Atomic crystal lattice in solid state. Ionic crystal lattice

Most substances are characterized by the ability, depending on conditions, to be in one of three states of aggregation: solid, liquid or gaseous.

For example, water at normal pressure in the temperature range 0-100 o C it is a liquid, at temperatures above 100 o C it can exist only in a gaseous state, and at temperatures below 0 o C it is a solid.
Substances in the solid state are divided into amorphous and crystalline.

A characteristic feature of amorphous substances is the absence of a clear melting point: their fluidity gradually increases with increasing temperature. Amorphous substances include compounds such as wax, paraffin, most plastics, glass, etc.

Still, crystalline substances have a specific melting point, i.e. a substance with a crystalline structure passes from a solid to a liquid state not gradually, but abruptly, upon reaching a specific temperature. Examples of crystalline substances include table salt, sugar, and ice.

The difference in the physical properties of amorphous and crystalline solids is primarily due to the structural features of such substances. What is the difference between a substance in an amorphous and a crystalline state can be most easily understood from the following illustration:

As you can see, in an amorphous substance, unlike a crystalline one, there is no order in the arrangement of particles. If in a crystalline substance you mentally connect two atoms close to each other with a straight line, then you can find that the same particles will lie on this line at strictly defined intervals:

Thus, in the case of crystalline substances, we can talk about such a concept as a crystal lattice.

Crystal lattice called a spatial framework connecting the points in space in which the particles that form the crystal are located.

The points in space at which the particles forming the crystal are located are called crystal lattice nodes .

Depending on which particles are located at the nodes of the crystal lattice, they are distinguished: molecular, atomic, ionic And metal crystal lattices .

In nodes molecular crystal lattice

Ice crystal lattice as an example of a molecular lattice

There are molecules within which the atoms are connected by strong covalent bonds, but the molecules themselves are held near each other by weak intermolecular forces. Due to such weak intermolecular interactions, crystals with a molecular lattice are fragile. Such substances differ from substances with other types of structure by significantly lower melting and boiling points; they do not conduct electric current, may or may not be soluble in various solvents. Solutions of such compounds may or may not conduct electric current, depending on the class of the compound. Compounds with a molecular crystal lattice include many simple substances - nonmetals (hardened H 2, O 2, Cl 2, orthorhombic sulfur S 8, white phosphorus P 4), as well as many complex substances– hydrogen compounds of non-metals, acids, oxides of non-metals, most organic matter. It should be noted that if a substance is in a gaseous or liquid state, it is inappropriate to talk about a molecular crystal lattice: it is more correct to use the term molecular type of structure.

Diamond crystal lattice as an example of an atomic lattice

In nodes atomic crystal lattice

there are atoms. Moreover, all the nodes of such a crystal lattice are “linked” together through strong covalent bonds into a single crystal. In fact, such a crystal is one giant molecule. Due to their structural features, all substances with an atomic crystal lattice are solid, have high melting points, are chemically inactive, insoluble in either water or organic solvents, and their melts do not conduct electric current. It should be remembered that substances with an atomic type of structure include boron B, carbon C (diamond and graphite), silicon Si from simple substances, and silicon dioxide SiO 2 (quartz), silicon carbide SiC, boron nitride BN from complex substances.

For substances with ionic crystal lattice

lattice sites contain ions connected to each other through ionic bonds.
Because ionic bonds are quite strong, substances with an ionic lattice have relatively high hardness and refractoriness. Most often, they are soluble in water, and their solutions, like melts, conduct electric current.
Substances with an ionic crystal lattice include metal and ammonium salts (NH 4 +), bases, and metal oxides. A sure sign of the ionic structure of a substance is the presence in its composition of both atoms of a typical metal and a non-metal.

Crystal lattice of sodium chloride as an example of an ionic lattice

observed in crystals of free metals, for example, sodium Na, iron Fe, magnesium Mg, etc. In the case of a metal crystal lattice, its nodes contain cations and metal atoms, between which electrons move. In this case, moving electrons periodically attach to cations, thus neutralizing their charge, and individual neutral metal atoms in return “release” some of their electrons, turning, in turn, into cations. In fact, the “free” electrons do not belong to individual atoms, but to the entire crystal.

Such structural features lead to the fact that metals conduct heat and electric current well and often have high ductility (malleability).
The spread of melting temperatures of metals is very large. For example, the melting point of mercury is approximately minus 39 ° C (liquid under normal conditions), and tungsten is 3422 ° C. It should be noted that under normal conditions all metals except mercury are solids.

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 do 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 are distinguished crystal lattices: 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).

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When performing many physical and chemical reactions the 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 line up in in a certain order, which is called the crystal lattice. In another, due to the complex interaction of nuclei and electrons, they combine into molecules various types and complexity.

Definition of crystal lattice

In total various types Crystal 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 the 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. Studying the principles internal structure Chemistry deals with 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 electric charge. Thanks to the electromagnetic field, the forces of interionic interaction are quite strong, and this causes physical properties substances. Common characteristics are refractoriness, density, hardness and the ability to conduct electric current. Ionic types 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 the creation electric field around the grate. 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. Types of crystal lattices various elements determine almost the entire range of their properties. By influencing crystals and changing their internal structure, it is possible to achieve enhancement required properties substances and remove unwanted ones, transform chemical elements. Thus, studying internal structure the surrounding world can help to understand the essence and principles of the structure of the universe.

Structure of matter.

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 stable for substances 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, and the character chemical bond is not violated. 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 are open in late XIX centuries, but have been studied in the last 20-25 years. Many display devices modern technology, for example, some electronic watches, 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 2000 C. The unusualness of the combination “liquid crystals” is that the liquid state indicates the mobility of the structure, and the crystal implies strict order.

If a substance consists of polyatomic molecules of an elongated or lamellar shape and having an asymmetric 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 consumption, which is what is used in modern technology.

Types of crystal lattices.

Any chemical substance educated a large number identical particles that are connected to each other.
At low temperatures, when thermal movement is difficult, the particles are strictly oriented in space and form a crystal lattice.

Crystal lattice is a 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 different 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 polymers of carbon. 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, impermeable 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

Properties of substances with different crystal lattices (table)

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

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

Between the position of the element in periodic table and the crystal lattice of its corresponding simple substance there is a close relationship.

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:
- What is a crystal lattice?
- What types of crystal lattices exist?
- Describe 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:

- What type of crystal lattice does the following substances commonly used in everyday life have: water, acetic acid (CH3 COOH), sugar (C12 H22 O11), potash fertilizer(KCl), river sand (SiO2) – melting point 1710 0C, ammonia (NH3), table salt? Make a general conclusion: by what properties of a substance can one determine the type of its crystal lattice?
Using the formulas of the given substances: SiC, CS2, NaBr, C2 H2 - 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.
Trainer No. 1. "Crystal lattices"
Trainer No. 2. "Test tasks"
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” is not applicable to the structural unit of a substance:

b). oxygen

V). diamond

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, it has high temperature melting, electrically conductive, then its crystal lattice:

A). molecular

b). atomic

V). ionic

G). metal

Any substance in nature is known to consist of more fine 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 blood 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 various 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 equal distance from the atom being studied.

3. Lattice basis. Also a quantity characterizing the density of the lattice. Represents total number atoms that belong to the specific 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.