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Crystal lattice na. How to determine the type of crystal lattice




Let's talk about solids. Solids can be divided into two large groups: amorphous and crystalline. We will separate them according to the principle whether there is order or not.

AT amorphous substances molecules are arranged randomly. There are no regularities in their spatial arrangement. In fact, amorphous substances are very viscous liquids, so viscous that they are solid.

Hence the name: “a-” is a negative particle, “morphe” is a form. Amorphous substances include: glasses, resins, wax, paraffin, soap.

The lack of order in the arrangement of particles determines the physical properties of amorphous bodies: they do not have fixed melting points. As they heat up, their viscosity gradually decreases, and they also gradually become liquid.

In contrast to amorphous substances, there are crystalline ones. Particles of a crystalline substance are spatially ordered. This is the correct structure of the spatial arrangement of particles in a crystalline substance is called crystal lattice.

Unlike amorphous bodies, crystalline substances have fixed melting points.

Depending on which particles are in lattice nodes, and from what bonds hold them distinguish: molecular, nuclear, ionic and metal gratings.

Why is it fundamentally important to know what the crystal lattice of a substance is? What does she define? All. Structure defines how chemical and physical properties of matter.

The simplest example is DNA. In all organisms on earth, it is built from the same set of structural components: four types of nucleotides. And what a variety of life. It's all determined by structure: the order in which these nucleotides are arranged.

Molecular crystal lattice.

A typical example is water in the solid state (ice). The lattice sites contain whole molecules. And hold them together intermolecular interactions: hydrogen bonds, van der Waals forces.

These bonds are weak, so the molecular lattice is the most fragile, the melting point of such substances is low.

A good diagnostic sign: if a substance has a liquid or gaseous state under normal conditions and / or has an odor, then most likely this substance has a molecular crystal lattice. After all, the liquid and gaseous states are a consequence of the fact that the molecules on the surface of the crystal do not hold well (the bonds are weak). And they are "blown away". This property is called volatility. And the deflated molecules, diffusing in the air, reach our olfactory organs, which is subjectively felt as a smell.

The molecular crystal lattice has:

  1. Some simple substances of non-metals: I 2, P, S (that is, all non-metals that do not have an atomic lattice).
  2. Almost all organic matter ( except for salts).
  3. And as mentioned earlier, substances under normal conditions are liquid or gaseous (being frozen) and / or having an odor (NH 3, O 2, H 2 O, acids, CO 2).

Atomic crystal lattice.

In the nodes of the atomic crystal lattice, in contrast to the molecular one, there are individual atoms. It turns out that covalent bonds hold the lattice (after all, they bind neutral atoms).

A classic example is the standard of hardness strength - diamond (by chemical nature, it is a simple substance carbon). Connections: covalent non-polar, since only carbon atoms form the lattice.

But, for example, in a quartz crystal (whose chemical formula is SiO 2) there are Si and O atoms. Therefore, bonds covalent polar.

Physical properties of substances with an atomic crystal lattice:

  1. strength, hardness
  2. high melting points (refractory)
  3. non-volatile substances
  4. insoluble (neither in water nor in other solvents)

All these properties are due to the strength of covalent bonds.

There are few substances in the atomic crystal lattice. There is no special pattern, so you just need to remember them:

  1. Allotropic modifications of carbon (C): diamond, graphite.
  2. Boron (B), silicon (Si), germanium (Ge).
  3. Only two allotropic modifications of phosphorus have an atomic crystal lattice: red phosphorus and black phosphorus. (White phosphorus has a molecular crystal lattice).
  4. SiC - carborundum (silicon carbide).
  5. BN is boron nitride.
  6. Silica, rock crystal, quartz, river sand - all these substances have the composition SiO 2.
  7. Corundum, ruby, sapphire - these substances have the composition Al 2 O 3.

Surely the question arises: C is both diamond and graphite. But they are completely different: graphite is opaque, stains, conducts electric current, and diamond is transparent, does not stain and does not conduct current. They differ in structure.

And then, and then - the atomic lattice, but different. Therefore, the properties are different.

Ionic crystal lattice.

A classic example: table salt: NaCl. At the nodes of the lattice are individual ions: Na+ and Cl–. Holds the lattice electrostatic forces of attraction between ions ("plus" is attracted to "minus"), that is ionic bond.

Ionic crystal lattices are quite strong, but brittle, the melting points of such substances are quite high (higher than that of representatives of a metal one, but lower than that of substances with an atomic lattice). Many are water soluble.

As a rule, there are no problems with the definition of the ionic crystal lattice: where there is an ionic bond, there is an ionic crystal lattice. It: all salts, metal oxides, alkalis(and other basic hydroxides).

Metallic crystal lattice.

The metal grating is realized in simple substances metals. Earlier we said that all the splendor of the metallic bond can only be understood together with the metallic crystal lattice. The hour has come.

The main property of metals: electrons on outer energy level poorly held, so they are easily given. Having lost an electron, the metal turns into a positively charged ion - a cation:

Na 0 – 1e → Na +

In a metal crystal lattice, processes of recoil and electron attachment are constantly taking place: an electron is detached from a metal atom at one lattice site. A cation is formed. The detached electron is attracted by another cation (or the same one): a neutral atom is formed again.

The nodes of the metal crystal lattice contain both neutral atoms and metal cations. And free electrons travel between nodes:

These free electrons are called electron gas. It is they that determine the physical properties of simple substances of metals:

  1. thermal and electrical conductivity
  2. metallic luster
  3. malleability, plasticity

This is a metallic bond: metal cations are attracted to neutral atoms and all this is “glued together” by free electrons.

How to determine the type of crystal lattice.

P.S. There is something in the school curriculum and the USE program on this topic that we do not quite agree with. Namely: a generalization that any metal-nonmetal bond is an ionic bond. This assumption is deliberately made, apparently to simplify the program. But this leads to distortion. The boundary between ionic and covalent bonds is conditional. Each bond has its own percentage of "ionic" and "covalent". The bond with a low-active metal has a small percentage of "ionicity", it is more like a covalent one. But according to the USE program, it is "rounded" towards the ionic one. It gives rise to sometimes absurd things. For example, Al 2 O 3 is a substance with an atomic crystal lattice. What kind of ionicity are we talking about here. Only a covalent bond can hold atoms in this way. But according to the "metal-non-metal" standard, we qualify this bond as ionic. And it turns out a contradiction: the lattice is atomic, and the bond is ionic. This is what oversimplification leads to.

Crystal cell- a system of points located at equal, parallel oriented vertices and parallelepipeds adjacent along the faces without gaps, filling the space of points, called nodes, straight lines - rows, planes - grids, parallelepipeds are called elementary cells.

Types of crystal lattices: atomic - if atoms are located at the nodes, ionic - if ions are located at the nodes, molecular - if molecules are located at the nodes

2. Properties of crystalline substances - uniformity, anisotropy, the ability to self-cut.

Uniformity- two identical elementary volumes of matter oriented in parallel in space, but isolated at different points of the substance, are absolutely identical in properties (beryl - tourmaline).

Anisotropy- in different directions of the crystal lattice in non-parallel directions, many properties (eg, strength, hardness, refractive index) are different.

The ability to self-limit- the property of crystals during free growth to form correctly faceted polyhedra.

Permanence property of dihedral knots– the m/y angles of the corresponding faces and edges in all crystals of the same substance are the same.

3. The concept of syngony. What categories are syngonies divided into.

Syngony - a set of types of symmetries that has 1 or more common symmetry elements, with an equal number of unit directions. S. to. is characterized by the relationship between the axes a, b, c and the corners of the cell.

There are 7 divided into:

Inferior( do not have axes of symmetry higher than the second order)

middle ( they have one axis of symmetry of higher order)

Single destinations are directions that are not repeated in crystals.

Being the largest classificatory subdivision in the symmetry of crystals, each crystal structure includes several point symmetry groups and Bravais lattices.

4. Simple shapes and combinations. The physical meaning of the selection of simple forms in a crystal.

In appearance, crystals are divided into simple shapes and combinations. simple shapes- crystals obtained from one face by the action of a symmetry element on it.

Elements of symmetry:

    geometric image

    plane of symmetry- a plane perpendicular to the image, dividing the figure into 2 parts, correlating as an object and its mirror image.

    Axis of symmetry- this is a straight line perpendicular to the image, when rotated around it by 360 about, the figure is combined with itself n times.

    Center of symmetry- a point inside the crystal characterized by the fact that each line drawn through it meets identical points on both sides at the same distance.

Combinations- crystals consisting of faces of various types, differing in shape and size. Formed by a combination of two or more simple forms. How many types of faces on a uniformly developed crystal are so many simple forms in it.

The selection of faces of different types has physical meaning , since different faces grow at different rates and have different properties (hardness, density, refractive index).

Simple forms are open and closed. A closed simple form with the help of faces of the same type independently closes the space (tetragonal dipyramid), an open simple form can close the space only in combination with other simple forms (tetragonal pyramid + plane.) There are 47 simple forms in total. All of them are divided into categories:

A monohedron is a simple shape represented by a single face.

Pinacoid - two equal parallel faces that can be reversed.

Dihedron - two equal intersecting faces (can intersect on their continuation).

Rhombic prism - four equal pairwise parallel faces; form a rhombus in cross section.

A rhombic pyramid has four equal intersecting faces; also form a rhombus in cross section. The listed simple forms are open, since they do not close the space. The presence in the crystal of open simple forms, such as a rhombic prism, necessarily causes the presence of other simple forms, such as a pinacoid or a rhombic dipyramid, necessary to obtain a closed form.

Of the closed simple forms of the lower syngonies, we note the following. Rhombic dipyramid two rhombic pyramids folded at the bases; the shape has eight different faces, giving a rhombus in cross section; A rhombic tetrahedron has four faces that enclose space and are shaped like oblique triangles.

    Middle category(systems: triclinic, tetragonal, hexagonal) - 27 p.f.: monohedron, pinocoid, 6 dipyramids, 6 pyramids, 6 prisms, tetrahedron, rhombohedron, 3 trapezoids (faces in the shape of a trapezoid), 2 scalenoids (formed by doubling the faces of a tetrahedron and rhombohedron).

    Top category- 15 p.f.: the main ones are the tetrahedron, octahedron, cube. If instead of one face 3 faces appear - tritetrahedron, if 6 - hexatetrahedron, if 4 - tetratetrahedron. The faces can be 3x, 4x, 5-angled: 3x - trigon, 4x - tetragon, 5 - pentagon.

A simple form of a crystal is a family of faces interconnected by symmetrical operations of a given class of symmetry. All faces forming one simple crystal shape must be equal in size and shape. One or more simple forms may be present in a crystal. The combination of several simple forms is called a combination.

Closed are called such forms, the faces of which completely close the space enclosed between them, such as, for example, a cube;

Open simple forms do not close the space and cannot exist independently, but only in combinations. For example, prism + pinacoid.

Fig.6. Simple forms of the lowest category: monohedron (1), pinacoid (2), dihedron (3).
In the lower syngonies, the following open simple forms are possible (Fig. 6):

 Monohedron (from the Greek "mono" - one, "hedra" - face) - a simple form, represented by one single face. A monohedron is, for example, the base of a pyramid.

 Pinacoid (from the Greek "pinax" - board) - a simple shape consisting of two equal parallel faces, often reversely oriented.

 Dihedron (from the Greek "di" - two, "hedron" - face) - a simple shape formed by two equal intersecting (sometimes on its continuation) faces, forming a "straight roof".

• Rhombic prism - a simple form that consists of four equal, pairwise parallel faces that form a rhombus in cross section.

• Rhombic pyramid - a simple shape consists of four equal intersecting faces; in cross section is also a rhombus. Of the closed simple forms of the lower syngonies, we note the following:

 Rhombic dipyramid two rhombic pyramids folded at the bases. The shape has eight equal faces, giving a rhombus in cross section.

Rhombic tetrahedron - a simple shape, the four faces of which are in the form of oblique triangles and close the space.

The open simple forms of syngonies of the middle category will be prisms and pyramids.

 Trigonal prism (from the Greek "gon" - angle) - three equal faces intersecting along parallel edges and forming an equilateral triangle in cross section;

 Tetragonal prism (from the Greek "tetra" - four) - four equal pairwise parallel faces forming a square in cross section;

 Hexagonal prism (from the Greek "hexa" - six) - six equal faces intersecting along parallel edges and forming a regular hexagon in cross section.

The names ditrigonal, ditetragonal and dihexagonal were given to prisms with a double number of faces, when all faces are equal, and the same angles between the faces alternate through one.

Pyramids - simple forms of crystals of the middle category can be, like prisms, trigonal (and ditrigonal), tetragonal (and ditrigonal), hexagonal (and dihexagonal). They form regular polygons in cross section. The faces of the pyramids are located at an oblique angle to the axis of symmetry of the highest order.

In crystals of the middle category, there are also closed simple forms. There are several such forms:

 Dipyramids - simple shapes formed by two equal pyramids, folded bases. In such forms, the pyramid is doubling with a horizontal plane of symmetry perpendicular to the main axis of symmetry of a higher order (Fig. 8). Dipyramids, like simple pyramids, depending on the order of the axis, can have different cross-sectional shapes. They can be trigonal, ditrigonal, tetragonal, ditetragonal, hexagonal, and dihexagonal.

• Rhombohedron - a simple shape that consists of six rhombus-shaped faces and resembles an elongated or diagonally flattened cube. It is possible only in trigonal syngony. The upper and lower group of faces are rotated relative to each other by an angle of 60o so that the lower faces are located symmetrically between the upper ones.

Most solids are crystalline. Crystal cell is built from repeating identical structural units, individual for each crystal. This structural unit is called the "elementary cell". In other words, the crystal lattice serves as a reflection of the spatial structure of a solid.

Crystal lattices can be classified in various ways.

I. According to the symmetry of crystals lattices are classified into cubic, tetragonal, rhombic, hexagonal.

This classification is convenient for estimating the optical properties of crystals, as well as their catalytic activity.

II. By the nature of the particles located at the lattice nodes and by type of chemical bond distinguish between them atomic, molecular, ionic and metallic crystal lattices. The type of bond in a crystal determines the difference in hardness, solubility in water, the magnitude of the heat of dissolution and heat of fusion, and electrical conductivity.

An important characteristic of a crystal is crystal lattice energy, kJ/mol – the energy required to destroy a given crystal.

molecular lattice

molecular crystals consist of molecules held in certain positions of the crystal lattice by weak intermolecular bonds (van der Waals forces) or hydrogen bonds. These lattices are characteristic of substances with covalent bonds.

There are a lot of substances with a molecular lattice. These are a large number of organic compounds (sugar, naphthalene, etc.), crystalline water (ice), solid carbon dioxide (“dry ice”), solid hydrogen halides, iodine, solid gases, including noble ones,

The minimum energy of the crystal lattice for substances with non-polar and low-polar molecules (CH 4, CO 2, etc.).

Lattices formed by more polar molecules also have a higher crystal lattice energy. Lattices with substances that form hydrogen bonds (H 2 O, NH 3) have the highest energy.

Due to the weak interaction between molecules, these substances are volatile, fusible, have low hardness, do not conduct electric current (dielectrics) and have low thermal conductivity.

atomic lattice

in knots atomic crystal lattice there are atoms of one or different elements linked by covalent bonds along all three axes. Such crystals, which are also called covalent are relatively few.

Examples of crystals of this type are diamond, silicon, germanium, tin, as well as crystals of complex substances such as boron nitride, aluminum nitride, quartz, silicon carbide. All these substances have a diamond-like lattice.

The energy of the crystal lattice in such substances practically coincides with the energy of the chemical bond (200 - 500 kJ/mol). This also determines their physical properties: high hardness, melting point and boiling point.

The electrically conductive properties of these crystals are varied: diamond, quartz, boron nitride are dielectrics; silicon, germanium - semiconductors; metallic gray tin conducts electricity well.

In crystals with an atomic crystal lattice, it is impossible to single out a separate structural unit. The entire single crystal is one giant molecule.

Ionic lattice

in knots ionic lattice positive and negative ions alternate, between which electrostatic forces act. Ionic crystals form compounds with ionic bonds, for example, sodium chloride NaCl, potassium fluoride and KF, etc. Ionic compounds can also include complex ions, for example, NO 3 - , SO 4 2 - .

Ionic crystals are also a giant molecule in which each ion is strongly affected by all the other ions.

The energy of the ionic crystal lattice can reach significant values. So, E (NaCl) \u003d 770 kJ / mol, and E (BeO) \u003d 4530 kJ / mol.

Ionic crystals have high melting and boiling points and high strength, but are brittle. Many of them are poor conductors of electric current at room temperature (about twenty orders of magnitude lower than those of metals), but with increasing temperature, an increase in electrical conductivity is observed.

metal grate

metal crystals give examples of the simplest crystal structures.

Metal ions in the lattice of a metal crystal can be approximately considered as spheres. In solid metals, these balls are packed with maximum density, as indicated by the significant density of most metals (from 0.97 g/cm3 for sodium, 8.92 g/cm3 for copper, to 19.30 g/cm3 for tungsten and gold). ). The densest packing of balls in one layer is the hexagonal packing, in which each ball is surrounded by six other balls (in the same plane). The centers of any three adjacent balls form an equilateral triangle.

Such properties of metals as high ductility and ductility indicate the absence of rigidity in metal lattices: their planes rather easily shift one relative to the other.

Valence electrons participate in the formation of bonds with all atoms, move freely throughout the entire volume of a piece of metal. This is indicated by high values ​​of electrical conductivity and thermal conductivity.

According to the energy of the crystal lattice, metals occupy an intermediate position between molecular and covalent crystals. The energy of the crystal lattice is:

Thus, the physical properties of solids essentially depend on the type of chemical bond and structure.

Structure and properties of solids

Characteristics crystals
metal Ionic Molecular Nuclear
Examples K, Al, Cr, Fe NaCl, KNO3 I 2, naphthalene diamond, quartz
Structural particles Positive ions and mobile electrons Cations and anions molecules atoms
Type of chemical bond metal Ionic In molecules - covalent; between molecules - van der Waals forces and hydrogen bonds Between atoms - covalent
melting temperature High High low Very high
boiling temperature High High low Very high
Mechanical properties Hard, malleable, malleable hard, brittle Soft Very hard
Electrical conductivity Good conductors In solid form - dielectrics; in melt or solution - conductors Dielectrics Dielectrics (except graphite)
Solubility
in water insoluble Soluble insoluble insoluble
in non-polar solvents insoluble insoluble Soluble insoluble

(All definitions, formulas, graphs and equations of reactions are given under the record.)

A crystal is a body whose particles (atoms, ions, molecules) are arranged not in a chaotic, but in a strictly defined order. This order is periodically repeated, forming, as it were, an imaginary "lattice". It is generally accepted that there are four types of crystal lattices: metallic, ionic, atomic and molecular. And how can you determine what type of crystal lattice a particular substance has?

Instruction

As you can easily guess from the name itself, the metallic type of lattice is found in metals. These substances are characterized, as a rule, by a high melting point, metallic luster, hardness, and are good conductors of electric current. Remember that at the sites of this type of lattice there are either neutral atoms or positively charged ions. In the gaps between the nodes there are electrons, the migration of which ensures the high electrical conductivity of such substances.

Ionic type of crystal lattice. It should be remembered that it is inherent in oxides and salts. A typical example is the crystals of the well-known table salt, sodium chloride. At the nodes of such lattices, positively and negatively charged ions alternate alternately. Such substances, as a rule, are refractory, with low volatility. As you might guess, they have an ionic type of chemical bond.

The atomic type of the crystal lattice is inherent in simple substances - non-metals, which under normal conditions are solids. For example, sulfur, phosphorus, carbon. At the sites of such lattices there are neutral atoms bound to each other by a covalent chemical bond. Such substances are characterized by infusibility, insolubility in water. Some (for example, carbon in the form of diamond) have exceptionally high hardness.

Finally, the last type of lattice is molecular. It occurs in substances that are under normal conditions in liquid or gaseous form. As again, it is easy to understand from the name, at the nodes of such lattices there are molecules. They can be either non-polar (for simple gases such as Cl2, O2) or polar (the most famous example is water H2O). Substances with this type of lattice do not conduct current, are volatile, and have low melting points.

Thus, in order to determine with certainty what type of crystal lattice a particular substance has, you should figure out what class of substances it belongs to and what physicochemical properties it has.


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The structure of matter is determined not only by the mutual arrangement of atoms in chemical particles, but also by the location of these chemical particles in space. The most ordered arrangement of atoms, molecules and ions in crystals(from Greek " crystallos"- ice), where chemical particles (atoms, molecules, ions) are arranged in a certain order, forming a crystal lattice in space. Under certain conditions of formation, they can have the natural shape of regular symmetrical polyhedra. The crystalline state is characterized by the presence of a long-range order in the arrangement of particles and symmetry crystal lattice.

The amorphous state is characterized by the presence of only short-range order. The structures of amorphous substances resemble liquids, but they have much less fluidity. The amorphous state is usually unstable. Under the action of mechanical loads or when the temperature changes, amorphous bodies can crystallize. The reactivity of substances in the amorphous state is much higher than in the crystalline state.

Amorphous substances

main feature amorphous(from Greek " amorphos"- formless) state of matter - the absence of an atomic or molecular lattice, that is, a three-dimensional periodicity of the structure characteristic of the crystalline state.

When a liquid substance is cooled, it does not always crystallize. under certain conditions, a non-equilibrium solid amorphous (glassy) state can form. The glassy state can contain simple substances (carbon, phosphorus, arsenic, sulfur, selenium), oxides (for example, boron, silicon, phosphorus), halides, chalcogenides, and many organic polymers.

In this state, the substance can be stable for a long period of time, for example, the age of some volcanic glasses is estimated at millions of years. The physical and chemical properties of a substance in a glassy amorphous state can differ significantly from the properties of a crystalline substance. For example, glassy germanium dioxide is chemically more active than crystalline. Differences in the properties of the liquid and solid amorphous state are determined by the nature of the thermal motion of particles: in the amorphous state, particles are only capable of oscillatory and rotational motions, but cannot move in the thickness of the substance.

There are substances that in solid form can only be in an amorphous state. This applies to polymers with an irregular sequence of units.

Amorphous bodies isotropic, that is, their mechanical, optical, electrical and other properties do not depend on direction. Amorphous bodies do not have a fixed melting point: melting occurs in a certain temperature range. The transition of an amorphous substance from a solid to a liquid state is not accompanied by an abrupt change in properties. A physical model of the amorphous state has not yet been created.

Crystalline substances

Solid crystals- three-dimensional formations characterized by strict repetition of the same element of the structure ( elementary cell) in all directions. The unit cell is the smallest volume of a crystal in the form of a parallelepiped, repeated in the crystal an infinite number of times.

The geometrically correct shape of crystals is primarily due to their strictly regular internal structure. If, instead of atoms, ions or molecules in a crystal, we represent points as the centers of gravity of these particles, then we get a three-dimensional regular distribution of such points, called the crystal lattice. The points themselves are called knots crystal lattice.

Types of crystal lattices

Depending on which particles the crystal lattice is built from and what is the nature of the chemical bond between them, different types of crystals are distinguished.

Ionic crystals are formed by cations and anions (for example, salts and hydroxides of most metals). They have an ionic bond between the particles.

Ionic crystals can be monatomic ions. This is how crystals are built sodium chloride, potassium iodide, calcium fluoride.
In the formation of ionic crystals of many salts, monatomic metal cations and polyatomic anions, for example, NO 3 - nitrate ion, SO 4 2 - sulfate ion, CO 3 2 - carbonate ion, participate in the formation of ionic crystals.

In an ionic crystal, it is impossible to isolate single molecules. Each cation is attracted to each anion and repelled by other cations. The whole crystal can be considered a huge molecule. The size of such a molecule is not limited, since it can grow by adding new cations and anions.

Most ionic compounds crystallize according to one of the structural types, which differ from each other in the value of the coordination number, that is, the number of neighbors around a given ion (4, 6 or 8). For ionic compounds with an equal number of cations and anions, four main types of crystal lattices are known: sodium chloride (the coordination number of both ions is 6), cesium chloride (the coordination number of both ions is 8), sphalerite and wurtzite (both structural types are characterized by the coordination number of the cation and anion equal to 4). If the number of cations is half the number of anions, then the coordination number of cations must be twice the coordination number of anions. In this case, the structural types of fluorite (coordination numbers 8 and 4), rutile (coordination numbers 6 and 3), and cristobalite (coordination numbers 4 and 2) are realized.

Typically, ionic crystals are hard but brittle. Their brittleness is due to the fact that even with a slight deformation of the crystal, cations and anions are displaced in such a way that the repulsive forces between like ions begin to prevail over the forces of attraction between cations and anions, and the crystal is destroyed.

Ionic crystals have high melting points. In the molten state, the substances that form ionic crystals are electrically conductive. When dissolved in water, these substances dissociate into cations and anions, and the resulting solutions conduct an electric current.

High solubility in polar solvents, accompanied by electrolytic dissociation, is due to the fact that in a solvent medium with a high dielectric constant ε, the attraction energy between ions decreases. The dielectric constant of water is 82 times higher than that of vacuum (conditionally existing in an ionic crystal), the attraction between ions in an aqueous solution decreases by the same amount. The effect is enhanced by the solvation of ions.

Atomic crystals are made up of individual atoms held together by covalent bonds. Of the simple substances, only boron and the elements of the IVA group have such crystal lattices. Often, compounds of non-metals with each other (for example, silicon dioxide) also form atomic crystals.

Just like ionic crystals, atomic crystals can be considered giant molecules. They are very strong and hard and do not conduct heat and electricity well. Substances that have atomic crystal lattices melt at high temperatures. They are practically insoluble in any solvents. They are characterized by low reactivity.

Molecular crystals are built from individual molecules, within which the atoms are connected by covalent bonds. Weaker intermolecular forces act between molecules. They are easily destroyed, so molecular crystals have low melting points, low hardness, and high volatility. Substances that form molecular crystal lattices do not have electrical conductivity, their solutions and melts also do not conduct electric current.

Intermolecular forces arise due to the electrostatic interaction of negatively charged electrons of one molecule with positively charged nuclei of neighboring molecules. The strength of intermolecular interaction is influenced by many factors. The most important among them is the presence of polar bonds, that is, the shift of electron density from one atom to another. In addition, intermolecular interaction is more pronounced between molecules with a large number of electrons.

Most non-metals in the form of simple substances (for example, iodine I 2 , argon Ar, sulfur S 8) and compounds with each other (for example, water, carbon dioxide, hydrogen chloride), as well as almost all organic solids form molecular crystals.

Metals have a metallic crystal lattice. It has a metallic bond between atoms. In metal crystals, the nuclei of atoms are arranged in such a way that their packing is as dense as possible. The bond in such crystals is delocalized and extends to the entire crystal. Metal crystals have high electrical and thermal conductivity, metallic luster and opacity, and easy deformability.

The classification of crystal lattices corresponds to limiting cases. Most crystals of inorganic substances belong to intermediate types - covalent-ionic, molecular-covalent, etc. For example, in a crystal graphite inside each layer, the bonds are covalent-metal, and between the layers - intermolecular.

Isomorphism and polymorphism

Many crystalline substances have the same structures. At the same time, the same substance can form different crystal structures. This is reflected in the phenomena isomorphism and polymorphism.

isomorphism is the ability of atoms, ions or molecules to replace each other in crystal structures. This term (from Greek " isos" - equal and " morphe"- form) was proposed by E. Mitscherlich in 1819. The law of isomorphism would be formulated by E. Mitscherlich in 1821 in this way: "The same number of atoms, connected in the same way, give the same crystalline forms; in this case, the crystalline form does not depend on the chemical nature of the atoms, but is determined only by their number and relative position.

While working in the chemical laboratory of the University of Berlin, Mitscherlich drew attention to the complete similarity of the crystals of lead, barium and strontium sulfates and the proximity of the crystalline forms of many other substances. His observations attracted the attention of the famous Swedish chemist J.-J. Berzelius, who suggested that Micherlich confirm the observed patterns using the example of compounds of phosphoric and arsenic acids. As a result of the study, it was concluded that "the two series of salts differ only in that one contains arsenic as an acid radical, and the other - phosphorus." Mitscherlich's discovery very soon attracted the attention of mineralogists, who began research on the problem of isomorphic substitution of elements in minerals.

In the case of joint crystallization of substances prone to isomorphism ( isomorphic substances), mixed crystals (isomorphic mixtures) are formed. This is possible only if the particles replacing each other differ little in size (no more than 15%). In addition, isomorphic substances must have a similar spatial arrangement of atoms or ions and, therefore, crystals similar in external form. Such substances include, for example, alum. In crystals of potassium alum KAl (SO 4) 2 . 12H 2 O potassium cations can be partially or completely replaced by rubidium or ammonium cations, and aluminum cations by chromium(III) or iron(III) cations.

Isomorphism is widespread in nature. Most minerals are isomorphic mixtures of complex variable composition. For example, in the mineral sphalerite ZnS, up to 20% of zinc atoms can be replaced by iron atoms (in this case, ZnS and FeS have different crystal structures). Isomorphism is associated with the geochemical behavior of rare and trace elements, their distribution in rocks and ores, where they are contained in the form of isomorphic impurities.

Isomorphic substitution determines many useful properties of artificial materials of modern technology - semiconductors, ferromagnets, laser materials.

Many substances can form crystalline forms that have different structures and properties, but the same composition ( polymorphic modifications). Polymorphism- the ability of solids and liquid crystals to exist in two or more forms with different crystal structures and properties with the same chemical composition. This word comes from the Greek polymorphos"- diverse. The phenomenon of polymorphism was discovered by M. Klaproth, who in 1798 discovered that two different minerals - calcite and aragonite - have the same chemical composition of CaCO 3.

Polymorphism of simple substances is usually called allotropy, while the concept of polymorphism does not apply to non-crystalline allotropic forms (for example, gaseous O 2 and O 3). A typical example of polymorphic forms is carbon modifications (diamond, lonsdaleite, graphite, carbines and fullerenes), which differ sharply in properties. The most stable form of existence of carbon is graphite, however, its other modifications under normal conditions can be preserved for an arbitrarily long time. At high temperatures, they turn into graphite. In the case of diamond, this occurs when heated above 1000° C. in the absence of oxygen. The reverse transition is much more difficult. Not only a high temperature (1200-1600 o C) is necessary, but also a gigantic pressure - up to 100 thousand atmospheres. The transformation of graphite into diamond is easier in the presence of molten metals (iron, cobalt, chromium, and others).

In the case of molecular crystals, polymorphism manifests itself in a different packing of molecules in a crystal or in a change in the shape of molecules, and in ionic crystals, in a different mutual arrangement of cations and anions. Some simple and complex substances have more than two polymorphs. For example, silicon dioxide has ten modifications, calcium fluoride has six, and ammonium nitrate has four. Polymorphic modifications are usually denoted by Greek letters α, β, γ, δ, ε, ... starting from modifications that are stable at low temperatures.

During crystallization from a vapor, solution or melt of a substance that has several polymorphic modifications, a modification is first formed that is less stable under the given conditions, which then turns into a more stable one. For example, when phosphorus vapor condenses, white phosphorus is formed, which under normal conditions slowly, and when heated, quickly turns into red phosphorus. When lead hydroxide is dehydrated, at first (about 70 o C) yellow β-PbO, which is less stable at low temperatures, is formed, at about 100 o C it turns into red α-PbO, and at 540 o C - again into β-PbO.

The transition of one polymorphic modification to another is called polymorphic transformations. These transitions occur with a change in temperature or pressure and are accompanied by an abrupt change in properties.

The process of transition from one modification to another can be reversible or irreversible. So, when a white soft graphite-like substance of composition BN (boron nitride) is heated at 1500-1800 o C and a pressure of several tens of atmospheres, its high-temperature modification is formed - borazon, close to diamond in hardness. When the temperature and pressure are lowered to values ​​corresponding to ordinary conditions, the borazone retains its structure. An example of a reversible transition is the mutual transformations of two sulfur modifications (rhombic and monoclinic) at 95 o C.

Polymorphic transformations can also take place without a significant change in the structure. Sometimes there is no change in the crystal structure at all, for example, during the transition of α-Fe to β-Fe at 769 o C, the structure of iron does not change, but its ferromagnetic properties disappear.