Give a definition of the concept of a crystal lattice. School encyclopedia

It is not individual atoms or molecules that enter into chemical interactions, but substances.

Our task is to get acquainted with the structure of matter.

At low temperatures, substances are in a stable solid state.

☼ The hardest substance in nature is diamond. He is considered the king of all gems and precious stones. And its name itself means “indestructible” in Greek. Diamonds have long been looked upon as miraculous stones. It was believed that a person wearing diamonds does not know stomach diseases, is not affected by poison, retains his memory and a cheerful mood until old age, and enjoys royal favor.

☼ A diamond that has been subjected to jewelry processing - cutting, polishing - is called a diamond.

When melting as a result of thermal vibrations, the order of the particles is disrupted, they become mobile, while the nature of the chemical bond is not disrupted. Thus, there are no fundamental differences between solid and liquid states.

The liquid acquires fluidity (i.e., the ability to take the shape of a vessel).

Liquid crystals

Liquid crystals were discovered at the end of the 19th century, but have been studied in the last 20-25 years. Many display devices of modern technology, for example, some electronic watches and mini-computers, operate on liquid crystals.

In general, the words “liquid crystals” sound no less unusual than “hot ice”. However, in reality, ice can also be hot, because... at a pressure of more than 10,000 atm. water ice melts at temperatures above 200 0 C. The unusualness of the combination “liquid crystals” is that the liquid state indicates the mobility of the structure, and the crystal implies strict ordering.

If a substance consists of polyatomic molecules of an elongated or lamellar shape and having an asymmetrical structure, then when it melts, these molecules are oriented in a certain way relative to each other (their long axes are parallel). In this case, the molecules can move freely parallel to themselves, i.e. the system acquires the property of fluidity characteristic of a liquid. At the same time, the system retains an ordered structure, which determines the properties characteristic of crystals.

The high mobility of such a structure makes it possible to control it through very weak influences (thermal, electrical, etc.), i.e. purposefully change the properties of a substance, including optical ones, with very little energy expenditure, which is what is used in modern technology.

Types of crystal lattices

Any chemical substance is formed by a large number of identical particles that are interconnected.

At low temperatures, when thermal movement is difficult, the particles are strictly oriented in space and form crystal lattice.

Crystal cell - This structure with a geometrically correct arrangement of particles in space.

In the crystal lattice itself, nodes and internodal space are distinguished.

The same substance depending on the conditions (p, t,...)exists in various crystalline forms (i.e. they have different crystal lattices) - allotropic modifications that differ in properties.

For example, four modifications of carbon are known: graphite, diamond, carbyne and lonsdaleite.

☼ The fourth variety of crystalline carbon, “lonsdaleite,” is little known. It was discovered in meteorites and obtained artificially, and its structure is still being studied.

☼ Soot, coke, and charcoal were classified as amorphous carbon polymers. However, it has now become known that these are also crystalline substances.

☼ By the way, shiny black particles were found in the soot, which were called “mirror carbon.” Mirror carbon is chemically inert, heat-resistant, impervious to gases and liquids, has a smooth surface and is absolutely compatible with living tissues.

☼ The name graphite comes from the Italian “graffito” - I write, I draw. Graphite is a dark gray crystal with a weak metallic luster and has a layered lattice. Individual layers of atoms in a graphite crystal, connected to each other relatively weakly, are easily separated from each other.

TYPES OF CRYSTAL LATTICES

	ionic			metal
What is in the nodes of the crystal lattice, structural unit	ions	atoms	molecules	atoms and cations
Type of chemical bond between particles of the node	ionic		covalent: polar and non-polar	metal
Interaction forces between crystal particles	electrostatic logical	covalent	intermolecular- new	electrostatic logical
Physical properties due to the crystal lattice	· the attractive forces between ions are strong, · T pl. (refractory), · easily dissolves in water, · melt and solution conducts electric current,	non-volatile (no odor) · covalent bonds between atoms are large, · T pl. · the melt does not conduct electric current	· the forces of attraction between molecules are small, · T pl. ↓, some are soluble in water, · have a volatile odor	· interaction forces are large, · T pl. , High heat and electrical conductivity
Aggregate state of a substance under normal conditions	hard	hard	hard, gaseous liquid	hard, liquid(N g)
Examples	most salts, alkalis, oxides of typical metals	C (diamond, graphite), Si, Ge, B, SiO 2, CaC 2, SiC (carborundum), BN, Fe 3 C, TaC (t pl. =3800 0 C) Red and black phosphorus. Oxides of some metals.	all gases, liquids, most non-metals: inert gases, halogens, H 2, N 2, O 2, O 3, P 4 (white), S 8. Hydrogen compounds of non-metals, oxides of non-metals: H 2 O, CO 2 "dry ice". Most organic compounds.	Metals, alloys

If the rate of crystal growth is low upon cooling, a glassy state (amorphous) is formed.

1. The relationship between the position of an element in the Periodic Table and the crystal lattice of its simple substance.

There is a close relationship between the position of an element in the periodic table and the crystal lattice of its corresponding elemental substance.

		group
				III				VII	VIII
P e R And O d								H 2
						N 2	O2	F 2
	III					P 4	S 8	Cl2
								BR 2
								I 2
Type crystal lattice		metal					atomic	molecular

The simple substances of the remaining elements have a metallic crystal lattice.

FIXING

Study the lecture material and answer the following questions in writing in your notebook:

1. What is a crystal lattice?
2. What types of crystal lattices exist?
3. Characterize each type of crystal lattice according to the plan: What is in the nodes of the crystal lattice, structural unit → Type of chemical bond between the particles of the node → Interaction forces between the particles of the crystal → Physical properties due to the crystal lattice → Aggregate state of the substance under normal conditions → Examples

Complete tasks on this topic:

1. What type of crystal lattice does the following substances widely used in everyday life have: water, acetic acid (CH 3 COOH), sugar (C 12 H 22 O 11), potassium fertilizer (KCl), river sand (SiO 2) - melting point 1710 0 C , ammonia (NH 3), table salt? Make a general conclusion: by what properties of a substance can one determine the type of its crystal lattice?
2. Using the formulas of the given substances: SiC, CS 2, NaBr, C 2 H 2 - determine the type of crystal lattice (ionic, molecular) of each compound and, based on this, describe the physical properties of each of the four substances.
   
3. Trainer No. 1. "Crystal lattices"
   
4. Trainer No. 2. "Test tasks"
   
5. Test (self-control):

1) Substances that have a molecular crystal lattice, as a rule:

a). refractory and highly soluble in water
b). fusible and volatile
V). Solid and electrically conductive
G). Thermally conductive and plastic

2) The concept of “molecule” not applicable in relation to the structural unit of a substance:

a). water

b). oxygen

V). diamond

G). ozone

3) The atomic crystal lattice is characteristic of:

a). aluminum and graphite

b). sulfur and iodine

V). silicon oxide and sodium chloride

G). diamond and boron

4) If a substance is highly soluble in water, has a high melting point, and is electrically conductive, then its crystal lattice is:

A). molecular

b). atomic

V). ionic

G). metal

Details Category: Molecular-kinetic theory Published 11/14/2014 17:19 Views: 14960

In solids, particles (molecules, atoms and ions) are located so close to each other that the interaction forces between them do not allow them to fly apart. These particles can only perform oscillatory movements around the equilibrium position. Therefore, solids retain their shape and volume.

Based on their molecular structure, solids are divided into crystalline And amorphous .

Structure of crystalline bodies

Crystal cell

Crystalline are those solids, molecules, atoms or ions in which they are arranged in a strictly defined geometric order, forming a structure in space called crystal lattice . This order is periodically repeated in all directions in three-dimensional space. It persists over long distances and is not limited in space. He is called in a long way .

Types of crystal lattices

A crystal lattice is a mathematical model that can be used to imagine how particles are arranged in a crystal. Mentally connecting the points in space where these particles are located with straight lines, we get a crystal lattice.

The distance between atoms located at the sites of this lattice is called lattice parameter .

Depending on which particles are located at the nodes, crystal lattices can be molecular, atomic, ionic and metallic .

The properties of crystalline bodies such as melting point, elasticity, and strength depend on the type of crystal lattice.

When the temperature rises to a value at which the melting of a solid begins, the crystal lattice is destroyed. The molecules gain more freedom, and the solid crystalline substance passes into the liquid stage. The stronger the bonds between molecules, the higher the melting point.

Molecular lattice

In molecular lattices, the bonds between molecules are not strong. Therefore, under normal conditions, such substances are in a liquid or gaseous state. The solid state is possible for them only at low temperatures. Their melting point (transition from solid to liquid) is also low. And under normal conditions they are in a gaseous state. Examples are iodine (I 2), “dry ice” (carbon dioxide CO 2).

Atomic lattice

In substances that have an atomic crystal lattice, the bonds between atoms are strong. Therefore, the substances themselves are very hard. They melt at high temperatures. Silicon, germanium, boron, quartz, oxides of some metals, and the hardest substance in nature, diamond, have a crystalline atomic lattice.

Ionic lattice

Substances with ionic crystal lattice include alkalis, most salts, and oxides of typical metals. Since the attractive force of ions is very strong, these substances can melt only at very high temperatures. They are called refractory. They have high strength and hardness.

Metal grill

At the nodes of the metal lattice, which all metals and their alloys have, both atoms and ions are located. Thanks to this structure, metals have good malleability and ductility, high thermal and electrical conductivity.

Most often, the crystal shape is a regular polyhedron. The faces and edges of such polyhedra always remain constant for a particular substance.

A single crystal is called single crystal . It has a regular geometric shape, a continuous crystal lattice.

Examples of natural single crystals are diamond, ruby, rock crystal, rock salt, Iceland spar, quartz. Under artificial conditions, single crystals are obtained through the process of crystallization, when, by cooling solutions or melts to a certain temperature, a solid substance in the form of crystals is isolated from them. With a slow crystallization rate, the cut of such crystals has a natural shape. In this way, under special industrial conditions, single crystals of semiconductors or dielectrics are obtained.

Small crystals randomly fused together are called polycrystals . The clearest example of a polycrystal is granite stone. All metals are also polycrystalline.

Anisotropy of crystalline bodies

In crystals, particles are located with different densities in different directions. If we connect atoms in one of the directions of the crystal lattice with a straight line, then the distance between them will be the same throughout this direction. In any other direction, the distance between the atoms is also constant, but its value may already differ from the distance in the previous case. This means that interaction forces of different magnitudes act between atoms in different directions. Therefore, the physical properties of the substance in these directions will also differ. This phenomenon is called anisotropy - dependence of the properties of matter on direction.

Electrical conductivity, thermal conductivity, elasticity, refractive index and other properties of a crystalline substance vary depending on the direction in the crystal. Electric current is conducted differently in different directions, the substance is heated differently, and light rays are refracted differently.

In polycrystals the phenomenon of anisotropy is not observed. The properties of the substance remain the same in all directions.

When carrying out many physical and chemical reactions, a substance passes into a solid state of aggregation. In this case, molecules and atoms tend to arrange themselves in such a spatial order in which the forces of interaction between particles of matter would be maximally balanced. This is how the strength of the solid substance is achieved. Atoms, once occupying a certain position, perform small oscillatory movements, the amplitude of which depends on temperature, but their position in space remains fixed. The forces of attraction and repulsion balance each other at a certain distance.

Modern ideas about the structure of matter

Modern science states that an atom consists of a charged nucleus, which carries a positive charge, and electrons, which carry negative charges. At a speed of several thousand trillion revolutions per second, electrons rotate in their orbits, creating an electron cloud around the nucleus. The positive charge of the nucleus is numerically equal to the negative charge of the electrons. Thus, the atom of the substance remains electrically neutral. Possible interactions with other atoms occur when electrons are detached from their parent atom, thereby disturbing the electrical balance. In one case, the atoms are arranged in a certain order, which is called a crystal lattice. In another, due to the complex interaction of nuclei and electrons, they are combined into molecules of various types and complexity.

Definition of crystal lattice

Taken together, various types of crystalline lattices of substances are networks with different spatial orientations, at the nodes of which ions, molecules or atoms are located. This stable geometric spatial position is called the crystal lattice of the substance. The distance between nodes of one crystal cell is called the identity period. The spatial angles at which the cell nodes are located are called parameters. According to the method of constructing bonds, crystal lattices can be simple, base-centered, face-centered, and body-centered. If the particles of matter are located only in the corners of the parallelepiped, such a lattice is called simple. An example of such a lattice is shown below:

If, in addition to the nodes, the particles of the substance are located in the middle of the spatial diagonals, then this arrangement of particles in the substance is called a body-centered crystal lattice. This type is clearly shown in the figure.

If, in addition to the nodes at the vertices of the lattice, there is a node at the place where the imaginary diagonals of the parallelepiped intersect, then you have a face-centered type of lattice.

Types of crystal lattices

The different microparticles that make up a substance determine the different types of crystal lattices. They can determine the principle of building connections between microparticles inside a crystal. Physical types of crystal lattices are ionic, atomic and molecular. This also includes various types of metal crystal lattices. Chemistry studies the principles of the internal structure of elements. The types of crystal lattices are presented in more detail below.

Ionic crystal lattices

These types of crystal lattices are present in compounds with an ionic type of bond. In this case, lattice sites contain ions with opposite electrical charges. Thanks to the electromagnetic field, the forces of interionic interaction are quite strong, and this determines the physical properties of the substance. Common characteristics are refractoriness, density, hardness and the ability to conduct electric current. Ionic types of crystal lattices are found in substances such as table salt, potassium nitrate and others.

Atomic crystal lattices

This type of structure of matter is inherent in elements whose structure is determined by covalent chemical bonds. Types of crystal lattices of this kind contain individual atoms at the nodes, connected to each other by strong covalent bonds. This type of bond occurs when two identical atoms “share” electrons, thereby forming a common pair of electrons for neighboring atoms. Thanks to this interaction, covalent bonds bind atoms evenly and strongly in a certain order. Chemical elements that contain atomic types of crystal lattices are hard, have a high melting point, are poor conductors of electricity, and are chemically inactive. Classic examples of elements with a similar internal structure include diamond, silicon, germanium, and boron.

Molecular crystal lattices

Substances that have a molecular type of crystal lattice are a system of stable, interacting, closely packed molecules that are located at the nodes of the crystal lattice. In such compounds, the molecules retain their spatial position in the gaseous, liquid and solid phases. At the nodes of the crystal, molecules are held together by weak van der Waals forces, which are tens of times weaker than the ionic interaction forces.

The molecules that form a crystal can be either polar or nonpolar. Due to the spontaneous movement of electrons and vibrations of nuclei in molecules, the electrical equilibrium can shift - this is how an instantaneous electric dipole moment arises. Appropriately oriented dipoles create attractive forces in the lattice. Carbon dioxide and paraffin are typical examples of elements with a molecular crystal lattice.

Metal crystal lattices

A metal bond is more flexible and ductile than an ionic bond, although it may seem that both are based on the same principle. The types of crystal lattices of metals explain their typical properties - such as mechanical strength, thermal and electrical conductivity, and fusibility.

A distinctive feature of a metal crystal lattice is the presence of positively charged metal ions (cations) at the sites of this lattice. Between the nodes there are electrons that are directly involved in creating an electric field around the lattice. The number of electrons moving around within this crystal lattice is called electron gas.

In the absence of an electric field, free electrons perform chaotic motion, randomly interacting with lattice ions. Each such interaction changes the momentum and direction of motion of the negatively charged particle. With their electric field, electrons attract cations to themselves, balancing their mutual repulsion. Although electrons are considered free, their energy is not enough to leave the crystal lattice, so these charged particles are constantly within its boundaries.

The presence of an electric field gives the electron gas additional energy. The connection with ions in the crystal lattice of metals is not strong, so electrons easily leave its boundaries. Electrons move along lines of force, leaving behind positively charged ions.

conclusions

Chemistry attaches great importance to the study of the internal structure of matter. The types of crystal lattices of various elements determine almost the entire range of their properties. By influencing crystals and changing their internal structure, it is possible to enhance the desired properties of a substance and remove undesirable ones and transform chemical elements. Thus, studying the internal structure of the surrounding world can help to understand the essence and principles of the structure of the universe.

Any substance in nature, as is known, consists of smaller particles. They, in turn, are connected and form a certain structure, which determines the properties of a particular substance.

Atomic is characteristic and occurs at low temperatures and high pressure. Actually, it is precisely thanks to this that metals and a number of other materials acquire their characteristic strength.

The structure of such substances at the molecular level looks like a crystal lattice, each atom in which is connected to its neighbor by the strongest connection existing in nature - a covalent bond. All the smallest elements that form the structures are arranged in an orderly manner and with a certain periodicity. Representing a grid in the corners of which atoms are located, always surrounded by the same number of satellites, the atomic crystal lattice practically does not change its structure. It is well known that the structure of a pure metal or alloy can be changed only by heating it. In this case, the higher the temperature, the stronger the bonds in the lattice.

In other words, the atomic crystal lattice is the key to the strength and hardness of materials. However, it is worth considering that the arrangement of atoms in different substances may also differ, which, in turn, affects the degree of strength. So, for example, diamond and graphite, which contain the same carbon atom, are extremely different from each other in terms of strength: diamond is on Earth, but graphite can exfoliate and break. The fact is that in the crystal lattice of graphite, atoms are arranged in layers. Each layer resembles a honeycomb, in which the carbon atoms are joined rather loosely. This structure causes layered crumbling of pencil leads: when broken, parts of the graphite simply peel off. Another thing is diamond, the crystal lattice of which consists of excited carbon atoms, that is, those that are capable of forming 4 strong bonds. It is simply impossible to destroy such a joint.

Crystal lattices of metals, in addition, have certain characteristics:

1. Lattice period- a quantity that determines the distance between the centers of two adjacent atoms, measured along the edge of the lattice. The generally accepted designation does not differ from that in mathematics: a, b, c are the length, width, height of the lattice, respectively. Obviously, the dimensions of the figure are so small that the distance is measured in the smallest units of measurement - a tenth of a nanometer or angstroms.

2. K - coordination number. An indicator that determines the packing density of atoms within a single lattice. Accordingly, its density is greater, the higher the number K. In fact, this figure represents the number of atoms that are as close as possible and at an equal distance from the atom under study.

3. Lattice basis. Also a quantity characterizing the density of the lattice. Represents the total number of atoms that belong to the particular cell being studied.

4. Compactness factor measured by calculating the total volume of the lattice divided by the volume occupied by all the atoms in it. Like the previous two, this value reflects the density of the lattice being studied.

We have considered only a few substances that have an atomic crystal lattice. Meanwhile, there are a great many of them. Despite its great diversity, the crystalline atomic lattice includes units that are always connected by means (polar or non-polar). In addition, such substances are practically insoluble in water and are characterized by low thermal conductivity.

In nature, there are three types of crystal lattices: body-centered cubic, face-centered cubic, and close-packed hexagonal.

The formation of molecules from atoms leads to a gain in energy, since under normal conditions the molecular state is more stable than the atomic state.

To consider this topic you need to know:

Electronegativity is the ability of an atom to shift a common electron pair towards itself. (The most electronegative element is fluorine.)

Crystal lattice - a three-dimensional ordered arrangement of particles.

There are three main types of chemical bonds: covalent, ionic and metallic.

Metal connection characteristic of metals that contain a small number of electrons at the outer energy level (1 or 2, less often 3). These electrons easily lose contact with the nucleus and move freely throughout the piece of metal, forming an “electron cloud” and providing communication with the positively charged ions formed after the electrons are removed. The crystal lattice is metal. This determines the physical properties of metals: high thermal and electrical conductivity, malleability and ductility, metallic luster.

Covalent bond is formed due to a common electron pair of non-metal atoms, with each of them achieving a stable configuration of an atom of an inert element.

If a bond is formed by atoms with the same electronegativity, that is, the difference in electronegativity of two atoms is zero, the electron pair is located symmetrically between the two atoms and the bond is called covalent nonpolar.

If a bond is formed by atoms with different electronegativity, and the difference in electronegativity of the two atoms lies in the range from zero to approximately two (most often these are different non-metals), then the shared electron pair is shifted to the more electronegative element. A partially negative charge arises on it (the negative pole of the molecule), and a partially positive charge arises on the other atom (the positive pole of the molecule). This connection is called covalent polar.

If a bond is formed by atoms with different electronegativity, and the difference in electronegativity of two atoms is more than two (most often it is a non-metal and a metal), then it is believed that the electron is completely transferred to the non-metal atom. As a result, this atom becomes a negatively charged ion. An atom that donates an electron is a positively charged ion. The bond between ions is called ionic bond.

Compounds with covalent bonds have two types of crystal lattices: atomic and molecular.

In an atomic crystal lattice, the nodes contain atoms connected by strong covalent bonds. Substances with such a crystal lattice have high melting points, are strong and hard, and are practically insoluble in liquids. for example, diamond, solid boron, silicon, germanium and compounds of certain elements with carbon and silicon.

In a molecular crystal lattice, the nodes contain molecules connected by weak intermolecular interactions.

Substances with such a lattice have low hardness and low melting points, are insoluble or slightly soluble in water, and solutions practically do not conduct electric current. For example, ice, solid carbon monoxide (IV) solid hydrogen halides, simple solids formed by one-(noble gases), two- (F 2, Cl 2, Br 2, I 2, H 2, O 2, N 2), three-(O 3), four-(P 4), eight-(S 8) atomic molecules. Most crystalline organic compounds have a molecular lattice. Compounds with ionic bonds have an ionic crystal lattice, in the nodes of which positively and negatively charged ions alternate. Substances with an ionic lattice refractory and low-volatile,

They have relatively high hardness, but are brittle. Melts and aqueous solutions of salts and alkalis conduct electric current.

Examples of tasks

1. In which molecule is the covalent bond “element - oxygen” most polar?

1) SO 2 2) NO 3) Cl 2 O 4) H 2 O

The polarity of a bond is determined by the difference in electronegativity between two atoms (in this case, an element and oxygen). Sulfur, nitrogen and chlorine are located next to oxygen, therefore their electronegativity differs slightly. And only hydrogen is located at a distance from oxygen, which means the difference in electronegativity will be large, and the bond will be the most polar.

Answer: 4)

2. Hydrogen bonds form between molecules

1) methanol 2) methanal 3) acetylene 4) methyl formate

1) SO 2 2) NO 3) Cl 2 O 4) H 2 O

Acetylene contains no highly electronegative elements at all. Methanal H 2 CO and methyl formate HCOOCH 3 do not contain hydrogen connected to a strongly electronegative element. The hydrogen in them is combined with carbon. But in methanol CH 3 OH, a hydrogen bond can form between the hydrogen atom of one hydroxo group and the oxygen atom of another molecule.

Answer: 1)