Types of crystal lattices. Crystal cell

The structure of matter is determined not only by the relative 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 is in crystals, where chemical particles are arranged in a certain order, forming in space crystal lattice.

Depending on what particles the crystal lattice is made of and what the nature of the chemical bond between them is, there are different types of crystal lattices:

· Nuclear

· Molecular

· Metal

· Ionic

Ionic crystal lattices are formed by ions - cations and anions. In nodes ionic lattice contains IONS - cations and anions, between which there is ELECTROSTATIC attraction.

This is a fairly durable type of grating.

‼ Characteristics of substances with an ionic crystal lattice:

· high melting points (refractoriness)– ionic compounds are always solid under normal conditions;

· solubility in water most ionic compounds;

· solutions and melts conduct electric current.

What substances have an IONIC lattice?

‼ An ionic lattice is characteristic of substances with an IONIC TYPE of bonding (salts, bases, metal oxides, other compounds containing metal and non-metal).

Atomic crystal lattices consist of individual atoms connected strong covalent bonds.

Graphite crystal

‼ Characteristics of substances with an atomic crystal lattice:

· atomic crystals are very strong and solid

· poorly conduct heat and electricity.

· melt at high temperatures.

· insoluble in any solvents.

· low reactivity.

What substances have an ATOMIC lattice?

‼ Substances with an atomic crystal lattice:

1) simple substances - boron, silicon, carbon (diamond and graphite).

2) silicon oxide (silica), silicon carbide (carborundum), as well as boron carbide and nitride.

Molecular crystal lattices consist of individual molecules, inside which atoms are connected by covalent bonds. Between molecules weaker intermolecular (van der Waals) forces operate. This is a very weak type of interaction.

Iodine molecule.

‼ Characteristics of substances with a molecular crystal lattice:

substances can be gaseous, liquid and solid

· low melting points

low lattice strength

· high volatility of substances

· do not have electrical conductivity

· their solutions and melts also do not conduct electric current.

What substances have a MOLECULAR lattice?

‼ Substances with a molecular lattice:

· simple diatomic substances - nonmetals

· non-metal compounds(except for boron and silicon oxides and carbides)

· all organic compounds, except salts.

A metal crystal lattice is characteristic of simple substances—metals. It takes place metal connection between atoms. At lattice nodes - metal cations; between them move socialized electrons (“electron gas”), which hold the metal cations, attracting them to themselves. The bonding in such crystals is delocalized and extends throughout the entire crystal.

In metal crystals, the nuclei of atoms are arranged in such a way that their packing is as dense as possible.

‼ Characteristics of substances with a metal crystal lattice:

· metallic luster and opacity

· malleability and ductility

Crystal lattices Type of lattice Types of particles at lattice sites Type of connection between particles Examples of substances Physical properties of substances Ionic IonicIonic These are binary compounds of metals (I A and II A), salts, oxides and hydroxides of typical metals. Solid, durable, non-volatile, brittle, refractory, many soluble in water, solutions and melts conduct electric current.

Crystal lattices Type of lattice Types of particles at lattice sites Type of bond between particles Examples of substances Physical properties of substances Atomic Atoms 1. Covalent non-polar - the bond is very strong 2. Covalent polar - the bond is strong Simple substances: diamond (C), graphite (C), boron ( B), silicon (Si), red phosphorus. Complex substances: aluminum oxide (Al 2 O 3), silicon oxide (IY) -SiO 2. Very hard, refractory, non-volatile, insoluble in water.

Crystal lattices Type of lattice Types of particles at lattice sites Type of bond between particles Examples of substances Physical properties of substances Molecular Molecules Between molecules there are weak forces of intermolecular attraction, and inside the molecules there is a strong covalent bond. Solids at low temperatures. With ordinary - gases or liquids - O 2, H 2, Cl 2, N 2, Br 2, H 2 O, CO 2, HCl, noble gases, sulfur, white phosphorus P 4, iodine; organic substances. Fragile, volatile, fusible, have low hardness.

Crystal lattices Type of lattice Types of particles at lattice sites Type of connection between particles Examples of substances Physical properties of substances Metallic Atoms, ions Metallic, of different strengths Metals and alloys. Metals that are in I A, II A, IIIA (except boron), and metals of the secondary subgroups are malleable, have a metallic luster, thermal and electrical conductivity

As we already know, a substance can exist in three states of aggregation: gaseous, hard And liquid. Oxygen, which under normal conditions is in a gaseous state, at a temperature of -194 ° C is transformed into a bluish liquid, and at a temperature of -218.8 ° C it turns into a snow-like mass with blue crystals.

The temperature range for the existence of a substance in the solid state is determined by the boiling and melting points. Solids are crystalline And amorphous.

U amorphous substances there is no fixed melting point - when heated, they gradually soften and turn into a fluid state. In this state, for example, various resins and plasticine are found.

Crystalline substances They are distinguished by the regular arrangement of the particles of which they consist: atoms, molecules and ions, at strictly defined points in space. When these points are connected by straight lines, a spatial framework is created, it is called a crystal lattice. The points at which crystal particles are located are called lattice nodes.

The nodes of the lattice we imagine can contain ions, atoms and molecules. These particles perform oscillatory movements. When the temperature increases, the range of these oscillations also increases, which leads to thermal expansion of bodies.

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 These are called crystal lattices in which ions are located at the nodes. They are formed by substances with ionic bonds, which can bind both simple ions Na+, Cl-, and complex SO24-, OH-. Thus, ionic crystal lattices have salts, some oxides and hydroxyls of metals, i.e. those substances in which an ionic chemical bond exists. Consider a sodium chloride crystal; it consists of positively alternating Na+ and negative CL- ions, together they form a cube-shaped lattice. The bonds between ions in such a crystal are extremely stable. Because of this, substances with an ionic lattice have relatively high strength and hardness; they are refractory and nonvolatile.

Atomic Crystal lattices are those crystal lattices whose nodes contain individual atoms. In such lattices, atoms are connected to each other by very strong covalent bonds. For example, diamond is one of the allotropic modifications of carbon.

Substances with an atomic crystal lattice are not very common in nature. These include crystalline boron, silicon and germanium, as well as complex substances, for example those containing silicon (IV) oxide - SiO 2: silica, quartz, sand, rock crystal.

The vast majority of substances with an atomic crystal lattice have very high melting points (for diamond it exceeds 3500 ° C), such substances are strong and hard, practically insoluble.

Molecular These are called crystal lattices in which molecules are located at the nodes. Chemical bonds in these molecules can also be polar (HCl, H 2 0) or non-polar (N 2, O 3). And although the atoms inside the molecules are connected by very strong covalent bonds, weak forces of intermolecular attraction act between the molecules themselves. That is why substances with molecular crystal lattices are characterized by low hardness, low melting point, and volatility.

Examples of such substances include solid water - ice, solid carbon monoxide (IV) - “dry ice”, solid hydrogen chloride and hydrogen sulfide, solid simple substances formed by one - (noble gases), two - (H 2, O 2, CL 2 , N 2 , I 2), three - (O 3), four - (P 4), eight-atomic (S 8) molecules. The vast majority of solid organic compounds have molecular crystal lattices (naphthalene, glucose, sugar).

blog.site, when copying material in full or in part, a link to the original source is required.

Solid crystals can be thought of as three-dimensional structures in which the same structure is clearly repeated in all directions. The geometrically correct shape of crystals is due to their strictly regular internal structure. If the centers of attraction of ions or molecules in a crystal are depicted as points, then we obtain a three-dimensional regular distribution of such points, which is called a crystal lattice, and the points themselves are nodes of the crystal lattice. The specific external shape of crystals is a consequence of their internal structure, which is associated specifically with the crystal lattice.

A crystal lattice is an imaginary geometric image for analyzing the structure of crystals, which is a volumetric-spatial network structure in the nodes of which atoms, ions or molecules of a substance are located.

To characterize the crystal lattice, the following parameters are used:

1. crystal lattice E cr [KJ/mol] is the energy released during the formation of 1 mole of a crystal from microparticles (atoms, molecules, ions) that are in a gaseous state and are separated from each other at such a distance that the possibility of their interaction is excluded.
2. Lattice constant d is the smallest distance between the centers of two particles at adjacent sites of the crystal lattice connected by .
3. Coordination number- the number of nearby particles surrounding the central particle in space and combining with it through a chemical bond.

The basis of the crystal lattice is the unit cell, which is repeated in the crystal an infinite number of times.

The unit cell is the smallest structural unit of a crystal lattice, which exhibits all the properties of its symmetry.

Simply put, a unit cell can be defined as a small part of a crystal lattice, which still reveals the characteristic features of its crystals. The characteristics of a unit cell are described using three Brevet rules:

• the symmetry of the unit cell must correspond to the symmetry of the crystal lattice;
• a unit cell must have the maximum number of identical edges A,b, With and equal angles between them a, b, g. ;
• provided that the first two rules are met, the unit cell must occupy a minimum volume.

To describe the shape of crystals, a system of three crystallographic axes is used a, b, c, which differ from ordinary coordinate axes in that they are segments of a certain length, the angles between which a, b, g can be either straight or indirect.

Crystal structure model: a) crystal lattice with a highlighted unit cell; b) unit cell with designations of facet angles

The shape of a crystal is studied by the science of geometric crystallography, one of the main provisions of which is the law of constancy of facet angles: for all crystals of a given substance, the angles between the corresponding faces always remain the same.

If you take a large number of elementary cells and fill a certain volume with them tightly to each other, maintaining the parallelism of the faces and edges, then a single crystal with an ideal structure is formed. But in practice, most often there are polycrystals in which regular structures exist within certain limits, along which the orientation of the regularity changes sharply.

Depending on the ratio of the lengths of the edges a, b, c and the angles a, b, g between the faces of the unit cell, seven systems are distinguished - the so-called crystal syngonies. However, an elementary cell can also be constructed in such a way that it has additional nodes that are located inside its volume or on all its faces - such lattices are called body-centered and face-centered, respectively. If the additional nodes are only on two opposite faces (top and bottom), then it is a base-centered lattice. Taking into account the possibility of additional nodes, there are a total of 14 types of crystal lattices.

The external shape and features of the internal structure of crystals are determined by the principle of dense “packing”: the most stable, and therefore the most probable structure will be the one that corresponds to the most dense arrangement of particles in the crystal and in which the smallest free space remains.

Types of crystal lattices

Depending on the nature of the particles contained in the nodes of the crystal lattice, as well as on the nature of the chemical bonds between them, there are four main types of crystal lattices.

Ionic lattices

Ionic lattices are constructed from unlike ions located at lattice sites and connected by forces of electrostatic attraction. Therefore, the structure of the ionic crystal lattice should ensure its electrical neutrality. Ions can be simple (Na +, Cl -) or complex (NH 4 +, NO 3 -). Due to the unsaturation and non-directionality of ionic bonds, ionic crystals are characterized by large coordination numbers. Thus, in NaCl crystals, the coordination numbers of Na + and Cl - ions are 6, and Cs + and Cl - ions in a CsCl crystal are 8, since one Cs + ion is surrounded by eight Cl - ions, and each Cl - ion is surrounded by eight Cs ions, respectively. + . Ionic crystal lattices are formed by a large number of salts, oxides and bases.

Examples of ionic crystal lattices: a) NaCl; b) CsCl

Substances with ionic crystal lattices have a relatively high hardness, they are quite refractory and non-volatile. In contrast, ionic compounds are very fragile, so even a small shift in the crystal lattice brings like-charged ions closer to each other, the repulsion between which leads to the breaking of ionic bonds and, as a result, to the appearance of cracks in the crystal or to its destruction. In the solid state, substances with an ionic crystal lattice are dielectrics and do not conduct electricity. However, when melted or dissolved in polar solvents, the geometrically correct orientation of the ions relative to each other is disrupted, chemical bonds are first weakened and then destroyed, and therefore the properties also change. As a consequence, both melts of ionic crystals and their solutions begin to conduct electric current.

Atomic lattices

These lattices are built from atoms connected to each other. They, in turn, are divided into three types: frame, layered and chain structures.

Frame structure has, for example, diamond - one of the hardest substances. Thanks to sp 3 hybridization of the carbon atom, a three-dimensional lattice is built, which consists exclusively of carbon atoms connected by covalent nonpolar bonds, the axes of which are located at the same bond angles (109.5 o).

Framework structure of the atomic crystal lattice of diamond

Layered structures can be considered as huge two-dimensional molecules. Layered structures are characterized by covalent bonds within each layer and weak van der Waals interactions between adjacent layers.

Layered structures of atomic crystal lattices: a) CuCl 2 ; b) PbO. Elementary cells are highlighted on the models using the outlines of parallelepipeds

A classic example of a substance with a layered structure is graphite, in which each carbon atom is in a state of sp 2 hybridization and forms three covalent s-bonds with three other C atoms in one plane. The fourth valence electrons of each carbon atom are unhybridized, due to which very weak van der Waals bonds between layers. Therefore, when even a small force is applied, the individual layers easily begin to slide along each other. This explains, for example, the ability of graphite to write. Unlike diamond, graphite conducts electricity well: under the influence of an electric field, non-localized electrons can move along the plane of the layers, and, conversely, graphite almost does not conduct electric current in the perpendicular direction.

Layered structure of the atomic crystal lattice of graphite

Chain structures characteristic, for example, of sulfur oxide (SO 3) n, cinnabar HgS, beryllium chloride BeCl 2, as well as many amorphous polymers and some silicate materials such as asbestos.

Chain structure of the atomic crystal lattice of HgS: a) side projection b) frontal projection

There are relatively few substances with the atomic structure of crystal lattices. These are, as a rule, simple substances formed by elements of the IIIA and IVA subgroups (Si, Ge, B, C). Often, compounds of two different nonmetals have atomic lattices, for example, some polymorphs of quartz (silicon oxide SiO 2) and carborundum (silicon carbide SiC).

All atomic crystals are distinguished by high strength, hardness, refractoriness and insolubility in almost any solvent. These properties are due to the strength of the covalent bond. Substances with an atomic crystal lattice have a wide range of electrical conductivity from insulators and semiconductors to electronic conductors.

Atomic crystal lattices of some polymorphic modifications of carborundum - silicon carbide SiC

Metal gratings

These crystal lattices contain metal atoms and ions at their nodes, between which electrons (electron gas) common to all of them move freely and form a metallic bond. A peculiarity of metal crystal lattices is their large coordination numbers (8-12), which indicate a significant packing density of metal atoms. This is explained by the fact that the “cores” of atoms, devoid of external electrons, are located in space like balls of the same radius. For metals, three types of crystal lattices are most often found: face-centered cubic with a coordination number of 12, body-centered cubic with a coordination number of 8, and hexagonal, close-packed with a coordination number of 12.

The special characteristics of metal bonds and metal lattices determine such important properties of metals as high melting points, electrical and thermal conductivity, malleability, ductility, and hardness.

Metal crystal lattices: a) body-centered cubic (Fe, V, Nb, Cr) b) face-centered cubic (Al, Ni, Ag, Cu, Au) c) hexagonal (Ti, Zn, Mg, Cd)

Molecular lattices

Molecular crystal lattices contain molecules at their nodes that are connected to each other by weak intermolecular forces—van der Waals or hydrogen bonds. For example, ice consists of molecules held in a crystal lattice by hydrogen bonds. The same type includes crystal lattices of many substances transferred to the solid state, for example: simple substances H 2, O 2, N 2, O 3, P 4, S 8, halogens (F 2, Cl 2, Br 2, I 2 ), “dry ice” CO 2, all noble gases and most organic compounds.

Molecular crystal lattices: a) iodine I2; b) ice H2O

Since the forces of intermolecular interaction are weaker than those of covalent or metallic bonds, molecular crystals have little hardness; They are fusible and volatile, insoluble in water and do not exhibit electrical conductivity.