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Each atom has a certain number of electrons.
Entering into chemical reactions, atoms donate, acquire, or socialize electrons, reaching the most stable electronic configuration. The configuration with the lowest energy is the most stable (as in noble gas atoms). This pattern is called the "octet rule" (Fig. 1).
Rice. one.
This rule applies to all connection types. Electronic bonds between atoms allow them to form stable structures, from the simplest crystals to complex biomolecules that eventually form living systems. They differ from crystals in their continuous metabolism. However, many chemical reactions proceed according to the mechanisms electronic transfer, which play an important role in the energy processes in the body.
A chemical bond is a force that holds together two or more atoms, ions, molecules, or any combination of them..
Nature chemical bond universal: it is the electrostatic force of attraction between negatively charged electrons and positively charged nuclei, determined by the configuration of the electrons in the outer shell of atoms. The ability of an atom to form chemical bonds is called valence, or oxidation state. The concept of valence electrons- electrons that form chemical bonds, that is, those located in the most high-energy orbitals. Accordingly, the outer shell of an atom containing these orbitals is called valence shell. At present, 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 theory of valency, 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 the opposite sign, a chemical bond is formed, called 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 of the periodic system and anions of non-metallic elements of groups VI and VII (16 and 17 subgroups - respectively, chalcogens and halogens). The bonds in ionic compounds are unsaturated and non-directional, so they retain the possibility of electrostatic interaction with other ions. On fig. 2 and 3 show examples of ionic bonds corresponding to the Kossel electron transfer model.
Rice. 2.
Rice. 3. Ionic bond in the sodium chloride (NaCl) molecule
Here it is appropriate to recall some of the properties that explain the behavior of substances in nature, in particular, to consider the concept of acids and grounds.
Aqueous solutions of all these substances are electrolytes. They change color in different ways. indicators. The mechanism of action of indicators was discovered by F.V. Ostwald. He showed that the indicators are weak acids or bases, the color of which in the undissociated and dissociated states is different.
Bases can neutralize acids. Not all bases are soluble in water (for example, some organic compounds that do not contain -OH groups are insoluble, in particular, triethylamine N (C 2 H 5) 3); soluble bases are called alkalis.
Aqueous solutions of acids enter into 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 H 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 H+ , 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 line with proton Bronsted and Lowry's theory, an acid is a substance containing molecules or ions that donate protons ( donors protons), and the base is a substance consisting of molecules or ions that accept protons ( acceptors protons). Note that in aqueous solutions, hydrogen ions exist in a hydrated form, that is, in the form of hydronium ions H3O+ . This theory describes reactions not only with water and hydroxide ions, but also carried out in the absence of a solvent or with a non-aqueous solvent.
For example, in the reaction between ammonia NH 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 conjugated pairs of acids and bases:
1)NH 4+ and NH 3
2) HCl and Cl ‑
Here, in each conjugated 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 Bronsted-Lowry theory makes it possible to 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 With acetic acid, water is a base, and with aqueous solutions of ammonia, it is an acid.
1) CH 3 COOH + H 2 O ↔ H 3 O + + CH 3 SOO- . Here the acetic acid molecule donates a proton to the water molecule;
2) NH3 + H 2 O ↔ NH4 + + HE- . Here the ammonia molecule accepts a proton from the water molecule.
Thus, water can form two conjugated pairs:
1) H 2 O(acid) and HE- (conjugate base)
2) H 3 O+ (acid) and H 2 O(conjugate base).
In the first case, water donates a proton, and in the second, it accepts it.
Such a property is called amphiprotonity. Substances that can react as both acids and bases are called amphoteric. Such substances are often found in nature. For example, amino acids can form salts with both acids and bases. Therefore, peptides readily form coordination compounds with the metal ions present.
Thus, the characteristic property of an ionic bond is the complete displacement of a bunch of binding electrons to one of the nuclei. This means that there is a region between the ions 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 each atom. In this case, the socialized bond electrons are distributed equally among the atoms. Examples covalent bond can be called homonuclear diatomic H molecules 2 , N 2 , F 2. Allotropes have the same type of bond. 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 FROM 2 H 5 HE, sulfur hexafluoride SF 6, acetylene FROM 2 H 2. All these molecules have the same common electrons, and their bonds are saturated and directed in the same way (Fig. 4).
For biologists, it is important that the covalent radii of atoms in double and triple bonds are reduced compared to a single bond.
Rice. four. Covalent bond in the Cl 2 molecule.
Ionic and covalent types of bonds are two limiting cases of many existing types of chemical bonds, and in practice most of the bonds are intermediate.
Compounds of two elements located at opposite ends of the same or different periods of the Mendeleev system predominantly form ionic bonds. As the elements approach each other within a period, the ionic nature of their compounds decreases, while the covalent character increases. For example, the halides and oxides of the 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 the 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 another modification.
In polyatomic ions and in complex biological molecules, both electrons can only come from one atom. It is called donor electron pair. An atom that socializes 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 most important d-elements for metabolism is largely described by coordination bonds.
Pic. 5.
As a rule, in a complex compound, a metal atom acts as an electron pair acceptor; 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 Bronsted-Lowry theory. The Lewis theory explains the nature of the formation of complex ions and the participation of substances in reactions nucleophilic substitution, that is, in the formation of the 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 pair of electrons, which, by donating electrons, forms a covalent bond with Lewis acid.
That is, the Lewis 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 able to accept an electron pair.
Therefore, according to this theory, cations are Lewis acids and anions are Lewis bases. The following reactions are examples:
It was noted above that the subdivision of substances into ionic and covalent ones is relative, since there is no complete transition of an electron from metal atoms to acceptor atoms in covalent molecules. In ionic compounds, 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; it is higher for anions than for cations. The highest polarizability among cations is for cations of larger charge and smaller size, for example, for Hg 2+ , Cd 2+ , Pb 2+ , Al 3+ , Tl 3+. Has a strong polarizing effect H+ . Since the effect of ion polarization is two-sided, it significantly changes the properties of the compounds they form.
The third type of connection -dipole-dipole connection
In addition to the listed types of communication, there are also dipole-dipole intermolecular interactions, also known as 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 ( dispersion 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 debye(1D \u003d 3.338 × 10 -30 coulomb meters - C × m).
In biochemistry, another type of bond is distinguished - hydrogen connection, which 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 a similar electronegativity (for example, with chlorine and sulfur), the hydrogen bond is much weaker. The hydrogen atom is distinguished by one essential feature: when the binding electrons are pulled away, its nucleus - the proton - is exposed and ceases to be screened 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, for stabilizing the structure of proteins in the form of an α-helix, or for the formation of a DNA double helix (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. one.
Table 1. Energy of intermolecular forces
Note: The degree of intermolecular interactions reflect the enthalpy of melting and evaporation (boiling). Ionic compounds require much more energy to separate ions than to separate molecules. The melting enthalpies of ionic compounds are much higher than those of molecular compounds.
The fourth type of connection -metallic bond
Finally, there is another type of intermolecular bonds - metal: connection of positive ions of the lattice of metals with free electrons. This type of connection does not occur in biological objects.
From a brief review of the types of bonds, one detail emerges: an important parameter of an atom or ion of a metal - an electron donor, as well as an atom - an electron acceptor is its the 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 in the groups of the periodic system increases. 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.
The most important for biologists and physicians are coordination(donor-acceptor) bonds considered by coordination chemistry.
Medical bioinorganics. G.K. Barashkov
A chemical bond is the interaction of atoms, which determines the stability of a chemical particle or crystal as a whole.
The nature of a chemical bond is the electrostatic attraction of oppositely charged particles (cations and anions, atomic nuclei and electron pairs, metal cations and electrons).
According to the mechanism of formation, there are:
a) ionic bond - a bond between a metal cation and a non-metal anion. Thus, the ionic type of bond occurs in substances formed by atoms of strong metals and strong non-metals. At the same time, metal atoms donate electrons from the external (sometimes from the pre-external) energy level and turn into positively charged ions (cations), and non-metal atoms accept electrons to the external energy level and turn into negatively charged ions (anions) (examples of substances: oxides of typical metals K2O, CaO, MgO, bases KOH, Ca(OH)2, salts NaNO3, CaSO4).
b) a covalent bond - a bond between atoms of non-metals. A covalent bond arises due to the formation of common electron pairs from unpaired electrons of the external energy level of each non-metal atom (calculated according to the formula 8 - the group number of the element). The number of bonds in a compound is equal to the number of shared electron pairs. If the compound is formed by atoms of one chemical element - non-metals, then the bond is called covalent non-polar (examples: N2, Cl2, O2, H2). A covalent non-polar bond exists in simple non-metal substances. If the compound is formed by atoms of different non-metal elements, then the bond is called covalent polar, because in this case, common electron pairs shift towards the element with greater electronegativity and partially positive and partially negative charges appear on the elements (examples of substances: HCl, NO, CCl4, H2SO4). A covalent polar bond exists in complex substances formed by non-metal atoms.
Valence - the ability of atoms of chemical elements to form chemical bonds. Numerically, valency coincides with the number of chemical bonds that atoms of a given chemical element form with atoms of another chemical element. The highest valence coincides with the group number of the element (exceptions: oxygen (II) and nitrogen (IV)).
c) a metallic bond - a bond between the atom-ions of metals and socialized electrons. A metallic bond arises as a result of the fact that metal atoms donate all the electrons from the external energy level to the common interatomic space and turn into positively charged ions (cations). Socialized electrons move freely in the interatomic space and bind all cations into a single whole due to electrostatic attraction. A metallic bond is observed in simple substances-metals or in metal alloys (examples of substances: Al, Fe, Cu, bronze, brass).
chemical bond
There are no single atoms in nature. All of them are in the composition of simple and complex compounds, where their combination into molecules is ensured by the formation of chemical bonds with each other.
The formation of chemical bonds between atoms is a natural, spontaneous process, since in this case the energy of the molecular system decreases, i.e. the energy of the molecular system is less than the total energy of the isolated atoms. This is the driving force behind the formation of a chemical bond.
The nature of chemical bonds is electrostatic, because Atoms are a collection of charged particles, between which the forces of attraction and repulsion act, which come into equilibrium.
Unpaired electrons located in outer atomic orbitals (or ready-made electron pairs) - valence electrons - participate in the formation of bonds. They say that when bonds are formed, electron clouds overlap, resulting in an area between the nuclei of atoms where the probability of finding electrons of both atoms is maximum.
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s, p - elements |
d - elements |
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Valence electrons are the outer level For example, H +1) 1 e 1s 1 1 valence electron O+8) 2e) 6 e 1s 2 2s 2 2p 4 Outer level not completed - 6 valence electrons |
Valence electrons are the outer level andd are electrons of the preexternal level For example , Cr +24) 2e) 8e) 8e+ 5e )1e 6 valence electrons (5e + 1e) |
chemical bond - this is the interaction of atoms, carried out by the exchange of electrons.
When a chemical bond is formed, atoms tend to acquire a stable eight-electron (or two-electron - H, He) outer shell, corresponding to the structure of the nearest inert gas atom, i.e. complete your outer level.
Classification of chemical bonds.
1. According to the mechanism of chemical bond formation.
a) exchange when both atoms that form a bond provide unpaired electrons for it.
For example, the formation of hydrogen molecules H 2 and chlorine Cl 2:
b) donor-acceptor , when one of the atoms provides a ready pair of electrons (donor) to form a bond, and the second atom provides an empty free orbital.
For example, the formation of an ammonium ion (NH 4) + (charged particle):
2. According to the way the electron orbitals overlap.
a) σ - connection (sigma), when the overlap maximum lies on the line connecting the centers of atoms.
For example,
H 2 σ (s-s)
Cl 2 σ(p-p)
HClσ(s-p)
b) π - connections (pi), if the overlap maximum does not lie on the line connecting the centers of atoms.
3. According to the method of achieving the completed electron shell.
Each atom tends to complete its outer electron shell, and there can be several ways to achieve such a state.
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Comparison sign |
covalent |
Ionic |
metal |
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non-polar |
polar |
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How is the completed electron shell achieved? |
Socialization of electrons |
Socialization of electrons |
Complete transfer of electrons, the formation of ions (charged particles). |
The socialization of electrons by all atoms in crist. lattice |
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What atoms are involved? |
nemeth - nemeth EO = EO |
1) Nemeth-Nemeth 1 2) Meth-Nemeth EO < ЭО |
meth+ [numb] - EO << EO |
The sites contain cationic metal atoms. Communication is carried out by electrons freely moving in the interstitial space. |
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∆c = EO 1 - EO 2 |
< 1,7 |
> 1,7 |
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Examples |
simple substances are non-metals. | |||
An ionic chemical bond is a bond that forms between atoms of chemical elements (positively or negatively charged ions). So what is an ionic bond, and how does it form?
General characteristics of the ionic chemical bond
Ions are charged particles that atoms become when they donate or accept electrons. They are attracted to each other quite strongly, it is for this reason that substances with this type of bond have high boiling and melting points.
Rice. 1. Ions.
An ionic bond is a chemical bond between dissimilar ions due to their electrostatic attraction. It can be considered the limiting case of a covalent bond, when the difference between the electronegativity of the bound atoms is so great that complete separation of charges occurs.
Rice. 2. Ionic chemical bond.
It is usually believed that the bond acquires an electronic character if EC > 1.7.
The difference in the value of electronegativity is greater, the further the elements are located from each other in the periodic system by period. This connection is characteristic of metals and non-metals, especially those located in the most remote groups, for example, I and VII.
Example: table salt, sodium chloride NaCl:
Rice. 3. Scheme of the ionic chemical bond of sodium chloride.
The ionic bond exists in crystals, it has strength, length, but is not saturated and not directed. Ionic bonding is characteristic only for complex substances, such as salts, alkalis, and some metal oxides. In the gaseous state, such substances exist in the form of ionic molecules.
An ionic chemical bond is formed between typical metals and non-metals. Electrons without fail pass from the metal to the non-metal, forming ions. As a result, an electrostatic attraction is formed, which is called an ionic bond.
In fact, a completely ionic bond does not occur. The so-called ionic bond is partly ionic, partly covalent. However, the bond of complex molecular ions can be considered ionic.
Examples of ionic bond formation
There are several examples of the formation of an ionic bond:
- interaction of calcium and fluorine
Ca 0 (atom) -2e \u003d Ca 2 + (ion)
It is easier for calcium to donate two electrons than to receive the missing ones.
F 0 (atom) + 1e \u003d F- (ion)
- Fluorine, on the contrary, is easier to accept one electron than to give seven electrons.
Let us find the least common multiple between the charges of the formed ions. It is equal to 2. Let's determine the number of fluorine atoms that will accept two electrons from a calcium atom: 2: 1 = 2. 4.
Let's make a formula for an ionic chemical bond:
Ca 0 +2F 0 →Ca 2 +F−2.
- interaction of sodium and oxygen
