The structure of a substance 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(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 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 have much less fluidity. The amorphous state is usually unstable. Under the influence of mechanical loads or 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 sign amorphous(from Greek " amorphos" - formless) state of matter - the absence of an atomic or molecular lattice, that is, the 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 nonequilibrium 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 one. Differences in the properties of the liquid and solid amorphous state are determined by the nature of the thermal movement of particles: in the amorphous state, particles are capable only of oscillatory and rotational movements, but cannot move through the thickness of the substance.
There are substances that can only exist in solid form in an amorphous state. This refers 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 repeatability of the same structural element ( unit 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 determined, first of all, by their strictly regular internal structure. If, instead of atoms, ions or molecules in a crystal, we depict points as the centers of gravity of these particles, we get a three-dimensional regular distribution of such points, called a crystal lattice. The points themselves are called nodes crystal lattice.
Types of crystal lattices
Depending on what particles the crystal lattice is made of and what the nature of the chemical bond between them is, different types of crystals are distinguished.
Ionic crystals are formed by cations and anions (for example, salts and hydroxides of most metals). In them there is an ionic bond between the particles.
Ionic crystals may consist of monatomic ions. This is how crystals are built sodium chloride, potassium iodide, calcium fluoride.
Monatomic metal cations and polyatomic anions, for example, nitrate ion NO 3 −, sulfate ion SO 4 2−, carbonate ion CO 3 2−, participate in the formation of ionic crystals of many salts.
It is impossible to isolate single molecules in an ionic crystal. Each cation is attracted to each anion and repelled by other cations. The entire 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 in 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 fragility is due to the fact that even with 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 attractive forces 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 electric current.
High solubility in polar solvents, accompanied by electrolytic dissociation, is due to the fact that in a solvent environment with a high dielectric constant ε, the energy of attraction between ions decreases. The dielectric constant of water is 82 times higher than that of vacuum (conditionally existing in an ionic crystal), and the attraction between ions in an aqueous solution decreases by the same amount. The effect is enhanced by solvation of ions.
Atomic crystals consist of individual atoms held together by covalent bonds. Of the simple substances, only boron and group IVA elements 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 durable 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, and their solutions and melts also do not conduct electric current.
Intermolecular forces arise due to the electrostatic interaction of the negatively charged electrons of one molecule with the positively charged nuclei of neighboring molecules. The strength of intermolecular interactions is influenced by many factors. The most important among them is the presence of polar bonds, that is, a shift in electron density from one atom to another. In addition, intermolecular interactions are stronger between molecules with a larger number of electrons.
Most nonmetals 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 solid organic substances form molecular crystals.
Metals are characterized by a metallic crystal lattice. It contains 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 bonding in such crystals is delocalized and extends throughout 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 Within each layer, the bonds are covalent-metallic, and between the layers they are intermolecular.
Isomorphism and polymorphism
Many crystalline substances have the same structure. At the same time, the same substance can form different crystal structures. This is reflected in the phenomena isomorphism And polymorphism.
Isomorphism lies in the ability of atoms, ions or molecules to replace each other in crystal structures. This term (from the Greek " isos" - equal and " morphe" - form) was proposed by E. Mitscherlich in 1819. The law of isomorphism was formulated by E. Mitscherlich in 1821 in this way: “The same numbers of atoms, connected in the same way, give the same crystalline forms; Moreover, the crystalline form does not depend on the chemical nature of the atoms, but is determined only by their number and relative position."
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 similarity of the crystalline forms of many other substances. His observations attracted the attention of the famous Swedish chemist J.-Ya. Berzelius, who suggested that Mitscherlich 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 contains phosphorus.” Mitscherlich's discovery very soon attracted the attention of mineralogists, who began research on the problem of isomorphic substitution of elements in minerals.
During the joint crystallization of substances prone to isomorphism ( isomorphic substances), mixed crystals (isomorphic mixtures) are formed. This is only possible 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, similar crystals in external shape. Such substances include, for example, alum. In potassium alum crystals 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 (while 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 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 modifications of carbon (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 persist indefinitely. At high temperatures they turn into graphite. In the case of diamond, this occurs when heated above 1000 o C in the absence of oxygen. The reverse transition is much more difficult to achieve. Not only high temperature is required (1200-1600 o C), but also enormous 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 different packing of molecules in the crystal or in changes in the shape of molecules, and in ionic crystals - in different relative positions of cations and anions. Some simple and complex substances have more than two polymorphs. For example, silicon dioxide has ten modifications, calcium fluoride - six, ammonium nitrate - four. Polymorphic modifications are usually denoted by the Greek letters α, β, γ, δ, ε,... starting with modifications that are stable at low temperatures.
When crystallizing from steam, solution or melt a substance that has several polymorphic modifications, a modification that is less stable under given conditions is first formed, which then turns into a more stable one. For example, when phosphorus vapor condenses, white phosphorus is formed, which under normal conditions slowly, but 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 it turns back into β-PbO.
The transition from one polymorph to another is called polymorphic transformation. These transitions occur when temperature or pressure changes and are accompanied by an abrupt change in properties.
The process of transition from one modification to another can be reversible or irreversible. Thus, 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 - borazone, close to diamond in hardness. When the temperature and pressure are lowered to values corresponding to normal conditions, borazone retains its structure. An example of a reversible transition is the mutual transformations of two modifications of sulfur (orthorhombic and monoclinic) at 95 o C.
Polymorphic transformations can occur without significant changes in 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.
One of the most common materials that people have always preferred to work with has been metal. In each era, preference was given to different types of these amazing substances. Thus, the IV-III millennium BC is considered the Chalcolithic, or Copper Age. Later it is replaced by bronze, and then the one that is still relevant today comes into force - iron.
Today it is generally difficult to imagine that it was once possible to do without metal products, because almost everything, from household items, medical instruments to heavy and light equipment, consists of this material or includes individual parts from it. Why did metals manage to gain such popularity? Let’s try to figure out what the features are and how this is inherent in their structure.
General concept of metals
"Chemistry. 9th grade" is a textbook used by schoolchildren. It is here that metals are studied in detail. A large chapter is devoted to the consideration of their physical and chemical properties, because their diversity is extremely great.
It is from this age that it is recommended to give children an idea of these atoms and their properties, because teenagers can already fully appreciate the significance of such knowledge. They see perfectly well that the variety of objects, machines and other things around them is based on a metallic nature.
What is metal? From the point of view of chemistry, these atoms are usually classified as those that have:
- small on the external level;
- exhibit strong restorative properties;
- have a large atomic radius;
- As simple substances, they have a number of specific physical properties.
The basis of knowledge about these substances can be obtained by considering the atomic-crystalline structure of metals. It is this that explains all the features and properties of these compounds.
In the periodic table, most of the entire table is allocated to metals, because they form all the secondary subgroups and the main ones from the first to the third group. Therefore, their numerical superiority is obvious. The most common are:
- calcium;
- sodium;
- titanium;
- iron;
- magnesium;
- aluminum;
- potassium.
All metals have a number of properties that allow them to be combined into one large group of substances. In turn, these properties are explained precisely by the crystalline structure of metals.
Properties of metals
The specific properties of the substances in question include the following.
- Metallic shine. All representatives of simple substances have it, and most are the same. Only a few (gold, copper, alloys) are different.
- Malleability and plasticity - the ability to deform and recover quite easily. It is expressed to different degrees in different representatives.
- Electrical and thermal conductivity are one of the main properties that determine the areas of application of the metal and its alloys.
The crystalline structure of metals and alloys explains the reason for each of the indicated properties and speaks about their severity in each specific representative. If you know the features of such a structure, then you can influence the properties of the sample and adjust it to the desired parameters, which is what people have been doing for many decades.
Atomic crystal structure of metals
What is this structure, what is it characterized by? The name itself suggests that all metals are crystals in the solid state, that is, under normal conditions (except for mercury, which is a liquid). What is a crystal?
This is a conventional graphic image constructed by intersecting imaginary lines through the atoms that line up the body. In other words, every metal is made up of atoms. They are located in it not chaotically, but very correctly and consistently. So, if you mentally combine all these particles into one structure, you will get a beautiful image in the form of a regular geometric body of some shape.
This is what is commonly called the crystal lattice of a metal. It is very complex and spatially voluminous, therefore, for simplicity, not all of it is shown, but only a part, an elementary cell. A set of such cells, collected together and reflected in and forms crystal lattices. Chemistry, physics and metallurgy are sciences that study the structural features of such structures.
Itself is a set of atoms that are located at a certain distance from each other and coordinate a strictly fixed number of other particles around themselves. It is characterized by packing density, distance between constituent structures, and coordination number. In general, all these parameters are characteristics of the entire crystal, and therefore reflect the properties exhibited by the metal.
There are several varieties. They all have one feature in common - the nodes contain atoms, and inside there is a cloud of electron gas, which is formed by the free movement of electrons inside the crystal.
Types of crystal lattices
Fourteen lattice structure options are usually combined into three main types. They are as follows:
- Body-centered cubic.
- Hexagonal close-packed.
- Face-centered cubic.
The crystalline structure of metals was studied only when it became possible to obtain high magnification images. And the classification of types of lattices was first given by the French scientist Bravais, by whose name they are sometimes called.
Body-centered lattice
The structure of the crystal lattice of metals of this type is the following structure. This is a cube with eight atoms at its nodes. Another one is located in the center of the free internal space of the cell, which explains the name “body-centered”.
This is one of the options for the simplest structure of the unit cell, and therefore the entire lattice as a whole. The following metals have this type:
- molybdenum;
- vanadium;
- chromium;
- manganese;
- alpha iron;
- beta iron and others.
The main properties of such representatives are a high degree of malleability and ductility, hardness and strength.
Face-centered lattice
The crystal structure of metals having a face-centered cubic lattice is the following structure. This is a cube that includes fourteen atoms. Eight of them form lattice nodes, and another six are located, one on each face.
They have a similar structure:
- aluminum;
- nickel;
- lead;
- gamma iron;
- copper.
The main distinctive properties are shine of different colors, lightness, strength, malleability, increased resistance to corrosion.
Hexagonal lattice
The crystal structure of metals with lattices is as follows. The unit cell is based on a hexagonal prism. There are 12 atoms at its nodes, two more at the bases, and three atoms lie freely inside the space in the center of the structure. There are seventeen atoms in total.
Metals such as:
- alpha titanium;
- magnesium;
- alpha cobalt;
- zinc.
The main properties are a high degree of strength, strong silver shine.
Defects in the crystal structure of metals
However, all types of cells considered may also have natural shortcomings, or so-called defects. This may be due to various reasons: foreign atoms and impurities in metals, external influences, and so on.
Therefore, there is a classification that reflects the defects that crystal lattices may have. Chemistry as a science studies each of them in order to identify the cause and method of elimination so that the properties of the material are not changed. So, the defects are as follows.
- Spot. They come in three main types: vacancies, impurities or dislocated atoms. Lead to deterioration of the magnetic properties of the metal, its electrical and thermal conductivity.
- Linear or dislocation. There are edge and screw ones. They deteriorate the strength and quality of the material.
- Surface defects. Affects the appearance and structure of metals.
Currently, methods have been developed to eliminate defects and obtain pure crystals. However, it is not possible to completely eradicate them; an ideal crystal lattice does not exist.
The importance of knowledge about the crystalline structure of metals
From the above material, it is obvious that knowledge about the fine structure and structure makes it possible to predict the properties of the material and influence them. And the science of chemistry allows you to do this. The 9th grade of a general education school places emphasis in the learning process on developing in students a clear understanding of the importance of the fundamental logical chain: composition - structure - properties - application.
Information about the crystalline structure of metals is very clearly illustrated and allows the teacher to clearly explain and show children how important it is to know the fine structure in order to correctly and competently use all the properties.
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, 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 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 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 consumption, 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 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, 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
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 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.
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 widely used in everyday life have: water, acetic acid (CH3 COOH), sugar (C12 H22 O11), potassium 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, has a high melting point, and is electrically conductive, then its crystal lattice is:
A). molecular
b). atomic
V). ionic
G). metal
Since ancient times, metals have played a huge role in the development of mankind. Their introduction into everyday life has produced a real revolution both in the methods of processing materials and in human perception of the surrounding reality. Modern industry and agriculture, transport and infrastructure are impossible without the use of metals and the use of their beneficial qualities and properties. These qualities, in turn, are determined by the internal structure of a given class of chemical compounds, which is based on a crystal lattice.
The concept and essence of the crystal lattice
From the point of view of the internal structure, any substance can be in one of three states - liquid, gaseous and solid. Moreover, it is the latter that is characterized by the greatest stability, which is due to the fact that the crystal lattice implies not only a clear arrangement of atoms or molecules in strictly defined places, but also the need to apply a sufficiently large force to break the bonds between these elementary particles.
Features of the ionic lattice
The structure of any substance in the solid state necessarily involves the periodic repetition of molecules and atoms in three dimensions at once. Moreover, depending on what is located at the nodal points, the crystal lattice can be ionic, atomic, molecular and metallic. As for the first type, here the basic components are oppositely charged ions, between which the so-called Coulomb forces arise and act. In this case, the force of interaction is directly dependent on the radii of charged particles.
Such a lattice is a complex system consisting of metal cations, in the space between which negatively charged electrons move. It is the presence of these elementary particles that gives the lattice stability and hardness, because they serve as a kind of compensators for positively charged cations.
Strength and weakness of the atomic lattice
The atomic crystal lattice is quite interesting from the point of view of structure. Already from the name we can conclude that its nodes contain atoms held together by covalent bonds. In recent years, many scientists have attributed this type of interaction to the family of inorganic polymers, since the structure of a given molecule is largely determined by the valence of its constituent atoms.
Main characteristics of a molecular lattice
The molecular crystal lattice is the least stable of all those presented. The thing is that the level of interaction between the molecules located in its nodes is extremely low, and the energy potential is determined by a number of factors, the main role in which is played by dispersion, induction and orientation forces.
The influence of the crystal lattice on the properties of objects
Thus, the crystal lattice largely determines the properties of a particular substance. For example, atomic crystals melt at extremely high temperatures and have increased hardness, and substances with a metal lattice are excellent conductors
Most solids have crystalline structure, which is characterized strictly defined arrangement of particles. If you connect the particles with conventional lines, you get a spatial framework called crystal lattice. The points at which crystal particles are located are called lattice nodes. The nodes of an imaginary lattice may contain atoms, ions or molecules.
Depending on the nature of the particles located at the nodes and the nature of the connection between them, four types of crystal lattices are distinguished: ionic, metallic, atomic and molecular.
Ionic are called lattices in whose nodes there are ions.
They are formed by substances with ionic bonds. At the nodes of such a lattice there are positive and negative ions connected to each other by electrostatic interaction.
Ionic crystal lattices have salts, alkalis, active metal oxides. Ions can be simple or complex. For example, at the lattice sites of sodium chloride there are simple sodium ions Na and chlorine Cl − , and at the lattice sites of potassium sulfate simple potassium ions K and complex sulfate ions S O 4 2 − alternate.
The bonds between ions in such crystals are strong. Therefore, ionic substances are solid, refractory, non-volatile. Such substances are good dissolve in water.
Crystal lattice of sodium chloride
Sodium chloride crystal
Metal called lattices, which consist of positive ions and metal atoms and free electrons.
They are formed by substances with metallic bonds. At the nodes of a metal lattice there are atoms and ions (either atoms or ions, into which atoms easily turn, giving up their outer electrons for common use).
Such crystal lattices are characteristic of simple substances of metals and alloys.
The melting points of metals can be different (from \(–37\) °C for mercury to two to three thousand degrees). But all metals have a characteristic metallic shine, malleability, ductility, conduct electricity well and warmth.
Metal crystal lattice
Hardware
Atomic lattices are called crystal lattices, at the nodes of which there are individual atoms connected by covalent bonds.
Diamond has this type of lattice - one of the allotropic modifications of carbon. Substances with an atomic crystal lattice include graphite, silicon, boron and germanium, as well as complex substances, for example carborundum SiC and silica, quartz, rock crystal, sand, which include silicon oxide (\(IV\)) Si O 2.
Such substances are characterized high strength and hardness. Thus, diamond is the hardest natural substance. Substances with an atomic crystal lattice have very high melting points and boiling. For example, the melting point of silica is \(1728\) °C, while for graphite it is higher - \(4000\) °C. Atomic crystals are practically insoluble.
Diamond crystal lattice
Diamond
Molecular are called lattices, at the nodes of which there are molecules connected by weak intermolecular interactions.
Despite the fact that the atoms inside the molecules are connected by very strong covalent bonds, weak forces of intermolecular attraction act between the molecules themselves. Therefore, molecular crystals have low strength and hardness, low melting points and boiling. Many molecular substances are liquids and gases at room temperature. Such substances are volatile. For example, crystalline iodine and solid carbon monoxide (\(IV\)) (“dry ice”) evaporate without turning into a liquid state. Some molecular substances have smell .
This type of lattice has simple substances in a solid state of aggregation: noble gases with monatomic molecules (He, Ne, Ar, Kr, Xe, Rn ), as well as non-metals with two- and polyatomic molecules (H 2, O 2, N 2, Cl 2, I 2, O 3, P 4, S 8).
They have a molecular crystal lattice also substances with covalent polar bonds: water - ice, solid ammonia, acids, non-metal oxides. Majority organic compounds are also molecular crystals (naphthalene, sugar, glucose).
