Molecular and non-molecular structure of substances. The structure of matter
It is not individual atoms or molecules that enter into chemical interactions, but substances. Substances are distinguished by the type of bond molecular And non-molecular structure. Substances made up of molecules are called molecular substances. The bonds between molecules in such substances are very weak, much weaker than between atoms inside a molecule, and already at relatively low temperatures they break - the substance turns into a liquid and then into a gas (iodine sublimation). The melting and boiling points of substances consisting of molecules increase with increasing molecular weight. TO molecular substances include substances with an atomic structure (C, Si, Li, Na, K, Cu, Fe, W), among them there are metals and non-metals. To substances non-molecular structure include ionic compounds. Most compounds of metals with non-metals have this structure: all salts (NaCl, K 2 SO 4), some hydrides (LiH) and oxides (CaO, MgO, FeO), bases (NaOH, KOH). Ionic (non-molecular) substances have high melting and boiling points.
Solids: amorphous and crystalline
Solids are divided into crystalline and amorphous.
Amorphous substances do not have a clear melting point - when heated, they gradually soften and become fluid. In the amorphous state, for example, are plasticine and various resins.
Crystalline substances are characterized by the correct arrangement of the particles of which they are composed: atoms, molecules and ions - at strictly defined points in space. When these points are connected by straight lines, a spatial frame is formed, called the crystal lattice. The points at which crystal particles are located are called lattice nodes. Depending on the type of particles located at the nodes of the crystal lattice, and the nature of the connection between them, four types of crystal lattices are distinguished: ionic, atomic, molecular and metallic.
Crystal lattices are called ionic, at the sites of which there are ions. They are formed by substances with an ionic bond, which can be associated with both simple ions Na +, Cl -, and complex SO 4 2-, OH -. Consequently, salts, some oxides and hydroxides of metals have ionic crystal lattices. For example, a sodium chloride crystal is built from alternating positive Na + and negative Cl - ions, forming a cube-shaped lattice. The bonds between ions in such a crystal are very stable. Therefore, substances with an ionic lattice are characterized by relatively high hardness and strength, they are refractory and non-volatile.
Crystal lattice - a) and amorphous lattice - b).
Crystal lattice - a) and amorphous lattice - b). Atomic crystal lattices
nuclear called crystal lattices, in the nodes of which there are individual atoms. In such lattices, atoms are connected to each other very strong covalent bonds. An example of substances with this type of crystal lattice is diamond, one of the allotropic modifications of carbon. Most substances with an atomic crystal lattice have very high melting points (for example, in diamond it is over 3500 ° C), they are strong and hard, practically insoluble.
Molecular crystal lattices
Molecular called crystal lattices, at the nodes of which molecules are located. Chemical bonds in these molecules can be both polar (HCl, H 2 O) and non-polar (N 2 , O 2). Despite the fact that atoms within molecules are bound by very strong covalent bonds, weak forces of intermolecular attraction act between the molecules themselves. Therefore, substances with molecular crystal lattices have low hardness, low melting points, and are volatile. Most solid organic compounds have molecular crystal lattices (naphthalene, glucose, sugar).
Molecular crystal lattice (carbon dioxide) Metallic crystal lattices
Substances with metallic bond have metallic crystal lattices. At the nodes of such lattices are atoms and ions(either atoms, or ions, into which metal atoms easily turn, giving their outer electrons “for general use”). Such an internal structure of metals determines their characteristic physical properties: malleability, plasticity, electrical and thermal conductivity, and a characteristic metallic luster.
cheat sheets
The structure of matter is determined not only by the mutual arrangement of atoms in chemical particles, but also by the location of these chemical particles in space. The most ordered arrangement of atoms, molecules and ions in crystals(from Greek " crystallos"- ice), where chemical particles (atoms, molecules, ions) are arranged in a certain order, forming a crystal lattice in space. Under certain conditions of formation, they can have the natural shape of regular symmetrical polyhedra. The crystalline state is characterized by the presence of a long-range order in the arrangement of particles and symmetry crystal lattice.
The amorphous state is characterized by the presence of only short-range order. The structures of amorphous substances resemble liquids, but they have much less fluidity. The amorphous state is usually unstable. Under the action of mechanical loads or when the temperature changes, amorphous bodies can crystallize. The reactivity of substances in the amorphous state is much higher than in the crystalline state.
Amorphous substances
main feature amorphous(from Greek " amorphos"- formless) state of matter - the absence of an atomic or molecular lattice, that is, a three-dimensional periodicity of the structure characteristic of the crystalline state.
When a liquid substance is cooled, it does not always crystallize. under certain conditions, a non-equilibrium solid amorphous (glassy) state can form. The glassy state can contain simple substances (carbon, phosphorus, arsenic, sulfur, selenium), oxides (for example, boron, silicon, phosphorus), halides, chalcogenides, and many organic polymers.
In this state, the substance can be stable for a long period of time, for example, the age of some volcanic glasses is estimated at millions of years. The physical and chemical properties of a substance in a glassy amorphous state can differ significantly from the properties of a crystalline substance. For example, glassy germanium dioxide is chemically more active than crystalline. Differences in the properties of the liquid and solid amorphous state are determined by the nature of the thermal motion of particles: in the amorphous state, particles are only capable of oscillatory and rotational motions, but cannot move in the thickness of the substance.
There are substances that in solid form can only be in an amorphous state. This applies to polymers with an irregular sequence of units.
Amorphous bodies isotropic, that is, their mechanical, optical, electrical and other properties do not depend on direction. Amorphous bodies do not have a fixed melting point: melting occurs in a certain temperature range. The transition of an amorphous substance from a solid to a liquid state is not accompanied by an abrupt change in properties. A physical model of the amorphous state has not yet been created.
Crystalline substances
Solid crystals- three-dimensional formations characterized by strict repetition of the same element of the structure ( elementary cell) in all directions. The unit cell is the smallest volume of a crystal in the form of a parallelepiped, repeated in the crystal an infinite number of times.
The geometrically correct shape of crystals is primarily due to their strictly regular internal structure. If, instead of atoms, ions or molecules in a crystal, we represent points as the centers of gravity of these particles, then we get a three-dimensional regular distribution of such points, called the crystal lattice. The points themselves are called knots crystal lattice.
Types of crystal lattices
Depending on which particles the crystal lattice is built from and what is the nature of the chemical bond between them, different types of crystals are distinguished.
Ionic crystals are formed by cations and anions (for example, salts and hydroxides of most metals). They have an ionic bond between the particles.
Ionic crystals can be monatomic ions. This is how crystals are built sodium chloride, potassium iodide, calcium fluoride.
In the formation of ionic crystals of many salts, monatomic metal cations and polyatomic anions, for example, NO 3 - nitrate ion, SO 4 2 - sulfate ion, CO 3 2 - carbonate ion, participate in the formation of ionic crystals.
In an ionic crystal, it is impossible to isolate single molecules. Each cation is attracted to each anion and repelled by other cations. The whole crystal can be considered a huge molecule. The size of such a molecule is not limited, since it can grow by adding new cations and anions.
Most ionic compounds crystallize according to one of the structural types, which differ from each other in the value of the coordination number, that is, the number of neighbors around a given ion (4, 6 or 8). For ionic compounds with an equal number of cations and anions, four main types of crystal lattices are known: sodium chloride (the coordination number of both ions is 6), cesium chloride (the coordination number of both ions is 8), sphalerite and wurtzite (both structural types are characterized by the coordination number of the cation and anion equal to 4). If the number of cations is half the number of anions, then the coordination number of cations must be twice the coordination number of anions. In this case, the structural types of fluorite (coordination numbers 8 and 4), rutile (coordination numbers 6 and 3), and cristobalite (coordination numbers 4 and 2) are realized.
Typically, ionic crystals are hard but brittle. Their brittleness is due to the fact that even with a slight deformation of the crystal, cations and anions are displaced in such a way that the repulsive forces between like ions begin to prevail over the forces of attraction between cations and anions, and the crystal is destroyed.
Ionic crystals have high melting points. In the molten state, the substances that form ionic crystals are electrically conductive. When dissolved in water, these substances dissociate into cations and anions, and the resulting solutions conduct an electric current.
High solubility in polar solvents, accompanied by electrolytic dissociation, is due to the fact that in a solvent medium with a high dielectric constant ε, the attraction energy between ions decreases. The dielectric constant of water is 82 times higher than that of vacuum (conditionally existing in an ionic crystal), the attraction between ions in an aqueous solution decreases by the same amount. The effect is enhanced by the solvation of ions.
Atomic crystals are made up of individual atoms held together by covalent bonds. Of the simple substances, only boron and the elements of the IVA group have such crystal lattices. Often, compounds of non-metals with each other (for example, silicon dioxide) also form atomic crystals.
Just like ionic crystals, atomic crystals can be considered giant molecules. They are very strong and hard and do not conduct heat and electricity well. Substances that have atomic crystal lattices melt at high temperatures. They are practically insoluble in any solvents. They are characterized by low reactivity.
Molecular crystals are built from individual molecules, within which the atoms are connected by covalent bonds. Weaker intermolecular forces act between molecules. They are easily destroyed, so molecular crystals have low melting points, low hardness, and high volatility. Substances that form molecular crystal lattices do not have electrical conductivity, their solutions and melts also do not conduct electric current.
Intermolecular forces arise due to the electrostatic interaction of negatively charged electrons of one molecule with positively charged nuclei of neighboring molecules. The strength of intermolecular interaction is influenced by many factors. The most important among them is the presence of polar bonds, that is, the shift of electron density from one atom to another. In addition, intermolecular interaction is more pronounced between molecules with a large number of electrons.
Most non-metals in the form of simple substances (for example, iodine I 2 , argon Ar, sulfur S 8) and compounds with each other (for example, water, carbon dioxide, hydrogen chloride), as well as almost all organic solids form molecular crystals.
Metals have a metallic crystal lattice. It has a metallic bond between atoms. In metal crystals, the nuclei of atoms are arranged in such a way that their packing is as dense as possible. The bond in such crystals is delocalized and extends to the entire crystal. Metal crystals have high electrical and thermal conductivity, metallic luster and opacity, and easy deformability.
The classification of crystal lattices corresponds to limiting cases. Most crystals of inorganic substances belong to intermediate types - covalent-ionic, molecular-covalent, etc. For example, in a crystal graphite inside each layer, the bonds are covalent-metal, and between the layers - intermolecular.
Isomorphism and polymorphism
Many crystalline substances have the same structures. At the same time, the same substance can form different crystal structures. This is reflected in the phenomena isomorphism And polymorphism.
isomorphism is the ability of atoms, ions or molecules to replace each other in crystal structures. This term (from Greek " isos" - equal and " morphe"- form) was proposed by E. Mitscherlich in 1819. The law of isomorphism would be formulated by E. Mitscherlich in 1821 in this way: "The same number of atoms, connected in the same way, give the same crystalline forms; in this case, the crystalline form does not depend on the chemical nature of the atoms, but is determined only by their number and relative position.
While working in the chemical laboratory of the University of Berlin, Mitscherlich drew attention to the complete similarity of the crystals of lead, barium and strontium sulfates and the proximity of the crystalline forms of many other substances. His observations attracted the attention of the famous Swedish chemist J.-J. Berzelius, who suggested that Micherlich confirm the observed patterns using the example of compounds of phosphoric and arsenic acids. As a result of the study, it was concluded that "the two series of salts differ only in that one contains arsenic as an acid radical, and the other - phosphorus." Mitscherlich's discovery very soon attracted the attention of mineralogists, who began research on the problem of isomorphic substitution of elements in minerals.
In the case of joint crystallization of substances prone to isomorphism ( isomorphic substances), mixed crystals (isomorphic mixtures) are formed. This is possible only if the particles replacing each other differ little in size (no more than 15%). In addition, isomorphic substances must have a similar spatial arrangement of atoms or ions and, therefore, crystals similar in external form. Such substances include, for example, alum. In crystals of potassium alum KAl (SO 4) 2 . 12H 2 O potassium cations can be partially or completely replaced by rubidium or ammonium cations, and aluminum cations by chromium(III) or iron(III) cations.
Isomorphism is widespread in nature. Most minerals are isomorphic mixtures of complex variable composition. For example, in the mineral sphalerite ZnS, up to 20% of zinc atoms can be replaced by iron atoms (in this case, ZnS and FeS have different crystal structures). Isomorphism is associated with the geochemical behavior of rare and trace elements, their distribution in rocks and ores, where they are contained in the form of isomorphic impurities.
Isomorphic substitution determines many useful properties of artificial materials of modern technology - semiconductors, ferromagnets, laser materials.
Many substances can form crystalline forms that have different structures and properties, but the same composition ( polymorphic modifications). Polymorphism- the ability of solids and liquid crystals to exist in two or more forms with different crystal structures and properties with the same chemical composition. This word comes from the Greek polymorphos"- diverse. The phenomenon of polymorphism was discovered by M. Klaproth, who in 1798 discovered that two different minerals - calcite and aragonite - have the same chemical composition of CaCO 3.
Polymorphism of simple substances is usually called allotropy, while the concept of polymorphism does not apply to non-crystalline allotropic forms (for example, gaseous O 2 and O 3). A typical example of polymorphic forms is carbon modifications (diamond, lonsdaleite, graphite, carbines and fullerenes), which differ sharply in properties. The most stable form of existence of carbon is graphite, however, its other modifications under normal conditions can be preserved for an arbitrarily long time. At high temperatures, they turn into graphite. In the case of diamond, this occurs when heated above 1000° C. in the absence of oxygen. The reverse transition is much more difficult. Not only a high temperature (1200-1600 o C) is necessary, but also a gigantic pressure - up to 100 thousand atmospheres. The transformation of graphite into diamond is easier in the presence of molten metals (iron, cobalt, chromium, and others).
In the case of molecular crystals, polymorphism manifests itself in a different packing of molecules in a crystal or in a change in the shape of molecules, and in ionic crystals, in a different mutual arrangement of cations and anions. Some simple and complex substances have more than two polymorphs. For example, silicon dioxide has ten modifications, calcium fluoride has six, and ammonium nitrate has four. Polymorphic modifications are usually denoted by Greek letters α, β, γ, δ, ε, ... starting from modifications that are stable at low temperatures.
During crystallization from a vapor, solution or melt of a substance that has several polymorphic modifications, a modification is first formed that is less stable under the given conditions, which then turns into a more stable one. For example, when phosphorus vapor condenses, white phosphorus is formed, which under normal conditions slowly, and when heated, quickly turns into red phosphorus. When lead hydroxide is dehydrated, at first (about 70 o C) yellow β-PbO, which is less stable at low temperatures, is formed, at about 100 o C it turns into red α-PbO, and at 540 o C - again into β-PbO.
The transition of one polymorphic modification to another is called polymorphic transformations. These transitions occur with a change in temperature or pressure and are accompanied by an abrupt change in properties.
The process of transition from one modification to another can be reversible or irreversible. So, when a white soft graphite-like substance of composition BN (boron nitride) is heated at 1500-1800 o C and a pressure of several tens of atmospheres, its high-temperature modification is formed - borazon, close to diamond in hardness. When the temperature and pressure are lowered to values corresponding to ordinary conditions, the borazone retains its structure. An example of a reversible transition is the mutual transformations of two sulfur modifications (rhombic and monoclinic) at 95 o C.
Polymorphic transformations can also take place without a significant change in the structure. Sometimes there is no change in the crystal structure at all, for example, during the transition of α-Fe to β-Fe at 769 o C, the structure of iron does not change, but its ferromagnetic properties disappear.
Solid crystals can be thought of as three-dimensional structures in which the same structure is clearly repeated in all directions. The geometrically correct shape of crystals is due to their strictly regular internal structure. If the centers of attraction, ions or molecules in a crystal are depicted as points, then we get a three-dimensional regular distribution of such points, which is called the crystal lattice, and the points themselves are the nodes of the crystal lattice. A certain external shape of crystals is a consequence of their internal structure, which is associated precisely with the crystal lattice.
A crystal lattice is an imaginary geometric image for analyzing the structure of crystals, which is a volume-spatial mesh structure, at the nodes of which atoms, ions or molecules of a substance are located.
The following parameters are used to characterize the crystal lattice:
- crystal lattice E cr [KJ / mol] is the energy released during the formation of 1 mole of a crystal from microparticles (atoms, molecules, ions) that are in a gaseous state and are separated from each other by such a distance that the possibility of their interaction is excluded.
- Crystal lattice constant d is the smallest distance between the centers of two particles in adjacent nodes of the crystal lattice connected by .
- coordination number is the number of nearest particles that surround the central particle in space and are combined with it by a chemical bond.
The basis of the crystal lattice is the elementary cell, which is repeated in the crystal an infinite number of times.
The unit cell is the smallest structural unit of the crystal lattice, which reveals all the properties of its symmetry.
Simplified, the unit cell can be defined as a small part of the crystal lattice, which still reveals the characteristic features of its crystals. The features of an elementary cell are described using three Breve rules:
- the symmetry of the unit cell must correspond to the symmetry of the crystal lattice;
- elementary cell must have the maximum number of identical edges A,b, With and equal angles between them a, b, g. ;
- subject to the first two rules, an elementary cell should occupy a minimum volume.
To describe the shape of crystals, a system of three crystallographic axes is used a, b, c, which differ from the usual coordinate axes in that they are segments of a certain length, the angles between which a, b, g can be both direct and indirect.
Crystal structure model: a) crystal lattice with a selected unit cell; b) unit cell with notation of facet angles
The shape of a crystal is studied by the science of geometric crystallography, one of the main provisions of which is the law of constancy of face angles: for all crystals of a given substance, the angles between the corresponding faces always remain the same.
If we take a large number of elementary cells and fill a certain volume with them tightly to each other, while maintaining the parallelism of faces and edges, then a single crystal of an ideal structure is formed. But in practice, polycrystals are most often encountered, in which regular structures exist within certain limits, along which the orientation of the regularity changes dramatically.
Depending on the ratio of the lengths of the edges a, b, c and the angles a, b, g between the faces of the unit cell, seven systems are distinguished - the so-called syngonies of crystals. However, an elementary cell can also be constructed in such a way that it has additional nodes that are placed inside its volume or on all its faces - such lattices are called body-centered and face-centered, respectively. If additional nodes are only on two opposite faces (upper and lower), then this is a base-centered lattice. Taking into account the possibility of additional nodes, there are only 14 types of crystal lattices.
The external shape and features of the internal structure of crystals are determined by the principle of dense “packing”: the most stable, and therefore the most probable structure will be the one that corresponds to the densest arrangement of particles in the crystal and in which the smallest free space remains.
Types of crystal lattices
Depending on the nature of the particles contained in the nodes of the crystal lattice, as well as on the nature of the chemical bonds between them, there are four main types of crystal lattices.
Ionic lattices
Ionic lattices are built from ions of different names located at lattice sites and bound by electrostatic attraction forces. Therefore, the structure of the ionic crystal lattice should ensure its electrical neutrality. Ions can be simple (Na +, Cl -) or complex (NH 4 +, NO 3 -). Due to the unsaturation and non-directionality of the ionic bond, ionic crystals are characterized by large coordination numbers. So, in NaCl crystals, the coordination numbers of Na + and Cl - ions is 6, and of Cs + and Cl ions - in a CsCl crystal - 8, since one Cs + ion is surrounded by eight Cl - ions, and each Cl ion is surrounded by eight Cs ions, respectively. + . Ionic crystal lattices are formed by a large number of salts, oxides and bases.
Examples of ionic crystal lattices: a) NaCl; b) CsCl Substances with ionic crystal lattices have a relatively high hardness, they are quite refractory, non-volatile. Unlike ionic compounds, they are very fragile, therefore, even a slight shift in the crystal lattice brings like-charged ions closer to each other, repulsion between which leads to the breaking of ionic bonds and, as a result, to the appearance of cracks in the crystal or to its destruction. In the solid state, substances with an ionic crystal lattice are dielectrics and do not conduct electricity. However, during melting or dissolution in polar solvents, the geometrically correct orientation of ions relative to each other is violated, first weakened, and then chemical bonds are destroyed, so the properties also change. As a consequence, both melts of ionic crystals and their solutions begin to conduct electric current.
Atomic lattices
These lattices are built from atoms connected to each other. They, in turn, are divided into three types: frame, layered and chain structures.
frame structure has, for example, diamond - one of the hardest substances. Thanks to the sp 3 hybridization of the carbon atom, a three-dimensional lattice is built, which consists exclusively of carbon atoms connected by covalent non-polar bonds, the axes of which are located at the same bond angles (109.5 o).
Skeleton structure of the atomic crystal lattice of diamond Layered structures can be viewed as huge two-dimensional molecules. Layered structures are characterized by covalent bonds within each layer and a weak van der Waals interaction between adjacent layers.
Layered structures of atomic crystal lattices: a) CuCl 2 ; b) PbO. Elementary cells are selected on models using the outlines of parallelepipeds A classic example of a substance with a layered structure is graphite, in which each carbon atom is in a state of sp 2 hybridization and forms three covalent s-bonds with three other C atoms in one plane. The fourth valence electrons of each carbon atom are unhybridized; very weak van der Waals bonds between the layers. Therefore, when even a small effort is applied, the individual layers easily begin to slide along each other. This explains, for example, the property of graphite to write. Unlike diamond, graphite conducts electricity well: under the influence of an electric field, nonlocalized electrons can move along the plane of the layers, and, conversely, graphite almost does not conduct electric current in the perpendicular direction.
Layered structure of the atomic crystal lattice of graphite Chain structures typical, for example, for sulfur oxide (SO 3) n, cinnabar HgS, beryllium chloride BeCl 2, as well as for many amorphous polymers and for some silicate materials, such as asbestos.
Chain structure of the atomic crystal lattice of HgS: a) side projection b) frontal projection There are relatively few substances with an atomic structure of crystal lattices. These are, as a rule, simple substances formed by elements of the IIIA and IVA subgroups (Si, Ge, B, C). Often, compounds of two different non-metals have atomic lattices, for example, some polymorphic modifications of quartz (silicon oxide SiO 2) and carborundum (silicon carbide SiC).
All atomic crystals are characterized by high strength, hardness, refractoriness and insolubility in almost any solvent. Such properties are due to the strength of the covalent bond. Substances with an atomic crystal lattice have a wide range of electrical conductivity from insulators and semiconductors to electronic conductors.
Atomic crystal lattices of some polymorphic modifications of carborundum - silicon carbide SiC Metal gratings
These crystal lattices contain metal atoms and ions at the nodes, between which electrons common to all of them (electron gas) move freely, which form a metallic bond. A feature of the crystal lattices of metals lies in large coordination numbers (8-12), which indicate a significant packing density of metal atoms. This is explained by the fact that the "skeletons" of atoms, devoid of external electrons, are placed in space as balls of the same radius. For metals, three types of crystal lattices are most common: face-centered cubic with a coordination number of 12, body-centered cubic with a coordination number of 8, and hexagonal, close-packed with a coordination number of 12.
The special characteristics of metallic bonds and metal lattices determine such important properties of metals as high melting points, electrical and thermal conductivity, malleability, ductility, and hardness.
Metal crystal lattices: a) body-centered cubic (Fe, V, Nb, Cr) b) face-centered cubic (Al, Ni, Ag, Cu, Au) c) hexagonal (Ti, Zn, Mg, Cd) Molecular lattices
Molecular crystal lattices contain molecules at the nodes, interconnected by weak intermolecular forces - van der Waals or hydrogen bonds. For example, ice consists of water molecules held in a crystal lattice by hydrogen bonds. The crystal lattices of many substances converted to a solid state belong to the same type, for example: simple substances H 2, O 2, N 2, O 3, P 4, S 8, halogens (F 2, Cl 2, Br 2, I 2 ), "dry ice" CO 2 , all noble gases and most organic compounds.
Molecular crystal lattices: a) iodine I2; b) ice H2O Since the forces of intermolecular interaction are weaker than the forces of a covalent or metallic bond, molecular crystals have little hardness; they are fusible and volatile, insoluble in and do not show electrical conductivity.
Most substances are characterized by the ability, depending on the conditions, to be in one of three states of aggregation: solid, liquid or gaseous.
For example, water at normal pressure in the temperature range of 0-100 o C is a liquid, at temperatures above 100 o C it can only exist in a gaseous state, and at temperatures below 0 o C it is a solid.
Substances in the solid state distinguish between 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.
Nevertheless, crystalline substances have a specific melting point, i.e. a substance with a crystalline structure passes from a solid state to a liquid not gradually, but abruptly, when a specific temperature is reached. Examples of crystalline substances include table salt, sugar, 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 crystalline state, the easiest way to understand is 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, one mentally connects two atoms close to each other with a straight line, then one can find that the same particles will lie on this line at strictly defined intervals:
Thus, in the case of crystalline substances, one can speak of such a concept as a crystal lattice.
crystal lattice called a spatial frame connecting the points of space in which there are particles that form a crystal.
The points in space where the particles that form the crystal are located are called lattice nodes .
Depending on which particles are in the nodes of the crystal lattice, there are: molecular, atomic, ionic And metal crystal lattice .
in knots molecular crystal lattice
The crystal lattice of ice as an example of a molecular latticethere are molecules within which the atoms are bound 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, do not conduct electric current, and can either dissolve or not dissolve in various solvents. Solutions of such compounds may or may not conduct electricity, depending on the class of the compound. Compounds with a molecular crystal lattice include many simple substances - non-metals (hardened H 2, O 2, Cl 2, rhombic 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 substances. It should be noted that if the substance is in a gaseous or liquid state, it is inappropriate to talk about the molecular crystal lattice: it is more correct to use the term - the molecular type of structure.
The crystal lattice of diamond as an example of an atomic latticein knots atomic crystal lattice
there are atoms. In this case, all the nodes of such a crystal lattice are "crosslinked" to each other by means of strong covalent bonds into a single crystal. In fact, such a crystal is one giant molecule. Due to 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 from simple substances include boron B, carbon C (diamond and graphite), silicon Si, from complex substances - silicon dioxide SiO 2 (quartz), silicon carbide SiC, boron nitride BN.
For substances with ionic crystal lattice
at the lattice sites are ions connected to each other through ionic bonds.
Since ionic bonds are strong enough, substances with an ionic lattice have a relatively high hardness and refractoriness. Most often, they are soluble in water, and their solutions, like melts, conduct electricity.
Substances with an ionic type of crystal lattice include metal and ammonium salts (NH 4 +), bases, metal oxides. A true sign of the ionic structure of a substance is the presence in its composition of both atoms of a typical metal and non-metal.
The 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, at its nodes are 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 instead “release” some of their electrons, turning, in turn, into cations. In fact, "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, often have high ductility (ductility).
The scatter in the values of the melting temperatures of metals is very large. So, for example, the melting point of mercury is approximately minus 39 ° C (liquid under normal conditions), and tungsten - 3422 ° C. It should be noted that under normal conditions, all metals except mercury are solids.
As we already know, matter can exist in three states of aggregation: gaseous, solid And liquid. Oxygen, which under normal conditions is in a gaseous state, at a temperature of -194 ° C is converted into a bluish liquid, and at a temperature of -218.8 ° C it turns into a snowy mass with blue crystals.
The temperature interval for the existence of a substance in the solid state is determined by the boiling and melting points. Solids are crystalline And amorphous.
At amorphous substances there is no fixed melting point - when heated, they gradually soften and become fluid. In this state, for example, there are various resins, plasticine.
Crystalline substances differ in the regular arrangement of the particles of which they are composed: atoms, molecules and ions, at strictly defined points in space. When these points are connected by straight lines, a spatial frame is created, it is called a crystal lattice. The points where the crystal particles are located are called lattice nodes.
At the nodes of the lattice we imagine, there can be ions, atoms and molecules. These particles oscillate. When the temperature increases, the scope of these fluctuations also increases, which leads to thermal expansion of the bodies.
Depending on the type of particles located in the nodes of the crystal lattice, and the nature of the connection between them, four types of crystal lattices are distinguished: ionic, atomic, molecular And metal.
Ionic called such crystal lattices, at the nodes of which ions are located. They are formed by substances with an ionic bond, which can be associated with 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 there is an ionic chemical bond. Let's consider a crystal of sodium chloride, it consists of positively alternating Na+ and negative CL- ions, together they form a lattice in the form of a cube. The bonds between ions in such a crystal are extremely stable. Because of this, substances with an ionic lattice have a relatively high strength and hardness, they are refractory and non-volatile.
nuclear crystal lattices are called such crystal lattices, at the nodes of which there are individual atoms. In such lattices, atoms are interconnected 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 that contain silicon oxide (IV) - 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 called such crystal lattices, at the nodes of which molecules are located. Chemical bonds in these molecules can also be either 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 are 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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