Any interaction between atoms is possible only if there is a chemical bond. Such a connection is the reason for the formation of a stable polyatomic system - a molecular ion, molecule, crystal lattice. A strong chemical bond requires a lot of energy to break, which is why it is the basic quantity for measuring bond strength.
Conditions for the formation of a chemical bond
The formation of a chemical bond is always accompanied by the release of energy. This process occurs due to a decrease in the potential energy of a system of interacting particles - molecules, ions, atoms. The potential energy of the resulting system of interacting elements is always less than the energy of unbound outgoing particles. Thus, the basis for the emergence of a chemical bond in a system is the decrease in the potential energy of its elements.
Nature of chemical interaction
A chemical bond is a consequence of the interaction of electromagnetic fields that arise around the electrons and atomic nuclei of those substances that take part in the formation of a new molecule or crystal. After the discovery of the theory of atomic structure, the nature of this interaction became more accessible to study.
For the first time, the idea of the electrical nature of a chemical bond arose from the English physicist G. Davy, who suggested that molecules are formed due to the electrical attraction of oppositely charged particles. This idea interested the Swedish chemist and natural scientist I.Ya. Bercellius, who developed the electrochemical theory of the occurrence of chemical bonds.
The first theory, which explained the processes of chemical interaction of substances, was imperfect, and over time it had to be abandoned.
Butlerov's theory
A more successful attempt to explain the nature of the chemical bond of substances was made by the Russian scientist A.M. Butlerov. This scientist based his theory on the following assumptions:
- Atoms in the bonded state are connected to each other in a certain order. A change in this order causes the formation of a new substance.
- Atoms bond with each other according to the laws of valence.
- The properties of a substance depend on the order of connection of atoms in the molecule of the substance. A different arrangement causes a change in the chemical properties of the substance.
- Atoms connected to each other most strongly influence each other.
Butlerov's theory explained the properties of chemical substances not only by their composition, but also by the order of arrangement of atoms. This internal order of A.M. Butlerov called it “chemical structure”.
The theory of the Russian scientist made it possible to restore order in the classification of substances and provided the opportunity to determine the structure of molecules by their chemical properties. The theory also answered the question: why molecules containing the same number of atoms have different chemical properties.
Prerequisites for the creation of theories of chemical bonding
In his theory of chemical structure, Butlerov did not touch upon the question of what a chemical bond is. To do this, there was too little data on the internal structure of matter. Only after the discovery of the planetary model of the atom, the American scientist Lewis began to develop the hypothesis that a chemical bond arises through the formation of an electron pair that simultaneously belongs to two atoms. Subsequently, this idea became the foundation for the development of the theory of covalent bonds.
Covalent chemical bond
A stable chemical compound can be formed when the electron clouds of two neighboring atoms overlap. The result of such mutual intersection is an increasing electron density in the internuclear space. The nuclei of atoms, as we know, are positively charged, and therefore try to be drawn as close as possible to the negatively charged electron cloud. This attraction is much stronger than the repulsive forces between two positively charged nuclei, so this connection is stable.
Chemical bond calculations were first performed by chemists Heitler and London. They examined the bond between two hydrogen atoms. The simplest visual representation of it might look like this:
As you can see, the electron pair occupies a quantum place in both hydrogen atoms. This two-center arrangement of electrons is called a “covalent chemical bond.” Covalent bonds are typical of molecules of simple substances and their non-metal compounds. Substances created by covalent bonds usually do not conduct electricity or are semiconductors.
Ionic bond
An ionic chemical bond occurs when two oppositely charged ions attract each other. Ions can be simple, consisting of one atom of a substance. In compounds of this type, simple ions are most often positively charged metal atoms of groups 1 and 2 that have lost their electron. The formation of negative ions is inherent in the atoms of typical nonmetals and their acid bases. Therefore, among the typical ionic compounds there are many alkali metal halides, such as CsF, NaCl, and others.
Unlike a covalent bond, an ion is not saturated: a varying number of oppositely charged ions can join an ion or group of ions. The number of attached particles is limited only by the linear dimensions of the interacting ions, as well as the condition under which the attractive forces of oppositely charged ions must be greater than the repulsive forces of equally charged particles participating in the ionic type compound.
Hydrogen bond
Even before the creation of the theory of chemical structure, it was experimentally noticed that hydrogen compounds with various non-metals have somewhat unusual properties. For example, the boiling points of hydrogen fluoride and water are much higher than might be expected.
These and other features of hydrogen compounds can be explained by the ability of the H+ atom to form another chemical bond. This type of connection is called a “hydrogen bond.” The reasons for the occurrence of a hydrogen bond lie in the properties of electrostatic forces. For example, in a hydrogen fluoride molecule, the total electron cloud is so shifted towards fluorine that the space around an atom of this substance is saturated with a negative electric field. Around a hydrogen atom, deprived of its only electron, the field is much weaker and has a positive charge. As a result, an additional relationship arises between the positive fields of electron clouds H + and negative F - .
Chemical bond of metals
The atoms of all metals are located in space in a certain way. The arrangement of metal atoms is called a crystal lattice. In this case, electrons of different atoms weakly interact with each other, forming a common electron cloud. This type of interaction between atoms and electrons is called a “metallic bond.”
It is the free movement of electrons in metals that can explain the physical properties of metallic substances: electrical conductivity, thermal conductivity, strength, fusibility and others.
It is one of the cornerstones of an interesting science called chemistry. In this article we will analyze all aspects of chemical bonds, their importance in science, give examples and much more.
What is a chemical bond
In chemistry, a chemical bond is understood as the mutual adhesion of atoms in a molecule and, as a result of the force of attraction that exists between. It is thanks to chemical bonds that various chemical compounds are formed; this is the nature of a chemical bond.
Types of Chemical Bonds
The mechanism of formation of a chemical bond strongly depends on its type or type; in general, the following main types of chemical bonds differ:
- Covalent chemical bond (which in turn can be polar or non-polar)
- Ionic bond
- Chemical bond
like people.
As for, a separate article is devoted to it on our website, and you can read in more detail at the link. Next, we will examine in more detail all the other main types of chemical bonds.
Ionic chemical bond
The formation of an ionic chemical bond occurs due to the mutual electrical attraction of two ions having different charges. Ions in such chemical bonds are usually simple, consisting of one atom of the substance.
Scheme of ionic chemical bond.
A characteristic feature of the ionic type of chemical bond is its lack of saturation, and as a result, a very different number of oppositely charged ions can join an ion or even a whole group of ions. An example of an ionic chemical bond is the cesium fluoride compound CsF, in which the level of “ionicity” is almost 97%.
Hydrogen chemical bond
Long before the advent of the modern theory of chemical bonds in its modern form, chemists noticed that hydrogen compounds with non-metals have various amazing properties. Let's say the boiling point of water and together with hydrogen fluoride is much higher than it could be, here is a ready-made example of a hydrogen chemical bond.
The picture shows a diagram of the formation of a hydrogen chemical bond.
The nature and properties of a hydrogen chemical bond are determined by the ability of the hydrogen atom H to form another chemical bond, hence the name of this bond. The reason for the formation of such a connection is the properties of electrostatic forces. For example, the total electron cloud in a hydrogen fluoride molecule is so shifted towards fluorine that the space around an atom of this substance is saturated with a negative electric field. Around a hydrogen atom, especially one deprived of its only electron, everything is exactly the opposite; its electronic field is much weaker and, as a result, has a positive charge. And positive and negative charges, as you know, attract, and in this simple way a hydrogen bond arises.
Chemical bond of metals
What chemical bond is characteristic of metals? These substances have their own type of chemical bond - the atoms of all metals are not arranged anyhow, but in a certain way, the order of their arrangement is called a crystal lattice. Electrons of different atoms form a common electron cloud, and they weakly interact with each other.
This is what a metal chemical bond looks like.
An example of a metallic chemical bond can be any metal: sodium, iron, zinc, and so on.
How to determine the type of chemical bond
Depending on the substances taking part in it, if there is a metal and a non-metal, then the bond is ionic, if there are two metals, then it is metallic, if there are two non-metals, then it is covalent.
Properties of chemical bonds
To compare different chemical reactions, different quantitative characteristics are used, such as:
- length,
- energy,
- polarity,
- order of connections.
Let's look at them in more detail.
Bond length is the equilibrium distance between the nuclei of atoms that are connected by a chemical bond. Usually measured experimentally.
The energy of a chemical bond determines its strength. In this case, energy refers to the force required to break a chemical bond and separate atoms.
The polarity of a chemical bond shows how much electron density is shifted towards one of the atoms. The ability of atoms to shift electron density toward themselves, or in simple terms “to pull the blanket over themselves,” is called electronegativity in chemistry.
The order of a chemical bond (in other words, the multiplicity of a chemical bond) is the number of electron pairs that enter into a chemical bond. The order can be either whole or fractional; the higher it is, the greater the number of electrons that carry out the chemical bond and the more difficult it is to break it.
Chemical bond, video
And finally, an educational video about different types of chemical bonds.
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.
Atoms of most elements do not exist separately, as they can interact with each other. This interaction produces more complex particles.
The nature of a chemical bond is the action of electrostatic forces, which are the forces of interaction between electric charges. Electrons and atomic nuclei have such charges.
Electrons located on the outer electronic levels (valence electrons), being farthest from the nucleus, interact with it weakest, and therefore are able to break away from the nucleus. They are responsible for bonding atoms to each other.
Types of interactions in chemistry
Types of chemical bonds can be presented in the following table:
Characteristics of ionic bonding
Chemical reaction that occurs due to ion attraction having different charges is called ionic. This happens if the atoms being bonded have a significant difference in electronegativity (that is, the ability to attract electrons) and the electron pair goes to the more electronegative element. The result of this transfer of electrons from one atom to another is the formation of charged particles - ions. An attraction arises between them.
They have the lowest electronegativity indices typical metals, and the largest are typical non-metals. Ions are thus formed by the interaction between typical metals and typical nonmetals.
Metal atoms become positively charged ions (cations), donating electrons to their outer electron levels, and nonmetals accept electrons, thus turning into negatively charged ions (anions).
Atoms move into a more stable energy state, completing their electronic configurations.
The ionic bond is non-directional and non-saturable, since the electrostatic interaction occurs in all directions; accordingly, the ion can attract ions of the opposite sign in all directions.
The arrangement of the ions is such that around each there is a certain number of oppositely charged ions. The concept of "molecule" for ionic compounds doesn't make sense.
Examples of education
The formation of a bond in sodium chloride (nacl) is due to the transfer of an electron from the Na atom to the Cl atom to form the corresponding ions:
Na 0 - 1 e = Na + (cation)
Cl 0 + 1 e = Cl - (anion)
In sodium chloride, there are six chlorine anions around the sodium cations, and six sodium ions around each chloride ion.
When interaction is formed between atoms in barium sulfide, the following processes occur:
Ba 0 - 2 e = Ba 2+
S 0 + 2 e = S 2-
Ba donates its two electrons to sulfur, resulting in the formation of sulfur anions S 2- and barium cations Ba 2+.
Metal chemical bond
The number of electrons in the outer energy levels of metals is small; they are easily separated from the nucleus. As a result of this detachment, metal ions and free electrons are formed. These electrons are called "electron gas". Electrons move freely throughout the volume of the metal and are constantly bound and separated from atoms.
The structure of the metal substance is as follows: the crystal lattice is the skeleton of the substance, and between its nodes electrons can move freely.
The following examples can be given:
Mg - 2е<->Mg 2+
Cs-e<->Cs+
Ca - 2e<->Ca2+
Fe-3e<->Fe 3+
Covalent: polar and non-polar
The most common type of chemical interaction is a covalent bond. The electronegativity values of the elements that interact do not differ sharply; therefore, only a shift of the common electron pair to a more electronegative atom occurs.
Covalent interactions can be formed by an exchange mechanism or a donor-acceptor mechanism.
The exchange mechanism is realized if each of the atoms has unpaired electrons on the outer electronic levels and the overlap of atomic orbitals leads to the appearance of a pair of electrons that already belongs to both atoms. When one of the atoms has a pair of electrons on the outer electronic level, and the other has a free orbital, then when the atomic orbitals overlap, the electron pair is shared and interacts according to the donor-acceptor mechanism.
Covalent ones are divided by multiplicity into:
- simple or single;
- double;
- triples.
Double ones ensure the sharing of two pairs of electrons at once, and triple ones - three.
According to the distribution of electron density (polarity) between bonded atoms, a covalent bond is divided into:
- non-polar;
- polar.
A nonpolar bond is formed by identical atoms, and a polar bond is formed by different electronegativity.
The interaction of atoms with similar electronegativity is called a nonpolar bond. The common pair of electrons in such a molecule is not attracted to either atom, but belongs equally to both.
The interaction of elements differing in electronegativity leads to the formation of polar bonds. In this type of interaction, shared electron pairs are attracted to the more electronegative element, but are not completely transferred to it (that is, the formation of ions does not occur). As a result of this shift in electron density, partial charges appear on the atoms: the more electronegative one has a negative charge, and the less electronegative one has a positive charge.
Properties and characteristics of covalency
Main characteristics of a covalent bond:
- The length is determined by the distance between the nuclei of interacting atoms.
- Polarity is determined by the displacement of the electron cloud towards one of the atoms.
- Directionality is the property of forming bonds oriented in space and, accordingly, molecules having certain geometric shapes.
- Saturation is determined by the ability to form a limited number of bonds.
- Polarizability is determined by the ability to change polarity under the influence of an external electric field.
- The energy required to break a bond determines its strength.
An example of a covalent nonpolar interaction can be the molecules of hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2) and many others.
H· + ·H → H-H molecule has a single non-polar bond,
O: + :O → O=O molecule has a double nonpolar,
Ṅ: + Ṅ: → N≡N the molecule is triple nonpolar.
Examples of covalent bonds of chemical elements include molecules of carbon dioxide (CO2) and carbon monoxide (CO), hydrogen sulfide (H2S), hydrochloric acid (HCL), water (H2O), methane (CH4), sulfur oxide (SO2) and many others .
In the CO2 molecule, the relationship between carbon and oxygen atoms is covalent polar, since the more electronegative hydrogen attracts electron density. Oxygen has two unpaired electrons in its outer shell, while carbon can provide four valence electrons to form the interaction. As a result, double bonds are formed and the molecule looks like this: O=C=O.
In order to determine the type of bond in a particular molecule, it is enough to consider its constituent atoms. Simple metal substances form a metallic bond, metals with nonmetals form an ionic bond, simple nonmetal substances form a covalent nonpolar bond, and molecules consisting of different nonmetals form through a polar covalent bond.
Covalent chemical bond, its varieties and mechanisms of formation. Characteristics of covalent bonds (polarity and bond energy). Ionic bond. Metal connection. Hydrogen bond
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.
The division of chemical bonds into types is conditional, since they are all characterized by a certain unity.
An ionic bond can be considered as an extreme case of a polar covalent bond.
A metallic bond combines the covalent interaction of atoms using shared electrons and the electrostatic attraction between these electrons and metal ions.
Substances often lack limiting cases of chemical bonding (or pure chemical bonding).
For example, lithium fluoride $LiF$ is classified as an ionic compound. In fact, the bond in it is $80%$ ionic and $20%$ covalent. It is therefore more correct, obviously, to talk about the degree of polarity (ionicity) of a chemical bond.
In the series of hydrogen halides $HF—HCl—HBr—HI—HAt$, the degree of bond polarity decreases, because the difference in the electronegativity values of the halogen and hydrogen atoms decreases, and in astatine hydrogen the bond becomes almost nonpolar $(EO(H) = 2.1; EO(At) = 2.2)$.
Different types of bonds can be found in the same substances, for example:
- 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;
- 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;
- in ammonium, methylammonium salts, etc.: between nitrogen and hydrogen atoms - covalent polar, and between ammonium or methylammonium ions and the acid residue - ionic;
- in metal peroxides (for example, $Na_2O_2$), the bond between oxygen atoms is covalent nonpolar, and between the metal and oxygen is ionic, etc.
Different types of connections can transform into one another:
— during electrolytic dissociation of covalent compounds in water, the covalent polar bond becomes ionic;
- when metals evaporate, the metal bond turns into a nonpolar covalent bond, 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 covalent bonds. Characteristics of a covalent bond: bond length and energy
A covalent chemical bond is a bond formed between atoms through the formation of shared electron pairs.
The mechanism of formation of such a bond can be exchange or donor-acceptor.
I. Exchange mechanism operates when atoms form shared electron pairs by combining unpaired electrons.
1) $H_2$ - hydrogen:
The bond arises due to the formation of a common electron pair by $s$-electrons of hydrogen atoms (overlapping $s$-orbitals):
2) $HCl$ - hydrogen chloride:
The bond arises 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 a nitrogen molecule three common electron pairs are formed between the atoms:
II. Donor-acceptor mechanism Let us consider the formation of a covalent bond using the example of the ammonium ion $NH_4^+$.
The donor has an electron pair, the acceptor has an empty 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 - according to the donor-acceptor mechanism.
Covalent bonds can be 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 $σ$ -bonds (sigma bonds). The sigma bond is very strong.
$p-$orbitals can overlap in two regions, forming a covalent bond due to lateral overlap:
Chemical bonds formed as a result of “lateral” overlap of electron orbitals outside the communication line, i.e. in two areas are called $π$ -bonds (pi-bonds).
By degree of displacement shared electron pairs to one of the atoms they bond, a covalent bond can be polar And non-polar.
A covalent chemical bond formed between atoms with the same electronegativity is called non-polar. Electron pairs are not shifted to any of the atoms, because atoms have the same EO - the property of attracting valence electrons from other atoms. For example:
those. molecules of simple non-metal substances are formed through covalent non-polar bonds. A covalent chemical bond between atoms of elements whose electronegativity differs is called polar.
Length and energy of covalent bonds.
Characteristic properties of covalent bond- its length and energy. Link length is the distance between the nuclei of atoms. The shorter the length of a chemical bond, the stronger it is. However, a measure of the strength of the connection is binding 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 $H_2, Cl_2$ and $N_2$ molecules are respectively $0.074, 0.198$ and $0.109$ nm, and the bond energies are respectively $436, 242$ and $946$ kJ/mol.
Ions. Ionic bond
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.
The chemical bond that occurs between ions is called ionic.
Let's consider the formation of this bond using the example of the well-known compound sodium chloride (table salt):
The process of converting atoms into ions is depicted in the diagram:
This transformation of atoms into ions always occurs during the interaction of atoms of typical metals and typical non-metals.
Let's consider the algorithm (sequence) of reasoning when recording the formation of an ionic bond, for example, between calcium and chlorine atoms:
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 indexes.
Metal connection
Let's get acquainted with how atoms of metal elements interact with each other. Metals usually do not exist as isolated atoms, but in the form of a piece, ingot, or metal product. What holds metal atoms in a single volume?
The atoms of most metals contain a small number of electrons at the outer level - $1, 2, 3$. These electrons are easily stripped off and the atoms become 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. 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 figure schematically shows the structure of a sodium metal fragment.
In this case, a small number of shared electrons bind a large number of ions and atoms.
A metallic bond has some similarities with a 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.
Hydrogen bond
A chemical bond between positively polarized hydrogen atoms of one molecule (or part thereof) and negatively polarized atoms of strongly electronegative elements having lone electron pairs ($F, O, N$ and less commonly $S$ and $Cl$) of another molecule (or its part) is called hydrogen.
The mechanism of hydrogen bond formation is partly electrostatic, partly donor-acceptor in nature.
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).
Substances with hydrogen bonds have molecular crystal lattices.
Substances of molecular and non-molecular structure. Type of crystal lattice. Dependence of the properties of substances on their composition and structure
Molecular and non-molecular structure of substances
It is not individual atoms or molecules that enter into chemical interactions, but substances. Under given conditions, a substance can be in one of three states of aggregation: solid, liquid or gaseous. The properties of a substance also depend on the nature of the chemical bond between the particles that form it - molecules, atoms or ions. Based on the type of bond, substances of molecular and non-molecular structure are distinguished.
Substances made up of molecules are called molecular substances. The bonds between the molecules in such substances are very weak, much weaker than between the atoms inside the molecule, and even at relatively low temperatures they break - the substance turns into a liquid and then into a gas (sublimation of iodine). The melting and boiling points of substances consisting of molecules increase with increasing molecular weight.
Molecular substances include substances with an atomic structure ($C, Si, Li, Na, K, Cu, Fe, W$), among them there are metals and non-metals.
Let's consider the physical properties of alkali metals. The relatively low bond strength between atoms causes low mechanical strength: alkali metals are soft and can be easily cut with a knife.
Large atomic sizes lead to low densities of alkali metals: lithium, sodium and potassium are even lighter than water. In the group of alkali metals, the boiling and melting points decrease with increasing atomic number of the element, because Atom sizes increase and bonds weaken.
To substances non-molecular structures include ionic compounds. Most compounds of metals with nonmetals have this structure: all salts ($NaCl, K_2SO_4$), some hydrides ($LiH$) and oxides ($CaO, MgO, FeO$), bases ($NaOH, KOH$). Ionic (non-molecular) substances have high melting and boiling points.
Crystal lattices
Matter, as is known, can exist in three states of aggregation: gaseous, liquid and solid.
Solids: amorphous and crystalline.
Let us consider how the characteristics of chemical bonds influence the properties of solids. Solids are divided into crystalline And amorphous.
Amorphous substances do not have a clear melting point; when heated, they gradually soften and turn into a fluid state. For example, plasticine and various resins are in an amorphous state.
Crystalline substances are characterized by the correct arrangement of the particles of which they are composed: atoms, molecules and ions - at strictly defined points in space. When these points are connected by straight lines, a spatial framework is formed, called a crystal lattice. The points at which crystal particles are located are called lattice nodes.
Depending on the type of particles located at the nodes of the crystal lattice and the nature of the connection between them, four types of crystal lattices are distinguished: ionic, atomic, molecular And metal.
Ionic crystal lattices.
Ionic are called crystal lattices, in the nodes of which there are ions. They are formed by substances with ionic bonds, which can bind both simple ions $Na^(+), Cl^(-)$, and complex $SO_4^(2−), OH^-$. Consequently, salts and some oxides and hydroxides of metals have ionic crystal lattices. For example, a sodium chloride crystal consists of alternating positive $Na^+$ and negative $Cl^-$ ions, forming a cube-shaped lattice. The bonds between ions in such a crystal are very stable. Therefore, substances with an ionic lattice are characterized by relatively high hardness and strength, they are refractory and non-volatile.
Atomic crystal lattices.
Atomic are called crystal lattices, in the nodes of which there are individual atoms. In such lattices, the atoms are connected to each other by very strong covalent bonds. An example of substances with this type of crystal lattices is diamond, one of the allotropic modifications of carbon.
Most substances with an atomic crystal lattice have very high melting points (for example, for diamond it is above $3500°C), they are strong and hard, and practically insoluble.
Molecular crystal lattices.
Molecular called crystal lattices, in the nodes of which molecules are located. Chemical bonds in these molecules can be both polar ($HCl, H_2O$) and nonpolar ($N_2, O_2$). Despite the fact that the atoms inside the molecules are connected by very strong covalent bonds, weak intermolecular forces of attraction act between the molecules themselves. Therefore, substances with molecular crystal lattices have low hardness, low melting points, and are volatile. Most solid organic compounds have molecular crystal lattices (naphthalene, glucose, sugar).
Metal crystal lattices.
Substances with metallic bonds have metallic crystal lattices. At the sites of such lattices there are atoms and ions (either atoms or ions, into which metal atoms easily turn, giving up their outer electrons “for common use”). This internal structure of metals determines their characteristic physical properties: malleability, plasticity, electrical and thermal conductivity, characteristic metallic luster.
