The state for any atom with E min corresponds to the “outside-completed” electron shell (for atoms of the 1st period –H and He are two electrons, for all other elements there are eight electrons - the octet rule).Ways to achieve E min atoms have several.
Ionic bond formation diagram
Two methods are used to achieve the state with E min:
(a) reception of electrons missing before the completion of the outer layer of the electron shell;
(b) the release of outer electrons to “expose” the previously completed pre-outer layer of the electron shell.
Both ways of reaching by atoms E minare realized simultaneously when an ionic bond occurs:
As a result of the transfer of an electron from the Na atom to the Cl atom, both atoms acquired E min
(8 electrons “outside” each). The atoms have now become oppositely charged ions, which are attracted to each other electrostatically - ionic bond.
Covalent bond
The third way atoms reach E min - socialization of the electrons of the connecting atoms with the simultaneous completion of the electronic shells of the connecting atoms.
Common electron pairs arise between the connecting atoms, which are shared by both atoms. As a result, several atoms at once reached E min (here 2 electrons for the H atom and 8 electrons for the C atom).
Common (connecting) electron pairs - covalent bond , is the main type of bond in organic chemistry.
Connections between organogenic elements can be both simple (single) and multiple (double or triple, carried out, respectively, by two or three common electron pairs).
This, albeit very simplified, concept of the covalent bond from the point of view of PM is convenient to use to discuss (explain) the methods of breaking and forming bonds, mechanisms and types of organic reactions.
To explain the structure of molecules and, consequently, their chemical properties PM is insufficient. The structure of molecules can be explained only from the standpoint of a quantum mechanical model of the structure of the atom.
From the point of view of CMM, a chemical bond is the overlap of the orbitals of the connecting atoms with the formation of regions of increased electron density between their nuclei.
There are two ways to overlap the orbitals:
1) "frontal" overlap (s - connection);
2) "lateral" overlap (p - connection).
s - communication (sigma - communication)
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In education s - connections can be involved atomic orbitals of all types - both simple (s- and p-) and hybrid (sp, sp 2 and sp 3). The area of \u200b\u200bincreased electron density (overlapping area) lies on the communication line (an imaginary line connecting the centers of atoms) - Fig. 8.
Education schemess - connections by overlapping orbitals of various types
p - link (pi - link)
Only symmetric non-hybrid p-orbitals * of connecting atoms, oriented along the parallel axes of space, can participate in the formation of p - bonds.
With this method of overlapping orbitals, the region of increased electron density (overlapping region) does not lie on the communication line.
It is important to emphasize that the p-bond is not a double bond, it is a way of overlapping the orbitals. p - bond - a single covalent bond formed by one common electron pair (like s). But p - electrons are on the periphery of the molecule and are primarily subjected to the “attack” of the reagent. In addition, the p - bond is less strong than the s - bond, and therefore compounds with the p - bond have increased reactivity.
* In the formation of p - bonds, d - orbitals can also participate (not considered here).
Multiple links
From the point of view of the CMM, a multiple bond is the simultaneous presence of s - and p - bonds between two atoms.
If there is a chemical bond between atoms, there is always an s-bond. A double bond is one s - and one p - bond (see Figure 11). A triple bond is one s - and two p - bonds (see Figure 12). When depicting structural formulas, s - and p - bonds are shown with the same valence lines. It is important to remember about the different nature of the s - and p - bonds.
Molecular structure and models
a) the order of bonds of atoms in a molecule (“who is connected with whom”);
b) the nature of the mutual arrangement of atoms in space relative to each other;
c) types of bonds between atoms.
Structural is s - link since she directed.
The directionality of the s - bond is expressed in the fact that several atoms associated with the same (central) atom of the molecule are located in space strictly along the directions of orientation of the hybrid orbitals of the central atom.
Carbon in the first valence state (sp 3). (Н - С) - bonds - s (sp 3 - s). The H atoms are arranged in the directions of orientation of the hybrid sp 3 - orbitals of the C atom. The molecule is volumetric.
Structure (a) and model (b) of the ethylene molecule C 2 H 4.
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Carbon in the second valence state (sp 2). The H atoms are located in the directions of orientation of the hybrid sp 2 - carbon orbitals. All six atoms of the C 2 H 4 molecule lie in the same plane (XY). The ethylene molecule is flat. There are two bonds between carbon atoms: 1) s (sp 2 - sp 2) and 2) p (p z - p z). There are 4 s (sp 2 - s) bonds between carbon and hydrogen atoms.
Structure (a) and model (b) of the C 2 H 2 acetylene molecule.
Carbon in the third valence state (sp). H atoms are located in the directions of orientation of hybrid sp - orbitals of carbon; all four atoms of the molecule lie on the same line - the acetylene molecule is linear. There are three bonds between carbon atoms: 1) s (sp - sp), 2) p (р z - р z) and 3) p (р y - р y). There are 2 s (sp - s) bonds between carbon and hydrogen atoms.
4. Nature and types chemical bond... Covalent bond
4.5. Types of covalent bonds
During the formation of a covalent bond, AOs can overlap in different ways; therefore, covalent bonds of the σ- and π-types are distinguished.
In the case of a bond, σ-bonds AO overlap along the line connecting the atomic nuclei (axial overlap):
When a π-type bond is formed, AO overlaps outside the line connecting the atomic nuclei (lateral overlap):
Π-type bonds are formed with the participation of p - or d -AO; only σ-type bonds can be formed with the participation of s-AOs.
Distinguish between single (simple) and multiple links.
A single bond is a bond formed by one pair of electrons. As a rule, this is a σ-bond.
Double and triple bonds are called multiples, i.e. bonds formed by two and three common electron pairs, respectively. A double bond consists of one σ- and one π-bond, and a triple bond consists of one σ- and two π-bonds (as we can see, only one σ-bond and only two π-bonds can form between two atoms at most). Examples of molecular structures with different numbers of single and multiple bonds:
In fig. 4.4 shows in detail the formation of bonds in the nitrogen molecule.
Figure: 4.4. Diagram of the formation of a triple bond in a nitrogen molecule
Since the electron cloud of the σ-bond has a cylindrical symmetry, free rotation of atoms or atomic groups that does not break the bond is possible around the axis of this bond. However, such rotation is impossible around multiple bonds, since the energy of rotational motion is much less than the energy of a π-bond. In the case of alkenes, this leads to the appearance of cis, trans isomers.
With axial overlap of AO, the electron density in the internuclear space is higher than with the lateral one. Therefore, σ-bonds are stronger than π-bonds, and it is for this reason that they form in the first place.
Distinguish between covalent polar and covalent non-polar chemical bonds.
Covalent non-polar connection is a bond between atoms of non-metals with the same electronegativity (H 2, O 2, Cl 2, N 2, etc.). In molecules with this bond, the electron bond density equally distributed between atoms (there are no charge poles, the bond is non-polar), therefore the atoms are electrically neutral.
Covalent polar bond is a bond between atoms of non-metals with different electronegativity. In the case of such molecules, the total electron bond density is shifted towards the atom with a large value of χ. As a result, an excess partial positive charge arises on an atom with a lower χ value, and on an atom with a higher electronegativity - the same magnitude but excess negative charge δ - (H δ + –Cl δ–, P δ + –F δ–). Such partial charges are called effective.
Polar molecules are called dipoles (they have two poles - positive and negative), conventionally they are designated as an ellipse. Examples of polar molecules: NH 3, SO 2, H 2 O, HCl, HF, HBr, HI.
The ability of an electron cloud to shift to one of the bound atoms (polarization) is different for σ- and π-bonds. The electron cloud of the π-bond is more easily polarized, which has a significant effect on the structure of the products of reactions with the participation of alkenes (Markovnikov's rule).
The degree of bond polarity can be qualitatively estimated by comparing the χ values \u200b\u200bof the atoms that formed the bond: the larger the ∆χ of these atoms, the more polar connection and the greater the value of the partial effective charges δ on the atoms that formed the bond. For example, the О – Н bond is more polar than the N – H bond, since χ (O)\u003e χ (N), and the H – F bond is more polar than the Н – О bond, since χ (F)\u003e χ (O) ...
Example 4.2. What is the least polar chemical bond in the molecules:
Decision. We write in a series of increasing electronegativities χ the atoms of the elements forming the indicated substances,
The length of the segments ∆χ is the difference between the electronegativities of the atoms that formed the chemical bond: the longer the length of the segment, the more polar the Н – Э bond, the smaller it is, the less polar the Н – Э bond.
We see that the length of the segment ∆χ is the smallest in the case of the element phosphorus. Thus, the least polar H – P bond.
Answer: 4).
The polarity of individual bonds should be distinguished from the polarity of the molecule as a whole. In this case, you must be guided by the following rules:
a) diatomic molecules with polar link (CO, HF, NO) are always polar;
b) triatomic molecules of type A 2 B (H 2 O, CO 2, BeH 2, BeCl 2, SO 2, H 2 S) are non-polar, if they have a linear structure:
O \u003d C \u003d O H – Be – H Cl – Be – Cl
and polar, if they have an angular structure:
c) tetraatomic molecules of composition A 3 B are non-polar if they have the shape of a regular triangle:
and polar, if they have a pyramidal structure:
Due to the high symmetry, the molecules of benzene, methane and its tetrahalogenated derivatives with the same halogen are nonpolar. On the contrary, monohalogenated derivatives of benzene, as well as derivatives of methane with one to three atoms of the same halogen in the molecule will be polar.
11. In which row are substances with only a covalent polar bond listed:1) СН4 Н2 Сl2 2) NH3 HBr CO2 3) PCl3 KCl CCl4 4) H2S SO2 LiF
12. In which row are the substances listed only with ionic type communication:
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 by interaction
1) CH4 and O2 2) NH3 and HCl 3) C2H6 and HNO3 4) SO3 and H2O
14. In what substance are all chemical bonds - covalent non-polar?
1) Diamond 2) Carbon (IV) oxide 3) Gold 4) Methane
15. The relationship formed between elements with serial numbers 15 and 53
1) ionic 2) metal
3) covalent non-polar 4) covalent polar
16. A 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. In what substance is there simultaneously ionic and covalent chemical bonds?
1) Sodium chloride 2) Hydrogen chloride 3) Sodium sulfate 4) Phosphoric acid
19. A more pronounced ionic character has a chemical bond in the molecule
1) lithium bromide 2) copper chloride 3) calcium carbide 4) potassium fluoride
20. Three common electron pairs form a covalent bond in a 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? 1) 2 2) 3 3) 4 4) 18
22. Four covalent bonds are contained in a molecule: 1) CO2 2) C2H4 3) P4 4) C3H4
fluorine? By overlapping what orbitals are they formed?
(With a detailed solution!)
1) There is a chemical bond in the Na2SO4 molecule ...
a) only ionic
b) covalent polar and non-polar
c) ionic and covalent polar
d) ionic and covalent non-polar
2) What is the chemical bond in the K2S compound
3) How many common electron pairs are in a nitrogen molecule?
4) Which of the formulas has a double chemical bond: S2; H2; N2; CI2?
1. Formula of substance with ionic bond: A.HCI. B. KBr. B.P4. G. CH3OH. 2. Substance with a metallic bond: A. Potassium oxide. B. Silicon. B. Copper. G.Magnesium hydroxide. 3. The number of common electron pairs in the nitrogen molecule: A. One. B. Two. At three o'clok. G. Four. 4. The polarity of the chemical bond decreases in a number of compounds, the formulas of which are: A. CI2, H2S, CO2. B. NH3, PH3, S02. B. HCI, HBr, HI. G. BH3, NH3, HF. 5. Type of hybridization of electron orbitals of the sulfur atom in the molecule of the compound, the formula of which is H2S: A. sp3. B. sp2. B. sp. D. Not hybridized. 6. Crystal cell silicon oxide (IV): A. Atomic. B. Metallic. B. Ionic. G. Molecular. 7. Number of Sigma and Pi bonds in ethene molecule: A. 6 Sigma and Pi - no. B. 4 Sigma and 2 Pi. B. 3 Sigma and 3 Pi. D. 5 Sigma and 1 Pi 8. Substances, the formulas of which are CH2 \u003d CH-CH2-CH3 and CH2 \u003d C-CH3 are: CHs A Homologues. B. Isomers. B. The same substance. 9. The homologue of a substance, the formula of which is CH3-CH2-CH2-OH, is: A. Butanal. V. Ethanal. B. Butanol-2. G. Ethanol. 10. Substance, the formula of which is CH3 - C \u003d CH2,. .. | CH3-CH2 A. 2-methylbutene-1. B. 2-ethylpropene-2. B. 2-ethylpropene-1. D. 2-methylbutene-2. ... ... ... 11. Make a diagram of the formation of connections consisting of chemical elements: A. Calcium and fluorine. B. Arsenic and hydrogen. Indicate the type of chemical bond in each compound. 12. What is the geometric shape of the compound molecule with covalent bond from task 11? 13. Arrange the compounds whose formulas are CH3NH2, NH3, C6H5NH2, C2H5NH2, in ascending order of acidic properties. Explain the answer. 14. Make up structural formulas at least three possible isomers of substances with the composition С4H8O2. Name these substances. 15. What volume of oxygen is required for complete combustion of 1 m3 of propane?