3.3.1 Covalent bond - This is a two-center two-electron bond formed due to the overlap of electron clouds carrying unpaired electrons with antiparallel spins. As a rule, it is formed between atoms of one chemical element.
Quantitatively, it is characterized by valency. Element valency - this is its ability to form a certain number of chemical bonds due to free electrons located in the atomic valence zone.
A covalent bond is formed only by a pair of electrons located between atoms. It is called a divided pair. The remaining pairs of electrons are called lone pairs. They fill the shells and do not take part in binding. Communication between atoms can be carried out not only by one, but also by two or even three shared pairs. Such connections are called double and t swarm - multiple bonds.
3.3.1.1 Covalent non-polar bond. A bond carried out by the formation of electron pairs equally belonging to both atoms is called covalent non-polar. It arises between atoms with practically equal electronegativity (0.4 > ΔEO > 0) and, consequently, a uniform distribution of electron density between the nuclei of atoms in homonuclear molecules. For example, H 2 , O 2 , N 2 , Cl 2 , etc. The dipole moment of such bonds is zero. The CH bond in saturated hydrocarbons (for example, in CH 4) is considered practically non-polar, because ΔEO = 2.5 (C) - 2.1 (H) = 0.4.
3.3.1.2 Covalent polar bond. If a molecule is formed by two different atoms, then the overlap zone of electron clouds (orbitals) shifts towards one of the atoms, and such a bond is called polar . With such a connection, the probability of finding electrons near the nucleus of one of the atoms is higher. For example, HCl, H 2 S, PH 3.
Polar (asymmetric) covalent bond - connection between atoms with different electronegativity (2 > ΔEO > 0.4) and asymmetric distribution of a common electron pair. As a rule, it is formed between two non-metals.
The electron density of such a bond is shifted towards a more electronegative atom, which leads to the appearance on it of a partial negative charge (delta minus), and on a less electronegative atom - a partial positive charge (delta plus)
C Cl C O C N O H C Mg .
The direction of electron displacement is also indicated by an arrow:
CCl, CO, CN, OH, CMg.
The greater the difference in the electronegativity of the bonded atoms, the higher the polarity of the bond and the greater its dipole moment. Additional forces of attraction act between partial charges of opposite sign. Therefore, the more polar the bond, the stronger it is.
Except polarizability covalent bond has the property satiety - the ability of an atom to form as many covalent bonds as it has energetically available atomic orbitals. The third property of a covalent bond is its orientation.
3.3.2 Ionic bond. The driving force behind its formation is the same aspiration of atoms to the octet shell. But in a number of cases, such an “octet” shell can arise only when electrons are transferred from one atom to another. Therefore, as a rule, an ionic bond is formed between a metal and a non-metal.
Consider as an example the reaction between sodium (3s 1) and fluorine (2s 2 3s 5) atoms. Electronegativity difference in NaF compound
EO = 4.0 - 0.93 = 3.07
Sodium, having donated its 3s 1 electron to fluorine, becomes the Na + ion and remains with a filled 2s 2 2p 6 shell, which corresponds to the electronic configuration of the neon atom. Exactly the same electronic configuration is acquired by fluorine, having accepted one electron donated by sodium. As a result, electrostatic attraction forces arise between oppositely charged ions.
Ionic bond - an extreme case of a polar covalent bond, based on the electrostatic attraction of ions. Such a bond occurs when there is a large difference in the electronegativity of the bonded atoms (EO > 2), when a less electronegative atom almost completely gives up its valence electrons and turns into a cation, and another, more electronegative atom, attaches these electrons and becomes an anion. The interaction of ions of the opposite sign does not depend on the direction, and the Coulomb forces do not have the property of saturation. Because of this ionic bond has no space focus And satiety , since each ion is associated with a certain number of counterions (coordination number of the ion). Therefore, ionically bound compounds do not have a molecular structure and are solid substances that form ionic crystal lattices, with high melting and boiling points, they are highly polar, often salt-like, and electrically conductive in aqueous solutions. For example, MgS, NaCl, A 2 O 3. Compounds with purely ionic bonds practically do not exist, since there is always a certain amount of covalence due to the fact that a complete transition of one electron to another atom is not observed; in the most "ionic" substances, the proportion of bond ionicity does not exceed 90%. For example, in NaF, the bond polarization is about 80%.
In organic compounds, ionic bonds are quite rare, because. a carbon atom tends to neither lose nor gain electrons to form ions.
Valence elements in compounds with ionic bonds very often characterize oxidation state , which, in turn, corresponds to the charge of the ion of the element in the given compound.
Oxidation state is the conditional charge that an atom acquires as a result of the redistribution of electron density. Quantitatively, it is characterized by the number of electrons displaced from a less electronegative element to a more electronegative one. A positively charged ion is formed from the element that gave up its electrons, and a negative ion is formed from the element that received these electrons.
The element in highest oxidation state (maximally positive), has already given up all its valence electrons in the ABD. And since their number is determined by the number of the group in which the element is located, then highest oxidation state for most elements and will be equal to group number . Concerning lowest oxidation state (maximally negative), then it appears during the formation of an eight-electron shell, that is, in the case when the AVZ is completely filled. For non-metals it is calculated according to the formula group number - 8 . For metals is equal to zero because they cannot accept electrons.
For example, the AVZ of sulfur has the form: 3s 2 3p 4 . If an atom gives up all the electrons (six), then it will acquire the highest oxidation state +6 equal to the group number VI , if it takes the two necessary to complete the stable shell, it will acquire the lowest oxidation state –2 equal to Group number - 8 \u003d 6 - 8 \u003d -2.
3.3.3 Metal bond. Most metals have a number of properties that are general in nature and differ from the properties of other substances. Such properties are relatively high melting points, the ability to reflect light, high thermal and electrical conductivity. These features are explained by the existence in metals of a special type of interaction – metallic connection.
In accordance with the position in the periodic system, metal atoms have a small number of valence electrons, which are rather weakly bound to their nuclei and can easily be detached from them. As a result, positively charged ions appear in the crystal lattice of the metal, localized in certain positions of the crystal lattice, and a large number of delocalized (free) electrons move relatively freely in the field of positive centers and carry out the connection between all metal atoms due to electrostatic attraction.
This is an important difference between metallic bonds and covalent bonds, which have a strict orientation in space. The bonding forces in metals are not localized and not directed, and the free electrons that form the "electron gas" cause high thermal and electrical conductivity. Therefore, in this case it is impossible to talk about the direction of the bonds, since the valence electrons are distributed almost uniformly over the crystal. This is precisely what explains, for example, the plasticity of metals, i.e., the possibility of displacement of ions and atoms in any direction
3.3.4 Donor-acceptor bond. In addition to the mechanism for the formation of a covalent bond, according to which a common electron pair arises from the interaction of two electrons, there is also a special donor-acceptor mechanism . It lies in the fact that a covalent bond is formed as a result of the transition of an already existing (lone) electron pair donor (electron supplier) for the general use of the donor and acceptor (supplier of a free atomic orbital).
After formation, it is no different from covalent. The donor-acceptor mechanism is well illustrated by the scheme for the formation of an ammonium ion (Figure 9) (asterisks indicate the electrons of the outer level of the nitrogen atom):
Figure 9 - Scheme of the formation of the ammonium ion
The electronic formula of the AVZ of the nitrogen atom is 2s 2 2p 3, that is, it has three unpaired electrons that enter into a covalent bond with three hydrogen atoms (1s 1), each of which has one valence electron. In this case, an ammonia molecule NH 3 is formed, in which the unshared electron pair of nitrogen is preserved. If a hydrogen proton (1s 0) that does not have electrons approaches this molecule, then nitrogen will transfer its pair of electrons (donor) to this hydrogen atomic orbital (acceptor), resulting in the formation of an ammonium ion. In it, each hydrogen atom is connected to the nitrogen atom by a common electron pair, one of which is realized by the donor-acceptor mechanism. It is important to note that the H-N bonds formed by various mechanisms do not have any differences in properties. This phenomenon is due to the fact that at the moment of bond formation, the orbitals of the 2s– and 2p– electrons of the nitrogen atom change their shape. As a result, four completely identical orbitals arise.
The donors are usually atoms with a large number of electrons, but with a small number of unpaired electrons. For elements of period II, in addition to the nitrogen atom, oxygen (two lone pairs) and fluorine (three lone pairs) have such a possibility. For example, the hydrogen ion H + in aqueous solutions is never in a free state, since the hydronium ion H 3 O + is always formed from water molecules H 2 O and the ion H +. The hydronium ion is present in all aqueous solutions, although for simplicity the symbol H + is retained in writing.
3.3.5 Hydrogen bond. A hydrogen atom bonded to a strongly electronegative element (nitrogen, oxygen, fluorine, etc.), which “pulls” a common electron pair onto itself, experiences a shortage of electrons and acquires an effective positive charge. Therefore, it is able to interact with the lone pair of electrons of another electronegative atom (which acquires an effective negative charge) of the same (intramolecular bond) or another molecule (intermolecular bond). As a result, there is hydrogen bond , which is graphically indicated by dots:
This bond is much weaker than other chemical bonds (the energy of its formation is 10 – 40 kJ/mol) and mainly has a partly electrostatic, partly donor-acceptor character.
The hydrogen bond plays an extremely important role in biological macromolecules, such inorganic compounds as H 2 O, H 2 F 2, NH 3. For example, O-H bonds in H 2 O have a noticeable polar character with an excess of negative charge – on the oxygen atom. The hydrogen atom, on the contrary, acquires a small positive charge + and can interact with lone pairs of electrons of the oxygen atom of the neighboring water molecule.
The interaction between water molecules turns out to be quite strong, such that even in water vapor there are dimers and trimers of the composition (H 2 O) 2, (H 2 O) 3, etc. In solutions, long chains of associates of this type can occur:
because the oxygen atom has two lone pairs of electrons.
The presence of hydrogen bonds explains the high boiling points of water, alcohols, carboxylic acids. Due to hydrogen bonds, water is characterized by such high melting and boiling points compared to H 2 E (E = S, Se, Te). If there were no hydrogen bonds, then water would melt at –100°C and boil at –80°C. Typical cases of association are observed for alcohols and organic acids.
Hydrogen bonds can occur both between different molecules and within a molecule if this molecule contains groups with donor and acceptor abilities. For example, it is intramolecular hydrogen bonds that play the main role in the formation of peptide chains that determine the structure of proteins. H-bonds affect the physical and chemical properties of a substance.
Hydrogen bonds do not form atoms of other elements , since the forces of electrostatic attraction of the opposite ends of the dipoles of polar bonds (О-Н, N-H, etc.) are rather weak and act only at short distances. Hydrogen, having the smallest atomic radius, allows such dipoles to approach each other so much that attractive forces become noticeable. No other element with a large atomic radius is capable of forming such bonds.
3.3.6 Forces of intermolecular interaction (van der Waals forces). In 1873, the Dutch scientist I. van der Waals suggested that there are forces that cause attraction between molecules. These forces were later called van der Waals forces. – the most versatile form of intermolecular bonding. The energy of the van der Waals bond is less than the hydrogen bond and is 2–20 kJ/∙mol.
Depending on the way the force is generated, they are divided into:
1) orientational (dipole-dipole or ion-dipole) - arise between polar molecules or between ions and polar molecules. When polar molecules approach each other, they are oriented in such a way that the positive side of one dipole is oriented towards the negative side of the other dipole (Figure 10).
|
Figure 10 - Orientation interaction |
||||
2) induction (dipole - induced dipole or ion - induced dipole) - arise between polar molecules or ions and non-polar molecules, but capable of polarization. Dipoles can act on non-polar molecules, turning them into indicated (induced) dipoles. (Figure 11).
|
Figure 11 - Inductive interaction |
||||
3) dispersive (induced dipole - induced dipole) - arise between non-polar molecules capable of polarization. In any molecule or atom of a noble gas, electric density fluctuations arise, as a result of which instantaneous dipoles appear, which in turn induce instantaneous dipoles in neighboring molecules. The movement of instantaneous dipoles becomes coordinated, their appearance and decay occur synchronously. As a result of the interaction of instantaneous dipoles, the energy of the system decreases (Figure 12).
|
Figure 12 - Dispersion interaction |
|||||||
Help with chemistry please. Indicate the type of bond in the molecules NH3, CaCl2, Al2O3, BaS... and got the best answer
Answer from Olga Lyabina[guru]
1) NH3 connection type cov. polar. three unpaired electrons of nitrogen and one each of hydrogen take part in the bond formation. there are no pi bonds. sp3 hybridization. The shape of the molecule is pyramidal (one orbital does not participate in hybridization, the tetrahedron turns into a pyramid)
CaCl2 bond type is ionic. two calcium electrons per s orbital participate in the formation of the bond, which accept two chlorine atoms, completing their third level. no pi bonds, sp hybridization type. they are located in space at an angle of 180 degrees
Al2O3 bond type is ionic. three electrons from the s and p orbitals of aluminum participate in the formation of the bond, which oxygen accepts, completing its second level. O=Al-O-Al=O. there are pi bonds between oxygen and aluminum. sp hybridization type most likely.
BaS bond type is ionic. Sulfur accepts two barium electrons. Ba=S is one pi bond. hybridization sp. Flat molecule.
2) AgNO3
silver is reduced at the cathode
K Ag+ + e = Ag
water oxidizes at the anode
A 2H2O - 4e \u003d O2 + 4H +
according to Faraday's law (whatever ...) the mass (volume) of the substance released at the cathode is proportional to the amount of electricity that has passed through the solution
m (Ag) \u003d Me / zF * I * t \u003d 32.23 g
V (O2) \u003d Ve / F * I * t \u003d 1.67 l
Answer from 2 answers[guru]
Hello! Here is a selection of topics with answers to your question: Help me solve chemistry, please. Indicate the type of bond in the molecules NH3, CaCl2, Al2O3, BaS...
.
You know that atoms can combine with each other to form both simple and complex substances. In this case, various types of chemical bonds are formed: ionic, covalent (non-polar and polar), metallic and hydrogen. One of the most essential properties of the atoms of elements, which determine what kind of bond is formed between them - ionic or covalent, - is the electronegativity, i.e. the ability of atoms in a compound to attract electrons to itself.
A conditional quantitative assessment of electronegativity is given by the scale of relative electronegativity.
In periods, there is a general tendency for the growth of the electronegativity of the elements, and in groups - their decline. Electronegativity elements are arranged in a row, on the basis of which it is possible to compare the electronegativity of elements in different periods.
The type of chemical bond depends on how large the difference in the electronegativity values of the connecting atoms of the elements is. The more the atoms of the elements forming the bond differ in electronegativity, the more polar the chemical bond is. It is impossible to draw a sharp boundary between the types of chemical bonds. In most compounds, the type of chemical bond is intermediate; for example, a highly polar covalent chemical bond is close to an ionic bond. Depending on which of the limiting cases is closer in nature to the chemical bond, it is referred to as either an ionic or a covalent polar bond.
Ionic bond.An ionic bond is formed by the interaction of atoms that differ sharply from each other in electronegativity. For example, typical metals lithium (Li), sodium (Na), potassium (K), calcium (Ca), strontium (Sr), barium (Ba) form an ionic bond with typical non-metals, mainly halogens.
In addition to alkali metal halides, ionic bonds are also formed in compounds such as alkalis and salts. For example, in sodium hydroxide (NaOH) and sodium sulfate (Na 2 SO 4), ionic bonds exist only between sodium and oxygen atoms (the rest of the bonds are covalent polar).
Covalent non-polar bond.When atoms interact with the same electronegativity, molecules are formed with a covalent non-polar bond. Such a bond exists in the molecules of the following simple substances: H 2 , F 2 , Cl 2 , O 2 , N 2 . Chemical bonds in these gases are formed through common electron pairs, i.e. when the corresponding electron clouds overlap, due to the electron-nuclear interaction, which occurs when the atoms approach each other.
When compiling the electronic formulas of substances, it should be remembered that each common electron pair is a conditional image of an increased electron density resulting from the overlap of the corresponding electron clouds.
covalent polar bond.During the interaction of atoms, the values of the electronegativity of which differ, but not sharply, there is a shift of the common electron pair to a more electronegative atom. This is the most common type of chemical bond found in both inorganic and organic compounds.
Covalent bonds fully include those bonds that are formed by the donor-acceptor mechanism, for example, in hydronium and ammonium ions.
Metal connection.
The bond that is formed as a result of the interaction of relatively free electrons with metal ions is called a metallic bond. This type of bond is typical for simple substances - metals.
The essence of the process of formation of a metallic bond is as follows: metal atoms easily give up valence electrons and turn into positively charged ions. Relatively free electrons, detached from the atom, move between positive metal ions. A metallic bond arises between them, i.e., the electrons, as it were, cement the positive ions of the crystal lattice of metals.
Hydrogen bond.
A bond that forms between the hydrogen atoms of one molecule and an atom of a strongly electronegative element(O, N, F) another molecule is called a hydrogen bond.
The question may arise: why exactly does hydrogen form such a specific chemical bond?
This is because the atomic radius of hydrogen is very small. In addition, when its single electron is displaced or completely donated, hydrogen acquires a relatively high positive charge, due to which the hydrogen of one molecule interacts with atoms of electronegative elements that have a partial negative charge that is part of other molecules (HF, H 2 O, NH 3).
Let's look at some examples. Usually we represent the composition of water with the chemical formula H 2 O. However, this is not entirely accurate. It would be more correct to denote the composition of water by the formula (H 2 O) n, where n \u003d 2.3.4, etc. This is due to the fact that individual water molecules are interconnected through hydrogen bonds.
Hydrogen bonds are usually denoted by dots. It is much weaker than an ionic or covalent bond, but stronger than the usual intermolecular interaction.
The presence of hydrogen bonds explains the increase in the volume of water with decreasing temperature. This is due to the fact that as the temperature decreases, the molecules become stronger and therefore the density of their “packing” decreases.
When studying organic chemistry, the following question also arose: why are the boiling points of alcohols much higher than those of the corresponding hydrocarbons? This is explained by the fact that hydrogen bonds are also formed between alcohol molecules.
An increase in the boiling point of alcohols also occurs due to the enlargement of their molecules.
The hydrogen bond is also characteristic of many other organic compounds (phenols, carboxylic acids, etc.). From the courses of organic chemistry and general biology, you know that the presence of a hydrogen bond explains the secondary structure of proteins, the structure of the DNA double helix, i.e., the phenomenon of complementarity.
DEFINITION
Ammonia- hydrogen nitride.
Formula - NH 3. Molar mass - 17 g / mol.
Physical properties of ammonia
Ammonia (NH 3) is a colorless gas with a pungent odor (the smell of "ammonia"), lighter than air, highly soluble in water (one volume of water will dissolve up to 700 volumes of ammonia). The concentrated ammonia solution contains 25% (mass) ammonia and has a density of 0.91 g/cm 3 .
The bonds between atoms in the ammonia molecule are covalent. General view of the AB 3 molecule. All valence orbitals of the nitrogen atom enter into hybridization, therefore, the type of hybridization of the ammonia molecule is sp 3. Ammonia has a geometric structure of the AB 3 E type - a trigonal pyramid (Fig. 1).
Rice. 1. The structure of the ammonia molecule.
Chemical properties of ammonia
Chemically, ammonia is quite active: it reacts with many substances. The degree of oxidation of nitrogen in ammonia "-3" is minimal, so ammonia exhibits only reducing properties.
When ammonia is heated with halogens, heavy metal oxides and oxygen, nitrogen is formed:
2NH 3 + 3Br 2 = N 2 + 6HBr
2NH 3 + 3CuO \u003d 3Cu + N 2 + 3H 2 O
4NH 3 + 3O 2 \u003d 2N 2 + 6H 2 O
In the presence of a catalyst, ammonia is able to oxidize to nitric oxide (II):
4NH 3 + 5O 2 \u003d 4NO + 6H 2 O (catalyst - platinum)
Unlike hydrogen compounds of non-metals of groups VI and VII, ammonia does not exhibit acidic properties. However, hydrogen atoms in its molecule are still capable of being replaced by metal atoms. With the complete replacement of hydrogen with a metal, the formation of compounds called nitrides occurs, which can also be obtained by direct interaction of nitrogen with a metal at high temperature.
The main properties of ammonia are due to the presence of a lone pair of electrons at the nitrogen atom. Ammonia solution in water is alkaline:
NH 3 + H 2 O ↔ NH 4 OH ↔ NH 4 + + OH -
When ammonia reacts with acids, ammonium salts are formed, which decompose when heated:
NH 3 + HCl = NH 4 Cl
NH 4 Cl \u003d NH 3 + HCl (when heated)
Getting ammonia
Allocate industrial and laboratory methods for producing ammonia. In the laboratory, ammonia is obtained by the action of alkalis on solutions of ammonium salts when heated:
NH 4 Cl + KOH \u003d NH 3 + KCl + H 2 O
NH 4 + + OH - = NH 3 + H 2 O
This reaction is qualitative for ammonium ions.
Application of ammonia
Ammonia production is one of the most important technological processes worldwide. About 100 million tons of ammonia are produced annually in the world. The release of ammonia is carried out in liquid form or in the form of a 25% aqueous solution - ammonia water. The main areas of ammonia use are the production of nitric acid (production of nitrogen-containing mineral fertilizers later), ammonium salts, urea, urotropine, synthetic fibers (nylon and capron). Ammonia is used as a refrigerant in industrial refrigeration, as a bleach in the cleaning and dyeing of cotton, wool and silk.
Examples of problem solving
EXAMPLE 1
| Exercise | What is the mass and volume of ammonia required to produce 5 tons of ammonium nitrate? |
| Solution | Let's write the reaction equation for obtaining ammonium nitrate from ammonia and nitric acid: NH 3 + HNO 3 \u003d NH 4 NO 3 According to the reaction equation, the amount of ammonium nitrate substance is 1 mol - v (NH 4 NO 3) \u003d 1 mol. Then, the mass of ammonium nitrate, calculated according to the reaction equation: m(NH 4 NO 3) = v(NH 4 NO 3)×M(NH 4 NO 3); m(NH 4 NO 3) \u003d 1 × 80 \u003d 80 t According to the reaction equation, the amount of ammonia substance is also 1 mol - v (NH 3) \u003d 1 mol. Then, the mass of ammonia, calculated by the equation: m (NH 3) \u003d v (NH 3) × M (NH 3); m (NH 3) \u003d 1 × 17 \u003d 17 t Let's make a proportion and find the mass of ammonia (practical): x g NH 3 - 5 t NH 4 NO 3 17 t NH 3 – 80 t NH 4 NO 3 x \u003d 17 × 5 / 80 \u003d 1.06 m (NH 3) \u003d 1.06 t We will compose a similar proportion to find the volume of ammonia: 1.06 g NH 3 - xl NH 3 17 t NH 3 - 22.4 × 10 3 m 3 NH 3 x \u003d 22.4 × 10 3 × 1.06 / 17 \u003d 1.4 × 10 3 V (NH 3) \u003d 1.4 × 10 3 m 3 |
| Answer | Ammonia mass - 1.06 tons, ammonia volume - 1.4 × 10 m |
