a common electron pair. There are 2 types: polar and non-polar.
- Ionic bond - a chemical bond that forms between non-metal atoms, in which a shared electron pair goes to an atom with higher electronegativity. As a result, atoms attract like oppositely charged bodies.
- Hydrogen bond
- a chemical bond between an electronegative atom and a hydrogen atom H bonded covalently to another electronegative atom. Electronegative atoms can be N, O or F. Hydrogen bonds can be intermolecular or intramolecular.
PROGRESS OF THE LESSON
Metal chemical bond
Identify the elements that are in the wrong “queue”. Why?
Ca Fe P K Al Mg Na
Which elements from the table Mendeleev are called metals?
Today we will learn what properties metals have, and how they depend on the bond that is formed between the metal ions.
First, let's remember the location of metals in the periodic table?
Metals, as we all know, usually do not exist in the form of isolated atoms, but in the form of a piece, ingot or metal product. Let's find out what collects metal atoms in a complete volume.
In the example we see a piece of gold. And by the way, gold is a unique metal. Using forging, pure gold can be used to make foil 0.002 mm thick! This thin sheet of foil is almost transparent and has a green tint to the light. As a result, from an ingot of gold the size of a matchbox, you can get a thin foil that will cover the area of a tennis court.
Chemically, all metals are characterized by the ease of giving up valence electrons, and as a result, the formation of positively charged ions and exhibit only positive oxidation. That is why metals in a free state are reducing agents. Common feature Metal atoms are larger in size relative to non-metals. The outer electrons are located on long distances from the core and therefore weakly connected with it, therefore easily detached.
Atoms of a larger number of metals at the external level have a small number of electrons - 1,2,3. These electrons are easily stripped off and the metal atoms become ions.
Ме0 – n ē ⇆ Men+
metal atoms – electrons ext. orbits ⇆ metal ions
In this way, the detached electrons can move from one ion to another, that is, they become free, as if linking them into a single whole. Therefore, it turns out that all the detached electrons are common, since it is impossible to understand which electron belongs to which of the metal atoms.
Electrons can combine with cations, then atoms are temporarily formed, from which electrons are then torn off. This process occurs constantly and without stopping. It turns out that in the volume of the metal, atoms are continuously transformed into ions and vice versa. At the same time not large number shared electrons bind a large number of metal atoms and ions. But it is important that the number of electrons in the metal is equal to the total charge of the positive ions, that is, it turns out that in general the metal remains electrically neutral.
This process is represented as a model - metal ions are in a cloud of electrons. Such an electron cloud is called an “electron gas.”
For example, in this picture we see how electrons move among motionless ions inside the crystal lattice of metal. 
Rice. 2. Electron movement
In order to better understand what Electron gas is and how it behaves in chemical reactions of different metals, let’s look interesting video. (gold is only mentioned as a color in this video!)
Now we can write down the definition: a metallic bond is a bond in metals between atoms and ions, formed by sharing electrons.
Let's compare all the types of connections that we know and consolidate them in order to better distinguish them, for this we will watch the video.
Metallic bonding occurs not only in pure metals, but is also characteristic of mixtures of different metals and alloys in different states of aggregation.
The metallic bond is important and determines the basic properties of metals
- electrical conductivity – random movement of electrons in the volume of metal. But with a small potential difference, so that the electrons move in an orderly manner. Metals with the best conductivity are Ag, Cu, Au, Al.
- plasticity
The bonds between the metal layers are not very significant, this allows the layers to move under load (deform the metal without breaking it). The best deformable metals (soft) are Au, Ag, Cu.
- metallic shine
Electron gas reflects almost all light rays. This is why pure metals shine so much and most often have a gray or white color. Metals that are the best reflectors Ag, Cu, Al, Pd, Hg
Homework
Exercise 1
Choose the formulas of substances that have
a) covalent polar bond: Cl2, KCl, NH3, O2, MgO, CCl4, SO2;
b) with ionic bond: HCl, KBr, P4, H2S, Na2O, CO2, CaS.
Exercise 2
Cross out the extra:
a) CuCl2, Al, MgS
b) N2, HCl, O2
c) Ca, CO2, Fe
d) MgCl2, NH3, H2
Sodium metal, lithium metal, and other alkali metals change the color of the flame. Metallic lithium and its salts give the fire a red color, metallic sodium and sodium salts give it a yellow color, metallic potassium and its salts give it a purple color, and rubidium and cesium give it a purple color, but lighter.
Rice. 4. A piece of lithium metal 
Rice. 5. Flame coloring with metals
Lithium (Li). Lithium metal, like sodium metal, is an alkali metal. Both are soluble in water. Sodium, when dissolved in water, forms caustic soda, a very strong acid. When alkali metals are dissolved in water, a lot of heat and gas (hydrogen) are released. It is advisable not to touch such metals with your hands, as you may get burned.
References
1. Lesson on the topic “Metallic chemical bond”, chemistry teacher Tukhta Valentina Anatolyevna MOU "Yesenovichskaya Secondary School"
2. F. A. Derkach “Chemistry” - scientific and methodological manual. – Kyiv, 2008.
3. L. B. Tsvetkova “Inorganic chemistry” - 2nd edition, corrected and expanded. – Lvov, 2006.
4. V. V. Malinovsky, P. G. Nagorny “Inorganic chemistry” - Kyiv, 2009.
5. Glinka N.L. General chemistry. – 27th ed./Under. ed. V.A. Rabinovich. – L.: Chemistry, 2008. – 704 pp.
Edited and sent by Lisnyak A.V.
Worked on the lesson:
Tukhta V.A.
Lisnyak A.V.
Ask a question about modern education, express an idea or solve a pressing problem, you can Educational forum, where an educational council of fresh thought and action meets internationally. Having created blog, Chemistry 8th grade
Atoms of most elements do not exist separately, as they can interact with each other. This interaction produces more complex particles.
The nature of a chemical bond is the action of electrostatic forces, which are the forces of interaction between electric charges. Electrons and atomic nuclei have such charges.
Electrons located on the outer electronic levels (valence electrons), being farthest from the nucleus, interact with it weakest, and therefore are able to break away from the nucleus. They are responsible for bonding atoms to each other.
Types of interactions in chemistry
Types of chemical bonds can be presented in the following table:
Characteristics of ionic bonding
Chemical reaction that occurs due to ion attraction having different charges, is called ionic. This happens if the atoms being bonded have a significant difference in electronegativity (that is, the ability to attract electrons) and the electron pair goes to the more electronegative element. The result of this transfer of electrons from one atom to another is the formation of charged particles - ions. An attraction arises between them.
They have the lowest electronegativity indices typical metals, and the largest are typical non-metals. Ions are thus formed by the interaction between typical metals and typical nonmetals.
Metal atoms become positively charged ions (cations), donating electrons to their outer electron levels, and nonmetals accept electrons, thus turning into negatively charged ions (anions).
Atoms move into a more stable energy state, completing their electronic configurations.
The ionic bond is non-directional and non-saturable, since the electrostatic interaction occurs in all directions; accordingly, the ion can attract ions of the opposite sign in all directions.
The arrangement of the ions is such that around each there is a certain number oppositely charged ions. The concept of "molecule" for ionic compounds doesn't make sense.
Examples of education
The formation of a bond in sodium chloride (nacl) is due to the transfer of an electron from the Na atom to the Cl atom to form the corresponding ions:
Na 0 - 1 e = Na + (cation)
Cl 0 + 1 e = Cl - (anion)
In sodium chloride, there are six chloride anions around the sodium cations, and six sodium ions around each chloride ion.
When interaction is formed between atoms in barium sulfide, the following processes occur:
Ba 0 - 2 e = Ba 2+
S 0 + 2 e = S 2-
Ba donates its two electrons to sulfur, resulting in the formation of sulfur anions S 2- and barium cations Ba 2+.
Metal chemical bond
The number of electrons in the outer energy levels of metals is small; they are easily separated from the nucleus. As a result of this detachment, metal ions and free electrons are formed. These electrons are called "electron gas". Electrons move freely throughout the volume of the metal and are constantly bound and separated from atoms.
The structure of the metal substance is as follows: the crystal lattice is the skeleton of the substance, and between its nodes electrons can move freely.
The following examples can be given:
Mg - 2е<->Mg 2+
Cs-e<->Cs+
Ca - 2e<->Ca2+
Fe-3e<->Fe 3+
Covalent: polar and non-polar
The most common type of chemical interaction is a covalent bond. The electronegativity values of the elements that interact do not differ sharply; therefore, only a shift of the common electron pair to a more electronegative atom occurs.
Covalent interactions can be formed by an exchange mechanism or a donor-acceptor mechanism.
The exchange mechanism is realized if each of the atoms has unpaired electrons on the outer electronic levels and the overlap of atomic orbitals leads to the appearance of a pair of electrons that already belongs to both atoms. When one of the atoms has a pair of electrons on the outer electronic level, and the other has a free orbital, then when the atomic orbitals overlap, the electron pair is shared and interacts according to the donor-acceptor mechanism.
Covalent ones are divided by multiplicity into:
- simple or single;
- double;
- triples.
Double ones ensure the sharing of two pairs of electrons at once, and triple ones - three.
According to the distribution of electron density (polarity) between bonded atoms, a covalent bond is divided into:
- non-polar;
- polar.
A nonpolar bond is formed by identical atoms, and a polar bond is formed by different electronegativity.
The interaction of atoms with similar electronegativity is called a nonpolar bond. The common pair of electrons in such a molecule is not attracted to either atom, but belongs equally to both.
The interaction of elements differing in electronegativity leads to the formation of polar bonds. In this type of interaction, shared electron pairs are attracted to the more electronegative element, but are not completely transferred to it (that is, the formation of ions does not occur). As a result of this shift in electron density, partial charges appear on the atoms: the more electronegative one has a negative charge, and the less electronegative one has a positive charge.
Properties and characteristics of covalency
Main characteristics of a covalent bond:
- The length is determined by the distance between the nuclei of interacting atoms.
- Polarity is determined by the displacement of the electron cloud towards one of the atoms.
- Directionality is the property of forming bonds oriented in space and, accordingly, molecules having certain geometric shapes.
- Saturation is determined by the ability to form a limited number of bonds.
- Polarizability is determined by the ability to change polarity under the influence of an external electric field.
- The energy required to break a bond determines its strength.
An example of a covalent non-polar interaction can be the molecules of hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2) and many others.
H· + ·H → H-H molecule has a single non-polar bond,
O: + :O → O=O molecule has a double nonpolar,
Ṅ: + Ṅ: → N≡N the molecule is triple nonpolar.
As examples of covalent bonding chemical elements You can cite molecules of carbon dioxide (CO2) and carbon monoxide (CO), hydrogen sulfide (H2S), hydrochloric acid (HCL), water (H2O), methane (CH4), sulfur oxide (SO2) and many others.
In the CO2 molecule, the relationship between carbon and oxygen atoms is covalent polar, since the more electronegative hydrogen attracts electron density. Oxygen has two unpaired electrons in its outer shell, while carbon can provide four valence electrons to form the interaction. As a result, double bonds are formed and the molecule looks like this: O=C=O.
In order to determine the type of bond in a particular molecule, it is enough to consider its constituent atoms. Simple metal substances form a metallic bond, metals with nonmetals form an ionic bond, simple nonmetal substances form a covalent nonpolar bond, and molecules consisting of different nonmetals form through a polar covalent bond.
163120 0
Each atom has a certain number of electrons.
Entering chemical reactions, atoms donate, gain, or share electrons, achieving the most stable electronic configuration. The configuration with the lowest energy (as in noble gas atoms) turns out to be the most stable. This pattern is called the “octet rule” (Fig. 1).
Rice. 1.
This rule applies to everyone types of connections. Electronic connections between atoms allow them to form stable structures, from the simplest crystals to complex biomolecules that ultimately form living systems. They differ from crystals in their continuous metabolism. At the same time, many chemical reactions proceed according to the mechanisms electronic transfer, which play a critical role in energy processes in the body.
A chemical bond is the force that holds together two or more atoms, ions, molecules, or any combination thereof.
The nature of a chemical bond is universal: it is an electrostatic force of attraction between negatively charged electrons and positively charged nuclei, determined by the configuration of the electrons of the outer shell of atoms. The ability of an atom to form chemical bonds is called valency, or oxidation state. The concept of valence electrons- electrons that form chemical bonds, that is, located in the highest energy orbitals. Accordingly, the outer shell of the atom containing these orbitals is called valence shell. Currently, it is not enough to indicate the presence of a chemical bond, but it is necessary to clarify its type: ionic, covalent, dipole-dipole, metallic.
The first type of connection isionic connection
According to Lewis and Kossel's electronic valence theory, atoms can achieve a stable electronic configuration in two ways: first, by losing electrons, becoming cations, secondly, acquiring them, turning into anions. As a result of electron transfer, due to the electrostatic force of attraction between ions with charges of opposite signs, a chemical bond is formed, called by Kossel “ electrovalent"(now called ionic).
In this case, anions and cations form a stable electronic configuration with a filled outer electron shell. Typical ionic bonds are formed from cations of T and II groups periodic table and anions of nonmetallic elements VI and VII groups(16 and 17 subgroups - respectively, chalcogens And halogens). The bonds of ionic compounds are unsaturated and non-directional, so they retain the possibility of electrostatic interaction with other ions. In Fig. Figures 2 and 3 show examples of ionic bonds corresponding to the Kossel model of electron transfer.
Rice. 2.
Rice. 3. Ionic bond in a molecule of table salt (NaCl)
Here it is appropriate to recall some properties that explain the behavior of substances in nature, in particular, consider the idea of acids And reasons.
Aqueous solutions of all these substances are electrolytes. They change color differently indicators. The mechanism of action of indicators was discovered by F.V. Ostwald. He showed that indicators are weak acids or bases, the color of which differs in the undissociated and dissociated states.
Bases can neutralize acids. Not all bases are soluble in water (for example, some are insoluble organic compounds, not containing - OH groups, in particular, triethylamine N(C 2 H 5) 3); soluble bases are called alkalis.
Aqueous solutions of acids undergo characteristic reactions:
a) with metal oxides - with the formation of salt and water;
b) with metals - with the formation of salt and hydrogen;
c) with carbonates - with the formation of salt, CO 2 and N 2 O.
The properties of acids and bases are described by several theories. In accordance with the theory of S.A. Arrhenius, an acid is a substance that dissociates to form ions N+ , while the base forms ions HE- . This theory does not take into account the existence of organic bases that do not have hydroxyl groups.
In accordance with proton According to the theory of Brønsted and Lowry, an acid is a substance containing molecules or ions that donate protons ( donors protons), and a base is a substance consisting of molecules or ions that accept protons ( acceptors protons). Note that in aqueous solutions, hydrogen ions exist in hydrated form, that is, in the form of hydronium ions H3O+ . This theory describes reactions not only with water and hydroxide ions, but also those carried out in the absence of a solvent or with a non-aqueous solvent.
For example, in the reaction between ammonia N.H. 3 (weak base) and hydrogen chloride in the gas phase, solid ammonium chloride is formed, and in an equilibrium mixture of two substances there are always 4 particles, two of which are acids, and the other two are bases:
This equilibrium mixture consists of two conjugate pairs of acids and bases:
1)N.H. 4+ and N.H. 3
2) HCl And Cl ‑
Here, in each conjugate pair, the acid and base differ by one proton. Every acid has a conjugate base. A strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.
The Brønsted-Lowry theory helps explain the unique role of water for the life of the biosphere. Water, depending on the substance interacting with it, can exhibit the properties of either an acid or a base. For example, in reactions with aqueous solutions of acetic acid, water is a base, and in reactions with aqueous solutions of ammonia, it is an acid.
1) CH 3 COOH + H2O ↔ H3O + + CH 3 COO- . Here, an acetic acid molecule donates a proton to a water molecule;
2) NH 3 + H2O ↔ NH 4 + + HE- . Here, an ammonia molecule accepts a proton from a water molecule.
Thus, water can form two conjugate pairs:
1) H2O(acid) and HE- (conjugate base)
2) H 3 O+ (acid) and H2O(conjugate base).
In the first case, water donates a proton, and in the second, it accepts it.
This property is called amphiprotonism. Substances that can react as both acids and bases are called amphoteric. Such substances are often found in living nature. For example, amino acids can form salts with both acids and bases. Therefore, peptides easily form coordination compounds with the metal ions present.
Thus, a characteristic property of an ionic bond is the complete movement of the bonding electrons to one of the nuclei. This means that between the ions there is a region where the electron density is almost zero.
The second type of connection iscovalent connection
Atoms can form stable electronic configurations by sharing electrons.
Such a bond is formed when a pair of electrons is shared one at a time from everyone atom. In this case, the shared bond electrons are distributed equally between the atoms. Examples of covalent bonds include homonuclear diatomic molecules H 2 , N 2 , F 2. The same type of connection is found in allotropes O 2 and ozone O 3 and for a polyatomic molecule S 8 and also heteronuclear molecules hydrogen chloride HCl, carbon dioxide CO 2, methane CH 4, ethanol WITH 2 N 5 HE, sulfur hexafluoride SF 6, acetylene WITH 2 N 2. All these molecules share the same electrons, and their bonds are saturated and directed in the same way (Fig. 4).
It is important for biologists that double and triple bonds have reduced covalent atomic radii compared to a single bond.
Rice. 4. Covalent bond in a Cl 2 molecule.
Ionic and covalent types of bonds are two extreme cases of the many existing types of chemical bonds, and in practice most bonds are intermediate.
Compounds of two elements located at opposite ends of the same or different periods of the periodic system predominantly form ionic bonds. As elements move closer together within a period, the ionic nature of their compounds decreases, and the covalent character increases. For example, the halides and oxides of elements on the left side of the periodic table form predominantly ionic bonds ( NaCl, AgBr, BaSO 4, CaCO 3, KNO 3, CaO, NaOH), and the same compounds of elements on the right side of the table are covalent ( H 2 O, CO 2, NH 3, NO 2, CH 4, phenol C6H5OH, glucose C 6 H 12 O 6, ethanol C 2 H 5 OH).
The covalent bond, in turn, has one more modification.
In polyatomic ions and in complex biological molecules, both electrons can only come from one atom. It's called donor electron pair. An atom that shares this pair of electrons with a donor is called acceptor electron pair. This type of covalent bond is called coordination (donor-acceptor, ordative) communication(Fig. 5). This type of bond is most important for biology and medicine, since the chemistry of the d-elements most important for metabolism is largely described by coordination bonds.
Fig. 5.
As a rule, in a complex compound the metal atom acts as an acceptor of an electron pair; on the contrary, in ionic and covalent bonds, the metal atom is an electron donor.
The essence of the covalent bond and its variety - the coordination bond - can be clarified with the help of another theory of acids and bases proposed by GN. Lewis. He somewhat expanded the semantic concept of the terms “acid” and “base” according to the Brønsted-Lowry theory. Lewis's theory explains the nature of the formation of complex ions and the participation of substances in nucleophilic substitution reactions, that is, in the formation of CS.
According to Lewis, an acid is a substance capable of forming a covalent bond by accepting an electron pair from a base. A Lewis base is a substance that has a lone electron pair, which, by donating electrons, forms a covalent bond with Lewis acid.
That is, Lewis's theory expands the range of acid-base reactions also to reactions in which protons do not participate at all. Moreover, the proton itself, according to this theory, is also an acid, since it is capable of accepting an electron pair.
Therefore, according to this theory, the cations are Lewis acids and the anions are Lewis bases. An example would be the following reactions:
It was noted above that the division of substances into ionic and covalent is relative, since complete electron transfer from metal atoms to acceptor atoms does not occur in covalent molecules. In compounds with ionic bonds, each ion is in electric field ions of the opposite sign, so they are mutually polarized, and their shells are deformed.
Polarizability determined by the electronic structure, charge and size of the ion; for anions it is higher than for cations. The highest polarizability among cations is for cations of higher charge and smaller size, for example, Hg 2+, Cd 2+, Pb 2+, Al 3+, Tl 3+. Has a strong polarizing effect N+ . Since the influence of ion polarization is two-way, it significantly changes the properties of the compounds they form.
The third type of connection isdipole-dipole connection
In addition to the listed types of communication, there are also dipole-dipole intermolecular interactions, also called van der Waals .
The strength of these interactions depends on the nature of the molecules.
There are three types of interactions: permanent dipole - permanent dipole ( dipole-dipole attraction); permanent dipole - induced dipole ( induction attraction); instantaneous dipole - induced dipole ( dispersive attraction, or London forces; rice. 6).
Rice. 6.
Only molecules with polar covalent bonds have a dipole-dipole moment ( HCl, NH 3, SO 2, H 2 O, C 6 H 5 Cl), and the bond strength is 1-2 Debaya(1D = 3.338 × 10‑30 coulomb meters - C × m).
In biochemistry, there is another type of connection - hydrogen connection that is a limiting case dipole-dipole attraction. This bond is formed by the attraction between a hydrogen atom and a small electronegative atom, most often oxygen, fluorine and nitrogen. With large atoms that have similar electronegativity (such as chlorine and sulfur), the hydrogen bond is much weaker. The hydrogen atom is distinguished by one significant feature: when the bonding electrons are pulled away, its nucleus - the proton - is exposed and is no longer shielded by electrons.
Therefore, the atom turns into a large dipole.
A hydrogen bond, unlike a van der Waals bond, is formed not only during intermolecular interactions, but also within one molecule - intramolecular hydrogen bond. Hydrogen bonds play an important role in biochemistry, for example, to stabilize the structure of proteins in the form of an a-helix, or for the formation of a double helix of DNA (Fig. 7).
Fig.7.
Hydrogen and van der Waals bonds are much weaker than ionic, covalent and coordination bonds. The energy of intermolecular bonds is indicated in table. 1.
Table 1. Energy of intermolecular forces
Note: The degree of intermolecular interactions is reflected by the enthalpy of melting and evaporation (boiling). Ionic compounds require significantly more energy to separate ions than to separate molecules. The enthalpy of melting of ionic compounds is much higher than that of molecular compounds.
The fourth type of connection ismetal connection
Finally, there is another type of intermolecular bonds - metal: connection of positive ions of a metal lattice with free electrons. This type of connection does not occur in biological objects.
From brief overview types of bonds, one detail becomes clear: an important parameter of a metal atom or ion - an electron donor, as well as an atom - an electron acceptor, is its size.
Without going into details, we note that the covalent radii of atoms, the ionic radii of metals and the van der Waals radii of interacting molecules increase as their atomic number increases in groups of the periodic table. In this case, the values of the ion radii are the smallest, and the van der Waals radii are the largest. As a rule, when moving down the group, the radii of all elements increase, both covalent and van der Waals.
Of greatest importance for biologists and physicians are coordination(donor-acceptor) bonds considered by coordination chemistry.
Medical bioinorganics. G.K. Barashkov
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