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.
The 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 break 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.
5) to s-elements
6) to p-elements
7) to d-elements
8) to f - elements
2. How many electrons do atoms of alkaline earth metals contain at the outer energy level?
1) One 2) two 3) three 4) four
3. In chemical reactions, aluminum atoms exhibit
3) Oxidizing properties 2) acidic properties
4) 3) restorative properties 4) basic properties
4. The interaction of calcium with chlorine is a reaction
1) Decomposition 2) connection 3) substitution 4) exchange
5. The molecular weight of sodium bicarbonate is:
1) 84 2) 87 3) 85 4) 86
3. Which atom is heavier - iron or silicon - and by how much?4. Determine the relative molecular weights of simple substances: hydrogen, oxygen, chlorine, copper, diamond (carbon). Remember which of them consist of diatomic molecules, and which of atoms.
5.calculate the relative molecular masses of the following compounds: carbon dioxide CO2 sulfuric acid H2SO4 sugar C12H22O11 ethyl alcohol C2H6O marble CaCPO3
6.In hydrogen peroxide, there is one hydrogen atom for every oxygen atom. Determine the formula of hydrogen preoxide if it is known that its relative molecular weight is 34. What is the mass ratio of hydrogen and oxygen in this compound?
7. How many times is a carbon dioxide molecule heavier than an oxygen molecule?
Please help me, 8th grade assignment.
Task No. 1
From the proposed list, select two compounds in which there is an ionic chemical bond.
- 1. Ca(ClO 2) 2
- 2. HClO 3
- 3.NH4Cl
- 4. HClO 4
- 5.Cl2O7
Answer: 13
In the overwhelming majority of cases, the presence of an ionic type of bond in a compound can be determined by the fact that its structural units simultaneously include atoms of a typical metal and non-metal atoms.
On this basis, we establish that there is an ionic bond in compound number 1 - Ca(ClO 2) 2, because in its formula, one can see atoms of a typical calcium metal and atoms of non-metals - oxygen and chlorine.
However, there are no more compounds containing both metal and non-metal atoms in this list.
Among the compounds indicated in the assignment there is ammonium chloride, in which the ionic bond is realized between the ammonium cation NH 4 + and the chloride ion Cl − .
Task No. 2
From the proposed list, select two compounds in which the type of chemical bond is the same as in the fluorine molecule.
1) oxygen
2) nitric oxide (II)
3) hydrogen bromide
4) sodium iodide
Write down the numbers of the selected connections in the answer field.
Answer: 15
The fluorine molecule (F 2) consists of two atoms of one non-metal chemical element, therefore the chemical bond in this molecule is covalent non-polar.
A covalent non-polar bond can only be realized between atoms of the same chemical element of a non-metal.
Of the proposed options, only oxygen and diamond have a covalent non-polar type of bond. The oxygen molecule is diatomic, consists of atoms of one chemical element of a non-metal. Diamond has an atomic structure and in its structure each carbon atom, which is a non-metal, is bonded to 4 other carbon atoms.
Nitric oxide (II) is a substance consisting of molecules formed by atoms of two different non-metals. Since the electronegativity of different atoms is always different, the shared electron pair in the molecule is shifted towards the more electronegative element, in this case oxygen. Thus, the bond in the NO molecule is covalent polar.
Hydrogen bromide also consists of diatomic molecules made up of hydrogen and bromine atoms. The shared electron pair forming the H-Br bond is shifted to the more electronegative bromine atom. The chemical bond in the HBr molecule is also covalent polar.
Sodium iodide is an ionic substance formed by a metal cation and an iodide anion. The bond in the NaI molecule is formed due to the transfer of an electron from 3 s-orbitals of the sodium atom (the sodium atom turns into a cation) to an underfilled 5 p-orbital of the iodine atom (the iodine atom turns into an anion). Such a chemical bond is called ionic.
Task number 3
From the proposed list, select two substances between the molecules of which hydrogen bonds are formed.
- 1. C 2 H 6
- 2.C2H5OH
- 3.H2O
- 4. CH 3 OCH 3
- 5. CH 3 COCH 3
Write down the numbers of the selected connections in the answer field.
Answer: 23
Explanation:
Hydrogen bonds occur in substances with a molecular structure that contain covalent bonds H-O, H-N, H-F. Those. covalent bonds of a hydrogen atom with atoms of three chemical elements with the highest electronegativity.
Thus, obviously, there are hydrogen bonds between molecules:
2) alcohols
3) phenols
4) carboxylic acids
5) ammonia
6) primary and secondary amines
7) hydrofluoric acid
Task No. 4
From the proposed list, select two compounds with an ionic chemical bond.
- 1. PCl 3
- 2.CO2
- 3.NaCl
- 4. H 2 S
- 5. MgO
Write down the numbers of the selected connections in the answer field.
Answer: 35
Explanation:
In the vast majority of cases, it can be concluded that there is an ionic type of bond in a compound by the fact that the composition of the structural units of a substance simultaneously includes atoms of a typical metal and non-metal atoms.
On this basis, we establish that there is an ionic bond in compound number 3 (NaCl) and 5 (MgO).
Note*
In addition to the above feature, the presence of an ionic bond in a compound can be said if its structural unit contains an ammonium cation (NH 4 +) or its organic analogs - cations of alkylammonium RNH 3 +, dialkylammonium R 2 NH 2 + , trialkylammonium R 3 NH + or tetraalkylammonium R 4 N + , where R is some hydrocarbon radical. For example, the ionic type of bond takes place in the compound (CH 3) 4 NCl between the cation (CH 3) 4 + and the chloride ion Cl - .
Task number 5
From the proposed list, select two substances with the same type of structure.
4) table salt
Write down the numbers of the selected connections in the answer field.
Answer: 23
Task No. 8
From the proposed list, select two substances of non-molecular structure.
2) oxygen
3) white phosphorus
5) silicon
Write down the numbers of the selected connections in the answer field.
Answer: 45
Task No. 11
From the proposed list, select two substances in the molecules of which there is a double bond between carbon and oxygen atoms.
3) formaldehyde
4) acetic acid
5) glycerin
Write down the numbers of the selected connections in the answer field.
Answer: 34
Task No. 14
From the proposed list, select two substances with an ionic bond.
1) oxygen
3) carbon monoxide (IV)
4) sodium chloride
5) calcium oxide
Write down the numbers of the selected connections in the answer field.
Answer: 45
Task No. 15
From the proposed list, select two substances with the same type of crystal lattice as diamond.
1) silica SiO 2
2) sodium oxide Na 2 O
3) carbon monoxide CO
4) white phosphorus P 4
5) silicon Si
Write down the numbers of the selected connections in the answer field.
Answer: 15
Task No. 20
From the proposed list, select two substances in the molecules of which there is one triple bond.
- 1. HCOOH
- 2.HCOH
- 3. C 2 H 4
- 4. N 2
- 5.C2H2
Write down the numbers of the selected connections in the answer field.
Answer: 45
Explanation:
In order to find the correct answer, let's draw the structural formulas of the compounds from the list presented:
Thus, we see that the triple bond exists in the molecules of nitrogen and acetylene. Those. correct answers 45
Task No. 21
From the proposed list, select two substances in the molecules of which there is a covalent non-polar bond.
“Chemical bond” is the energy of destruction of the lattice into ions _Ekul = Uresh. Basic principles of the MO method. Types of overlap of atomic AOs. bonding and antibonding MOs with a combination of atomic orbitals s and s pz and pz px and px. H?C? C?H. ? - Repulsion coefficient. Qeff =. Ao. Basic theories of chemical bonding.
“Types of chemical bonds” - Substances with ionic bonds form an ionic crystal lattice. Atoms. Electronegativity. Municipal Educational Institution Lyceum No. 18 chemistry teacher Kalinina L.A. Ions. For example: Na1+ and Cl1-, Li1+ and F1- Na1+ + Cl1- = Na(:Cl:) . If e - are added, the ion becomes negatively charged. The atomic frame has high strength.
“The Life of Mendeleev” - July 18 D.I. Mendeleev graduated from the Tobolsk gymnasium. August 9, 1850 - June 20, 1855 while studying at the Main Pedagogical Institute. “If you do not know names, then the knowledge of things will die” K. Liney. Life and work of D.I. Mendeleev. Ivan Pavlovich Mendeleev (1783 - 1847), father of the scientist. Discovery of the periodic law.
“Types of chemical bonds” - H3N. Al2O3. The structure of matter." H2S. MgO. H2. Cu. Mg S.CS2. I. Write down the formulas of the substances: 1.c.N.S. 2.s K.P.S. 3. with I.S. K.N.S. NaF. C.K.P.S. Determine the type of chemical bond. Which of the molecules corresponds to the scheme: A A?
"Mendeleev" - Dobereiner's Triads of Elements. Gases. Work. Life and scientific feat. Periodic table of elements (long form). Newlands' "Law of Octaves" Scientific activity. Solutions. A new stage of life. The second version of Mendeleev's system of elements. Part of L. Meyer's table of elements. Discovery of the periodic law (1869).
“The Life and Work of Mendeleev” - Ivan Pavlovich Mendeleev (1783 - 1847), the scientist’s father. 1834, January 27 (February 6) - D.I. Mendeleev was born in the city of Tobolsk, in Siberia. 1907, January 20 (February 2) D.I. Mendeleev died of heart paralysis. DI. Menedeleev (South Kazakhstan region, Shymkent city). Industry. On July 18, 1849, D.I. Mendeleev graduated from the Tobolsk gymnasium.
There is no unified theory of chemical bonds; chemical bonds are conventionally divided into covalent (a universal type of bond), ionic (a special case of a covalent bond), metallic and hydrogen.
covalent bond
The formation of a covalent bond is possible by three mechanisms: exchange, donor-acceptor and dative (Lewis).
According to metabolic mechanism The formation of a covalent bond occurs due to the sharing of common electron pairs. In this case, each atom tends to acquire a shell of an inert gas, i.e. obtain a completed external energy level. The formation of a chemical bond by exchange type is depicted using Lewis formulas, in which each valence electron of an atom is represented by dots (Fig. 1).
Rice. 1 Formation of a covalent bond in the HCl molecule by the exchange mechanism
With the development of the theory of atomic structure and quantum mechanics, the formation of a covalent bond is represented as the overlap of electronic orbitals (Fig. 2).
Rice. 2. Formation of a covalent bond due to the overlap of electron clouds
The greater the overlap of atomic orbitals, the stronger the bond, the shorter the bond length, and the greater the bond energy. A covalent bond can be formed by overlapping different orbitals. As a result of the overlap of s-s, s-p orbitals, as well as d-d, p-p, d-p orbitals with lateral lobes, the formation of bonds occurs. A bond is formed perpendicular to the line connecting the nuclei of 2 atoms. One and one bond are capable of forming a multiple (double) covalent bond, characteristic of organic substances of the class of alkenes, alkadienes, etc. One and two bonds form a multiple (triple) covalent bond, characteristic of organic substances of the class of alkynes (acetylenes).
Formation of a covalent bond by donor-acceptor mechanism Let's look at the example of the ammonium cation:
NH 3 + H + = NH 4 +
7 N 1s 2 2s 2 2p 3
The nitrogen atom has a free lone pair of electrons (electrons not involved in the formation of chemical bonds within the molecule), and the hydrogen cation has a free orbital, so they are an electron donor and acceptor, respectively.
Let us consider the dative mechanism of covalent bond formation using the example of a chlorine molecule.
17 Cl 1s 2 2s 2 2p 6 3s 2 3p 5
The chlorine atom has both a free lone pair of electrons and vacant orbitals, therefore, it can exhibit the properties of both a donor and an acceptor. Therefore, when a chlorine molecule is formed, one chlorine atom acts as a donor and the other as an acceptor.
Main characteristics of a covalent bond are: saturation (saturated bonds are formed when an atom attaches as many electrons to itself as its valence capabilities allow; unsaturated bonds are formed when the number of attached electrons is less than the valence capabilities of the atom); directionality (this value is related to the geometry of the molecule and the concept of “bond angle” - the angle between bonds).
Ionic bond
There are no compounds with a pure ionic bond, although this is understood as a chemically bonded state of atoms in which a stable electronic environment of the atom is created when the total electron density is completely transferred to the atom of a more electronegative element. Ionic bonding is possible only between atoms of electronegative and electropositive elements that are in the state of oppositely charged ions - cations and anions.
DEFINITION
Ion are electrically charged particles formed by the removal or addition of an electron to an atom.
When transferring an electron, metal and nonmetal atoms tend to form a stable electron shell configuration around their nucleus. A non-metal atom creates a shell of the subsequent inert gas around its core, and a metal atom creates a shell of the previous inert gas (Fig. 3).
Rice. 3. Formation of an ionic bond using the example of a sodium chloride molecule
Molecules in which ionic bonds exist in their pure form are found in the vapor state of the substance. The ionic bond is very strong, and therefore substances with this bond have a high melting point. Unlike covalent bonds, ionic bonds are not characterized by directionality and saturation, since the electric field created by ions acts equally on all ions due to spherical symmetry.
Metal connection
The metallic bond is realized only in metals - this is the interaction that holds metal atoms in a single lattice. Only the valence electrons of the metal atoms belonging to its entire volume participate in the formation of a bond. In metals, electrons are constantly stripped from atoms and move throughout the entire mass of the metal. Metal atoms, deprived of electrons, turn into positively charged ions, which tend to accept moving electrons. This continuous process forms the so-called “electron gas” inside the metal, which firmly binds all the metal atoms together (Fig. 4).
The metallic bond is strong, therefore metals are characterized by a high melting point, and the presence of “electron gas” gives metals malleability and ductility.
Hydrogen bond
A hydrogen bond is a specific intermolecular interaction, because its occurrence and strength depend on the chemical nature of the substance. It is formed between molecules in which a hydrogen atom is bonded to an atom with high electronegativity (O, N, S). The occurrence of a hydrogen bond depends on two reasons: firstly, the hydrogen atom associated with an electronegative atom does not have electrons and can easily be incorporated into the electron clouds of other atoms, and, secondly, having a valence s-orbital, the hydrogen atom is able to accept a lone pair electrons of an electronegative atom and form a bond with it through the donor-acceptor mechanism.
