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Each atom has a certain number of electrons.
When entering into 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 of these.
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 valence, 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 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 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 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 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 the ammonia molecule accepts a proton from the water molecule.
Thus, water can form two conjugated 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 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. An example of a covalent bond is homonuclear diatomic molecules H 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 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 the 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 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 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 the electric field of 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 greater 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 a brief review of bond types, 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, ionic radii of metals and van der Waals radii of interacting molecules increase as their atomic number increases in groups of the periodic system. 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
Chemical bond.
Exercises.
1. Determine the type of chemical bond in the following substances:
Substance | Phosphorus chloride | Sulfuric acid | |||
Communication type | |||||
Substance | Barium oxide | ||||
Communication type |
2. Emphasize substances in which BETWEEN molecules exists hydrogen bond:
sulphur dioxide; ice; ozone; ethanol; ethylene; acetic acid; hydrogen fluoride.
3. How do they affect bond length, strength and polarity- atomic radii, their electronegativity, bond multiplicity?
A) The larger the radii atoms that form a bond, so link length _______
b) The higher the multiplicity (single, double or triple) bonds, so its strength ____________________
V) The greater the electronegativity difference between two atoms, the polarity of the bond ____________
4. Compare length, strength and polarity of bonds in molecules:
a) bond length: HCl ___HBr
b) bond strength PH3_______NH3
c) polarity of the CCl4 bond ______CH4
d) bond strength: N2 _______O2
e) bond length between carbon atoms in ethylene and acetylene: __________
f) polarity of bonds in NH3_________H2O
Tests. A4. Chemical bond.
1. The valence of an atom is
1) the number of chemical bonds formed by a given atom in a compound
2) oxidation state of the atom
3) the number of electrons given or received
4) the number of electrons missing to obtain the electron configuration of the nearest inert gas
A. When a chemical bond is formed, energy is always released
B. The energy of a double bond is less than that of a single bond.
1) only A is true 2) only B is true 3) both judgments are correct 4) both judgments are incorrect
3. In substances formed by combining identical atoms, chemical bond
1) ionic 2) covalent polar 3) hydrogen 4) covalent nonpolar
4. Compounds with a covalent polar and covalent nonpolar bond are respectively
1) water and hydrogen sulfide 2) potassium bromide and nitrogen
5. Due to the shared electron pair, a chemical bond is formed in the compound
1) KI 2) HBr 3) Li2O 4) NaBr
6. Select a pair of substances in which all bonds are covalent:
1) NaCl, HCl 2) CO2, BaO 3) CH3Cl, CH3Na 4) SO2, NO2
7. A substance with a polar covalent bond has the formula
1)KCl 2)HBr 3)P4 4)CaCl2
8. Compound with an ionic chemical bond
1) phosphorus chloride 2) potassium bromide 3) nitrogen oxide (II) 4) barium
9. In ammonia and barium chloride, the chemical bond is respectively
1) ionic and covalent polar 2) covalent nonpolar and ionic 3) covalent polar and ionic 4) covalent nonpolar and metallic
10. Substances with a covalent polar bond are
1) sulfur oxide (IV) 2) oxygen 3) calcium hydride 4) diamond
11. Which series lists substances with only polar covalent bonds:
1) CH4 H2 Cl2 2) NH3 HBr CO2 3) PCl3 KCl CCl4 4) H2S SO2 LiF
12. Which series lists substances with only ionic bonds:
1) F2O LiF SF4 2) PCl3 NaCl CO2 3) KF Li2O BaCl2 4) CaF2 CH4 CCl4
13. A compound with an ionic bond is formed when interacting
1) CH4 and O2 2) NH3 and HCl 3) C2H6 and HNO3 4) SO3 and H2O
14. In which substance are all chemical bonds covalent nonpolar?
1) Diamond 2) Carbon monoxide (IV) 3) Gold 4) Methane
15. The connection formed between elements with serial numbers 15 and 53
1) ionic 2) metallic
3) covalent non-polar 4) covalent polar
16. Hydrogen bond is formed between molecules
1) ethane 2) benzene 3) hydrogen 4) ethanol
17. In what substance is hydrogen bonds?
1) Hydrogen sulfide 2) Ice 3) Hydrogen bromide 4) Benzene
18.Which substance contains both ionic and covalent chemical bonds?
1) Sodium chloride 2) Hydrogen chloride 3) Sodium sulfate 4) Phosphoric acid
19. The chemical bond in the molecule has a more pronounced ionic character
1) lithium bromide 2) copper chloride 3) calcium carbide 4) potassium fluoride
20. Three common electron pairs form a covalent bond in the molecule of 1) nitrogen 2) hydrogen sulfide 3) methane 4) chlorine
21. How many electrons are involved in the formation of chemical bonds in a water molecule?4) 18
22. The molecule contains four covalent bonds: 1) CO2 2) C2H4 3) P4 4) C3H4
23. The number of bonds in molecules increases in a series
1) CHCl3, CH4 2) CH4, SO3 3) CO2, CH4 4) SO2, NH3
24. In what compound is a covalent bond formed between atoms? by donor-acceptor mechanism? 1) KCl 2) CCl4 3) NH4Cl 4) CaCl2
25. Which of the following molecules requires the least amount of energy to decompose into atoms? 1) HI 2) H2 3) O2 4) CO
26. Indicate the molecule in which the binding energy is the highest:
1) N≡N 2) H-H 3) O=O 4) H-F
27. Indicate the molecule in which the chemical bond is the strongest:
1) HF 2) HCl 3) HBr 4) HI
28. Indicate a series characterized by an increase in the length of a chemical bond
1)O2, N2, F2, Cl2 2)N2, O2, F2, Cl2 3)F2, N2, O2, Cl2 4)N2, O2, Cl2, F2
29. The length of the E-O bond increases in the series
1) silicon oxide (IV), carbon oxide (IV)
2) sulfur(IV) oxide, tellurium(IV) oxide
3) strontium oxide, beryllium oxide
4) sulfur oxide(IV), carbon monoxide(IV)
30. In the series CH4 – SiH4 occurs increase
1) bond strength 2) oxidative properties
3) bond lengths 4) bond polarities
31. In what row are the molecules arranged in order of increasing polarity of bonds?
1) HF, HCl, HBr 2) H2Se, H2S, H2O 3) NH3, PH3, AsH3 4) CO2, CS2, CSe2
32. The most polar covalent bond in a molecule is:
1) CH4 2) CF4 3) CCl4 4) CBr4
33.Indicate the series in which the polarity increases:
1)AgF, F2, HF 2)Cl2, HCl, NaCl 3)CuO, CO, O2 4) KBr, NaCl, KF
Covalent chemical bond, its varieties and mechanisms of formation. Characteristics of covalent bonds (polarity and bond energy). Ionic bond. Metal connection. Hydrogen bond.
1. In ammonia and barium chloride, the chemical bond is respectively
1) ionic and covalent polar
2) covalent polar and ionic
3) covalent nonpolar and metallic
4) covalent nonpolar and ionic
2. Substances with only ionic bonds are listed in the following series:
1) F2, CCl4, KC1
2) NaBr, Na2O, KI
3. A compound with an ionic bond is formed by interaction
3) С2Н6 and HNO3
4. In which series do all substances have a polar covalent bond?
1) HCl, NaCl. Cl2
4) NaBr. HBr. CO
5. In which series are the formulas of substances with only covalent polar
1) C12, NO2, HC1
6. Covalent nonpolar bond is characteristic of
1) C12 2) SO3 3) CO 4) SiO2
7. A substance with a polar covalent bond is
1) C12 2) NaBr 3) H2S 4) MgCl2
8. A substance with a covalent bond is
1) CaC12 2) MgS 3) H2S 4) NaBr
9. A substance with a covalent nonpolar bond has the formula
1) NH3 2) Cu 3) H2S 4) I2
10. Substances with non-polar covalent bonds are
1) water and diamond
2) hydrogen and chlorine
3) copper and nitrogen
4) bromine and methane
11. A chemical bond is formed between atoms with the same relative electronegativity
2) covalent polar
3) covalent nonpolar
4) hydrogen
12. Covalent polar bonds are characteristic of
1) KC1 2) HBr 3) P4 4) CaCl2
13. A chemical element in the atom of which the electrons are distributed among the layers as follows: 2, 8, 8, 2 forms a chemical bond with hydrogen
1) covalent polar
2) covalent nonpolar
4) metal
14. In the molecule of which substance does the bond between carbon atoms have the longest length?
1) acetylene 2) ethane 3) ethene 4) benzene
15. Three common electron pairs form a covalent bond in a molecule
2) hydrogen sulfide
16. Hydrogen bonds form between molecules
1) dimethyl ether
2) methanol
3) ethylene
4) ethyl acetate
17. Bond polarity is most pronounced in the molecule
1) HI 2) HC1 3) HF 4) NVg
18. Substances with non-polar covalent bonds are
1) water and diamond
2) hydrogen and chlorine
3) copper and nitrogen
4) bromine and methane
19. Hydrogen bonding is not typical for the substance
1) H2O 2) CH4 3) NH3 4) CH3OH
20. A covalent polar bond is characteristic of each of the two substances whose formulas are
2) CO2 and K2O
4) CS2 and RS15
21. The weakest chemical bond in a molecule
1) fluorine 2) chlorine 3) bromine 4) iodine
22. Which substance has the longest chemical bond in its molecule?
1) fluorine 2) chlorine 3) bromine 4) iodine
23. Each of the substances indicated in the series has covalent bonds:
1) C4H10, NO2, NaCl
2) CO, CuO, CH3Cl
4) C6H5NO2, F2, CC14
24. Each of the substances indicated in the series has a covalent bond:
1) CaO, C3H6, S8
2) Fe. NaNO3, CO
3) N2, CuCO3, K2S
4) C6H5N02, SO2, CHC13
25. Each of the substances indicated in the series has a covalent bond:
1) C3H4, NO, Na2O
2) CO, CH3C1, PBr3
3) Р2Оз, NaHSO4, Cu
4) C6H5NO2, NaF, CC14
26. Each of the substances indicated in the series has covalent bonds:
1) C3Ha, NO2, NaF
2) KC1, CH3Cl, C6H12O6
3) P2O5, NaHSO4, Ba
4) C2H5NH2, P4, CH3OH
27. Bond polarity is most pronounced in molecules
1) hydrogen sulfide
3) phosphine
4) hydrogen chloride
28. In the molecule of which substance are the chemical bonds the strongest?
29. Among the substances NH4Cl, CsCl, NaNO3, PH3, HNO3 - the number of compounds with ionic bonds is equal
30. Among the substances (NH4)2SO4, Na2SO4, CaI2, I2, CO2 - the number of compounds with a covalent bond is equal
Answers: 1-2, 2-2, 3-4, 4-3, 5-4, 6-1, 7-3, 8-3, 9-4, 10-2, 11-3, 12-2, 13-3, 14-2, 15-1, 16-2, 17-3, 18-2, 19-2, 20-4, 21-4, 22-4, 23-4, 24-4, 25- 2, 26-4, 27-4, 28-1, 29-3, 30-4
Characteristics of chemical bonds
The doctrine of chemical bonding forms the basis of all theoretical chemistry. A chemical bond is understood as the interaction of atoms that binds them into molecules, ions, radicals, and crystals. There are four types of chemical bonds: ionic, covalent, metallic and hydrogen. Different types of bonds can be found in the same substances.
1. In bases: between the oxygen and hydrogen atoms in hydroxo groups the bond is polar covalent, and between the metal and the hydroxo group it is ionic.
2. In salts of oxygen-containing acids: between the non-metal atom and the oxygen of the acidic residue - covalent polar, and between the metal and the acidic residue - ionic.
3. In ammonium, methylammonium, etc. salts, between the nitrogen and hydrogen atoms there is a polar covalent, and between ammonium or methylammonium ions and the acid residue - ionic.
4. In metal peroxides (for example, Na 2 O 2), the bond between the oxygen atoms is covalent, nonpolar, and between the metal and oxygen is ionic, etc.
The reason for the unity of all types and types of chemical bonds is their identical chemical nature - electron-nuclear interaction. The formation of a chemical bond in any case is the result of electron-nuclear interaction of atoms, accompanied by the release of energy.
Methods for forming a covalent bond
Covalent chemical bond is a bond that arises between atoms due to the formation of shared electron pairs.
Covalent compounds are usually gases, liquids, or relatively low-melting solids. One of the rare exceptions is diamond, which melts above 3,500 °C. This is explained by the structure of diamond, which is a continuous lattice of covalently bonded carbon atoms, and not a collection of individual molecules. In fact, any diamond crystal, regardless of its size, is one huge molecule.
A covalent bond occurs when the electrons of two nonmetal atoms combine. The resulting structure is called a molecule.
The mechanism of formation of such a bond can be exchange or donor-acceptor.
In most cases, two covalently bonded atoms have different electronegativity and the shared electrons do not belong to the two atoms equally. Most of the time they are closer to one atom than to another. In a hydrogen chloride molecule, for example, the electrons that form a covalent bond are located closer to the chlorine atom because its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not large enough for complete electron transfer from the hydrogen atom to the chlorine atom to occur. Therefore, the bond between hydrogen and chlorine atoms can be considered as a cross between an ionic bond (complete electron transfer) and a non-polar covalent bond (a symmetrical arrangement of a pair of electrons between two atoms). The partial charge on atoms is denoted by the Greek letter δ. Such a bond is called a polar covalent bond, and the hydrogen chloride molecule is said to be polar, that is, it has a positively charged end (hydrogen atom) and a negatively charged end (chlorine atom).
1. The exchange mechanism operates when atoms form shared electron pairs by combining unpaired electrons.
1) H 2 - hydrogen.
The bond occurs due to the formation of a common electron pair by the s-electrons of hydrogen atoms (overlapping s-orbitals).
2) HCl - hydrogen chloride.
The bond occurs due to the formation of a common electron pair of s- and p-electrons (overlapping s-p orbitals).
3) Cl 2: In a chlorine molecule, a covalent bond is formed due to unpaired p-electrons (overlapping p-p orbitals).
4) N 2: In the nitrogen molecule, three common electron pairs are formed between the atoms.
Donor-acceptor mechanism of covalent bond formation
Donor has an electron pair acceptor- free orbital that this pair can occupy. In the ammonium ion, all four bonds with hydrogen atoms are covalent: three were formed due to the creation of common electron pairs by the nitrogen atom and hydrogen atoms according to the exchange mechanism, one - through the donor-acceptor mechanism. Covalent bonds are classified by the way the electron orbitals overlap, as well as by their displacement towards one of the bonded atoms. Chemical bonds formed as a result of overlapping electron orbitals along a bond line are called σ - connections(sigma bonds). The sigma bond is very strong.
p orbitals can overlap in two regions, forming a covalent bond through lateral overlap.
Chemical bonds formed as a result of the “lateral” overlap of electron orbitals outside the bond line, i.e., in two regions, are called pi bonds.
According to the degree of displacement of common electron pairs to one of the atoms they connect, a covalent bond can be polar or non-polar. A covalent chemical bond formed between atoms with the same electronegativity is called non-polar. Electron pairs are not displaced towards any of the atoms, since atoms have the same electronegativity - the property of attracting valence electrons from other atoms. For example,
that is, molecules of simple non-metal substances are formed through a covalent non-polar bond. A covalent chemical bond between atoms of elements whose electronegativity differs is called polar.
For example, NH 3 is ammonia. Nitrogen is a more electronegative element than hydrogen, so the shared electron pairs are shifted towards its atom.
Characteristics of a covalent bond: bond length and energy
The characteristic properties of a covalent bond are its length and energy. Bond length is the distance between atomic nuclei. The shorter the length of a chemical bond, the stronger it is. However, a measure of bond strength is bond energy, which is determined by the amount of energy required to break the bond. It is usually measured in kJ/mol. Thus, according to experimental data, the bond lengths of the H 2, Cl 2 and N 2 molecules, respectively, are 0.074, 0.198 and 0.109 nm, and the bond energies, respectively, are 436, 242 and 946 kJ/mol.
Ions. Ionic bond
There are two main possibilities for an atom to obey the octet rule. The first of these is the formation of ionic bonds. (The second is the formation of a covalent bond, which will be discussed below). When an ionic bond is formed, a metal atom loses electrons, and a non-metal atom gains electrons.
Let's imagine that two atoms “meet”: an atom of a group I metal and a non-metal atom of group VII. A metal atom has a single electron at its outer energy level, while a non-metal atom just lacks one electron for its outer level to be complete. The first atom will easily give the second its electron, which is far from the nucleus and weakly bound to it, and the second will provide it with a free place on its outer electronic level. Then the atom, deprived of one of its negative charges, will become a positively charged particle, and the second will turn into a negatively charged particle due to the resulting electron. Such particles are called ions.
This is a chemical bond that occurs between ions. Numbers showing the number of atoms or molecules are called coefficients, and numbers showing the number of atoms or ions in a molecule are called indices.
Metal connection
Metals have specific properties that differ from the properties of other substances. Such properties are relatively high melting temperatures, the ability to reflect light, and high thermal and electrical conductivity. These features are due to the existence of a special type of bond in metals - a metallic bond.
Metallic bond is a bond between positive ions in metal crystals, carried out due to the attraction of electrons moving freely throughout the crystal. The atoms of most metals at the outer level contain a small number of electrons - 1, 2, 3. These electrons come off easily, and the atoms turn into positive ions. The detached electrons move from one ion to another, binding them into a single whole. Connecting with ions, these electrons temporarily form atoms, then break off again and combine with another ion, etc. A process occurs endlessly, which can be schematically depicted as follows:
Consequently, in the volume of the metal, atoms are continuously converted into ions and vice versa. The bond in metals between ions through shared electrons is called metallic. The metallic bond has some similarities with the covalent bond, since it is based on the sharing of external electrons. However, with a covalent bond, the outer unpaired electrons of only two neighboring atoms are shared, while with a metallic bond, all atoms take part in the sharing of these electrons. That is why crystals with a covalent bond are brittle, but with a metal bond, as a rule, they are ductile, electrically conductive and have a metallic luster.
Metallic bonding is characteristic of both pure metals and mixtures of various metals - alloys in solid and liquid states. However, in the vapor state, metal atoms are connected to each other by a covalent bond (for example, sodium vapor fills yellow light lamps to illuminate the streets of large cities). Metal pairs consist of individual molecules (monatomic and diatomic).
A metal bond also differs from a covalent bond in strength: its energy is 3-4 times less than the energy of a covalent bond.
Bond energy is the energy required to break a chemical bond in all molecules that make up one mole of a substance. The energies of covalent and ionic bonds are usually high and amount to values of the order of 100-800 kJ/mol.
Hydrogen bond
Chemical bond between positively polarized hydrogen atoms of one molecule(or parts thereof) and negatively polarized atoms of highly electronegative elements having shared electron pairs (F, O, N and less often S and Cl), another molecule (or parts thereof) is called hydrogen. The mechanism of hydrogen bond formation is partly electrostatic, partly d honoror-acceptor character.
Examples of intermolecular hydrogen bonding:
In the presence of such a connection, even low-molecular substances can, under normal conditions, be liquids (alcohol, water) or easily liquefied gases (ammonia, hydrogen fluoride). In biopolymers - proteins (secondary structure) - there is an intramolecular hydrogen bond between carbonyl oxygen and the hydrogen of the amino group:
Polynucleotide molecules - DNA (deoxyribonucleic acid) - are double helices in which two chains of nucleotides are linked to each other by hydrogen bonds. In this case, the principle of complementarity operates, i.e., these bonds are formed between certain pairs consisting of purine and pyrimidine bases: the thymine (T) is located opposite the adenine nucleotide (A), and the cytosine (C) is located opposite the guanine (G).
Substances with hydrogen bonds have molecular crystal lattices.
