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).
The structure of matter is determined not only by the relative arrangement of atoms in chemical particles, but also by the location of these chemical particles in space. The most ordered arrangement of atoms, molecules and ions is in crystals(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 of only oscillatory and rotational movements, but cannot move within 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 structures. 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.
Back forward
Attention! Slide previews are for informational purposes only and may not represent all the features of the presentation. If you are interested in this work, please download the full version.
Lesson type: Combined.
The main goal of the lesson: To give students specific ideas about amorphous and crystalline substances, types of crystal lattices, to establish the relationship between the structure and properties of substances.
Lesson objectives.
Educational: to form concepts about the crystalline and amorphous state of solids, to familiarize students with various types of crystal lattices, to establish the dependence of the physical properties of a crystal on the nature of the chemical bond in the crystal and the type of crystal lattice, to give students basic ideas about the influence of the nature of chemical bonds and types of crystal lattices on properties of matter, give students an idea of the law of constancy of composition.
Educational: continue to form the worldview of students, consider the mutual influence of the components of whole-structural particles of substances, as a result of which new properties appear, develop the ability to organize their educational work, and observe the rules of working in a team.
Developmental: develop the cognitive interest of schoolchildren using problem situations; improve students’ abilities to establish the cause-and-effect dependence of the physical properties of substances on chemical bonds and the type of crystal lattice, to predict the type of crystal lattice based on the physical properties of the substance.
Equipment: Periodic table of D.I. Mendeleev, collection “Metals”, non-metals: sulfur, graphite, red phosphorus, oxygen; Presentation “Crystal lattices”, models of crystal lattices of different types (table salt, diamond and graphite, carbon dioxide and iodine, metals), samples of plastics and products made from them, glass, plasticine, resins, wax, chewing gum, chocolate, computer, multimedia installation, video experiment “Sublimation of benzoic acid”.
During the classes
1. Organizational moment.
The teacher welcomes students and records those who are absent.
Then he tells the topic of the lesson and the purpose of the lesson. Students write down the topic of the lesson in their notebook. (Slide 1, 2).
2. Checking homework
(2 students at the blackboard: Determine the type of chemical bond for substances with the formulas:
1) NaCl, CO 2, I 2; 2) Na, NaOH, H 2 S (write the answer on the board and include it in the survey).
3. Analysis of the situation.
Teacher: What does chemistry study? Answer: Chemistry is the science of substances, their properties and transformations of substances.
Teacher: What is a substance? Answer: Matter is what the physical body is made of. (Slide 3).
Teacher: What states of matter do you know?
Answer: There are three states of aggregation: solid, liquid and gaseous. (Slide 4).
Teacher: Give examples of substances that can exist in all three states of aggregation at different temperatures.
Answer: Water. Under normal conditions, water is in a liquid state, when the temperature drops below 0 0 C, water turns into a solid state - ice, and when the temperature rises to 100 0 C we get water vapor (gaseous state).
Teacher (addition): Any substance can be obtained in solid, liquid and gaseous form. In addition to water, these are metals that, under normal conditions, are in a solid state, when heated, they begin to soften, and at a certain temperature (t pl) they turn into a liquid state - they melt. With further heating, to the boiling point, the metals begin to evaporate, i.e. go into a gaseous state. Any gas can be converted into a liquid and solid state by lowering the temperature: for example, oxygen, which at a temperature (-194 0 C) turns into a blue liquid, and at a temperature (-218.8 0 C) solidifies into a snow-like mass consisting of blue crystals. Today in class we will look at the solid state of matter.
Teacher: Name what solid substances are on your tables.
Answer: Metals, plasticine, table salt: NaCl, graphite.
Teacher: What do you think? Which of these substances is excess?
Answer: Plasticine.
Teacher: Why?
Assumptions are made. If students find it difficult, then with the help of the teacher they come to the conclusion that plasticine, unlike metals and sodium chloride, does not have a certain melting point - it (plasticine) gradually softens and turns into a fluid state. Such, for example, is chocolate that melts in the mouth, or chewing gum, as well as glass, plastics, resins, wax (when explaining, the teacher shows the class samples of these substances). Such substances are called amorphous. (slide 5), and metals and sodium chloride are crystalline. (Slide 6).
Thus, two types of solids are distinguished : amorphous and crystalline. (slide7).
1) Amorphous substances do not have a specific melting point and the arrangement of particles in them is not strictly ordered.
Crystalline substances have a strictly defined melting point and, most importantly, are characterized by the correct arrangement of the particles from which they are built: atoms, molecules and ions. These particles are located at strictly defined points in space, and if these nodes are connected by straight lines, then a spatial frame is formed - crystal cell.
The teacher asks problematic issues
How to explain the existence of solids with such different properties?
2) Why do crystalline substances split in certain planes upon impact, while amorphous substances do not have this property?
Listen to the students' answers and lead them to conclusion:
The properties of substances in the solid state depend on the type of crystal lattice (primarily on what particles are in its nodes), which, in turn, is determined by the type of chemical bond in a given substance.
Checking homework:
1) NaCl – ionic bond,
CO 2 – covalent polar bond
I 2 – covalent nonpolar bond
2) Na – metal bond
NaOH - ionic bond between Na + ion - (O and H covalent)
H 2 S - covalent polar
Frontal survey.
- Which bond is called ionic?
- What kind of bond is called covalent?
- Which bond is called a polar covalent bond? non-polar?
- What is electronegativity called?
Conclusion: There is a logical sequence, the relationship of phenomena in nature: Structure of the atom -> EO -> Types of chemical bonds -> Type of crystal lattice -> Properties of substances . (slide 10).
Teacher: Depending on the type of particles and the nature of the connection between them, they distinguish four types of crystal lattices: ionic, molecular, atomic and metallic. (Slide 11).
The results are presented in the following table - a sample table at the students’ desks. (see Appendix 1). (Slide 12).
Ionic crystal lattices
Teacher: What do you think? For substances with what type of chemical bond will this type of lattice be characteristic?
Answer: Substances with ionic chemical bonds will be characterized by an ionic lattice.
Teacher: What particles will be at the lattice nodes?
Answer: Jonah.
Teacher: What particles are called ions?
Answer: Ions are particles that have a positive or negative charge.
Teacher: What are the compositions of ions?
Answer: Simple and complex.
Demonstration - model of sodium chloride (NaCl) crystal lattice.
Teacher's explanation: At the nodes of the sodium chloride crystal lattice there are sodium and chlorine ions.
In NaCl crystals there are no individual sodium chloride molecules. The entire crystal should be considered as a giant macromolecule consisting of an equal number of Na + and Cl - ions, Na n Cl n, where n is a large number.
The bonds between ions in such a crystal are very strong. Therefore, substances with an ionic lattice have a relatively high hardness. They are refractory, non-volatile, and fragile. Their melts conduct electric current (Why?) and easily dissolve in water.
Ionic compounds are binary compounds of metals (I A and II A), salts, and alkalis.
Atomic crystal lattices
Demonstration of crystal lattices of diamond and graphite.
The students have graphite samples on the table.
Teacher: What particles will be located at the nodes of the atomic crystal lattice?
Answer: At the nodes of the atomic crystal lattice there are individual atoms.
Teacher: What chemical bond will arise between atoms?
Answer: Covalent chemical bond.
Teacher's explanations.
Indeed, at the sites of atomic crystal lattices there are individual atoms connected to each other by covalent bonds. Since atoms, like ions, can be arranged differently in space, crystals of different shapes are formed.
Atomic crystal lattice of diamond
There are no molecules in these lattices. The entire crystal should be considered as a giant molecule. An example of substances with this type of crystal lattices are allotropic modifications of carbon: diamond, graphite; as well as boron, silicon, red phosphorus, germanium. Question: What are these substances in composition? Answer: Simple in composition.
Atomic crystal lattices have not only simple, but also complex ones. For example, aluminum oxide, silicon oxide. All these substances have very high melting points (diamond has over 3500 0 C), are strong and hard, non-volatile, and practically insoluble in liquids.
Metal crystal lattices
Teacher: Guys, you have a collection of metals on your tables, let’s look at these samples.
Question: What chemical bond is characteristic of metals?
Answer: Metal. Bonding in metals between positive ions through shared electrons.
Question: What general physical properties are characteristic of metals?
Answer: Luster, electrical conductivity, thermal conductivity, ductility.
Question: Explain what is the reason that so many different substances have the same physical properties?
Answer: Metals have a single structure.
Demonstration of models of metal crystal lattices.
Teacher's explanation.
Substances with metallic bonds have metallic crystal lattices
At the sites of such lattices there are atoms and positive ions of metals, and valence electrons move freely in the volume of the crystal. The electrons electrostatically attract positive metal ions. This explains the stability of the lattice.
Molecular crystal lattices
The teacher demonstrates and names the substances: iodine, sulfur.
Question: What do these substances have in common?
Answer: These substances are non-metals. Simple in composition.
Question: What is the chemical bond inside molecules?
Answer: The chemical bond inside molecules is covalent nonpolar.
Question: What physical properties are characteristic of them?
Answer: Volatile, fusible, slightly soluble in water.
Teacher: Let's compare the properties of metals and non-metals. Students answer that the properties are fundamentally different.
Question: Why are the properties of non-metals very different from the properties of metals?
Answer: Metals have metallic bonds, while non-metals have covalent, nonpolar bonds.
Teacher: Therefore, the type of lattice is different. Molecular.
Question: What particles are located at lattice points?
Answer: Molecules.
Demonstration of crystal lattices of carbon dioxide and iodine.
Teacher's explanation.
Molecular crystal lattice
As we see, not only solids can have a molecular crystal lattice. simple substances: noble gases, H 2, O 2, N 2, I 2, O 3, white phosphorus P 4, but also complex: solid water, solid hydrogen chloride and hydrogen sulfide. Most solid organic compounds have molecular crystal lattices (naphthalene, glucose, sugar).
The lattice sites contain nonpolar or polar molecules. Despite the fact that the atoms inside the molecules are connected by strong covalent bonds, weak intermolecular forces act between the molecules themselves.
Conclusion: The substances are fragile, have low hardness, a low melting point, are volatile, and are capable of sublimation.
Question : Which process is called sublimation or sublimation?
Answer : The transition of a substance from a solid state of aggregation directly to a gaseous state, bypassing the liquid state, is called sublimation or sublimation.
Demonstration of the experiment: sublimation of benzoic acid (video experiment).
Working with a completed table.
Appendix 1. (Slide 17)
Crystal lattices, type of bond and properties of substances
| Grille type | Types of particles at lattice sites |
Type of connection between particles | Examples of substances | Physical properties of substances |
| Ionic | Ions | Ionic – strong bond | Salts, halides (IA, IIA), oxides and hydroxides of typical metals | Solid, strong, non-volatile, brittle, refractory, many soluble in water, melts conduct electric current |
| Nuclear | Atoms | 1. Covalent nonpolar - the bond is very strong 2. Covalent polar - the bond is very strong |
Simple substances A: diamond(C), graphite(C), boron(B), silicon(Si).
Complex substances: aluminum oxide (Al 2 O 3), silicon oxide (IY)-SiO 2 |
Very hard, very refractory, durable, non-volatile, insoluble in water |
| Molecular | Molecules | Between molecules there are weak forces of intermolecular attraction, but inside the molecules there is a strong covalent bond | Solids under special conditions that under normal conditions are gases or liquids (O 2 , H 2 , Cl 2 , N 2 , Br 2 , H 2 O, CO 2, HCl); sulfur, white phosphorus, iodine; organic matter |
Fragile, volatile, fusible, capable of sublimation, have low hardness |
| Metal | Atom ions | Metal of different strengths | Metals and alloys | Malleable, shiny, ductile, thermally and electrically conductive |
Question: Which type of crystal lattice from those discussed above is not found in simple substances?
Answer: Ionic crystal lattices.
Question: What crystal lattices are characteristic of simple substances?
Answer: For simple substances - metals - a metal crystal lattice; for non-metals - atomic or molecular.
Working with the Periodic Table of D.I.Mendeleev.
Question: Where are the metal elements located in the Periodic Table and why? Non-metal elements and why?
Answer: If you draw a diagonal from boron to astatine, then in the lower left corner of this diagonal there will be metal elements, because at the last energy level they contain from one to three electrons. These are elements I A, II A, III A (except boron), as well as tin and lead, antimony and all elements of secondary subgroups.
Non-metal elements are located in the upper right corner of this diagonal, because at the last energy level they contain from four to eight electrons. These are the elements IY A, Y A, YI A, YII A, YIII A and boron.
Teacher: Let's find non-metal elements whose simple substances have an atomic crystal lattice (Answer: C, B, Si) and molecular ( Answer: N, S, O , halogens and noble gases ).
Teacher: Formulate a conclusion on how you can determine the type of crystal lattice of a simple substance depending on the position of the elements in D.I. Mendeleev’s Periodic Table.
Answer: For metal elements that are in I A, II A, IIIA (except for boron), as well as tin and lead, and all elements of secondary subgroups in a simple substance, the type of lattice is metal.
For the nonmetal elements IY A and boron in a simple substance, the crystal lattice is atomic; and the elements Y A, YI A, YII A, YIII A in simple substances have a molecular crystal lattice.
We continue to work with the completed table.
Teacher: Look carefully at the table. What pattern can be observed?
We listen carefully to the students’ answers, and then together with the class we draw the following conclusion:
There is the following pattern: if the structure of substances is known, then their properties can be predicted, or vice versa: if the properties of substances are known, then the structure can be determined. (Slide 18).
Teacher: Look carefully at the table. What other classification of substances can you suggest?
If the students find it difficult, the teacher explains that substances can be divided into substances of molecular and non-molecular structure. (Slide 19).
Substances with a molecular structure are made up of molecules.
Substances of non-molecular structure consist of atoms and ions.
Law of Constancy of Composition
Teacher: Today we will get acquainted with one of the basic laws of chemistry. This is the law of constancy of composition, which was discovered by the French chemist J.L. Proust. The law is valid only for substances of molecular structure. Currently, the law reads like this: “Molecular chemical compounds, regardless of the method of their preparation, have a constant composition and properties.” But for substances with a non-molecular structure this law is not always true.
The theoretical and practical significance of the law is that on its basis the composition of substances can be expressed using chemical formulas (for many substances of non-molecular structure, the chemical formula shows the composition of not a real existing, but a conditional molecule).
Conclusion: The chemical formula of a substance contains a lot of information.(Slide 21)
For example, SO 3:
1. The specific substance is sulfur dioxide, or sulfur oxide (YI).
2.Type of substance - complex; class - oxide.
3. Qualitative composition - consists of two elements: sulfur and oxygen.
4. Quantitative composition - the molecule consists of 1 sulfur atom and 3 oxygen atoms.
5.Relative molecular weight - M r (SO 3) = 32 + 3 * 16 = 80.
6. Molar mass - M(SO 3) = 80 g/mol.
7. Lots of other information.
Consolidation and application of acquired knowledge
(Slide 22, 23).
Tic-tac-toe game: cross out substances that have the same crystal lattice vertically, horizontally, diagonally.
Reflection.
The teacher asks the question: “Guys, what new did you learn in class?”
Summing up the lesson
Teacher: Guys, let's summarize the main results of our lesson - answer the questions.
1. What classifications of substances did you learn?
2. How do you understand the term crystal lattice?
3. What types of crystal lattices do you now know?
4. What regularities in the structure and properties of substances did you learn about?
5. In what state of aggregation do substances have crystal lattices?
6. What basic law of chemistry did you learn in class?
Homework: §22, notes.
1. Make up the formulas of the substances: calcium chloride, silicon oxide (IY), nitrogen, hydrogen sulfide.
Determine the type of crystal lattice and try to predict what the melting points of these substances should be.
2. Creative task -> make up questions for the paragraph.
The teacher thanks you for the lesson. Gives marks to students.
Crystal lattices
8TH GRADE
*According to the textbook: Gabrielyan O.S. Chemistry-8. M.: Bustard, 2003.
Goals. Educational. Give the concept of the crystalline and amorphous state of solids; get acquainted with the types of crystal lattices, their relationship with the types of chemical bonds and the effect on the physical properties of substances; give an idea of the law of constancy of the composition of substances.
Developmental. Develop logical thinking, observation skills and drawing conclusions.
Educational. To form aesthetic taste and collectivism, to broaden one’s horizons.
Equipment and reagents. Models of crystal lattices, filmstrip “Dependence of the properties of substances on composition and structure”, transparencies “Chemical bond. Structure of matter"; plasticine, chewing gum, resins, wax, table salt NaCl, graphite, sugar, water.
Form of work organization. Group.
Methods and techniques. Independent work, demonstration experience, laboratory work.
Epigraph.
DURING THE CLASSES
TEACHER. Crystals are found everywhere. We walk on crystals, build with crystals, create devices and products from crystals, widely use crystals in technology and science, eat crystals, heal with crystals, find crystals in living organisms, go out into the vastness of space roads with the help of devices made of crystals...
What are crystals?
Imagine for a moment that your eyes began to see atoms or molecules; the growth decreased and you were able to enter the crystal. The purpose of our lesson is to understand what the crystalline and amorphous states of solids are, to get acquainted with the types of crystal lattices, and to gain an understanding of the law of constancy of the composition of substances.
What aggregative states of substances are known? Solid, liquid and gaseous. They are interconnected (Scheme 1).
The Tale of Greedy Chlorine
In a certain kingdom, a chemical state, there lived Chlorine. And although he belonged to the ancient family of Halogens, and received a considerable inheritance (he had seven electrons at the external energy level), he was very greedy and envious, and even turned yellow-green from anger. Day and night he was tormented by the desire to become like Argon. He thought and thought and finally came up with: “Argon has eight electrons on the outer level, and I only have seven. So, I need to get one more electron, then I will also be noble.” The next day, Chlorus got ready to go on the road for the treasured electron, but he didn’t have to go far: near the house he met an atom that was like him like two peas in a pod.
“Listen, brother, give me your electron,” Chlorus spoke.
“No, you’d better give me an electron,” answered the twin.
“Okay, then let’s combine our electrons so that no one will be offended,” said the greedy Chlorine, hoping that later he would take the electron for himself.
But that was not the case: both atoms shared the same electrons equally, despite the desperate efforts of the greedy Chlorine to win them over to his side.
TEACHER. Look at the substances on your tables and divide them into two groups. Plasticine, chewing gum, resin, wax are amorphous substances. They often do not have a constant melting point, fluidity is observed, and there is no ordered structure (crystal lattice). On the contrary, salt NaCl , graphite and sugar are crystalline substances. They are characterized by clear melting temperatures, regular geometric shapes, and symmetry.
Both amorphous and crystalline substances are used. We will become familiar with the types of crystal lattices and their influence on the physical properties of substances. The creative tasks you have prepared - fairy tales - will help in repeating the types of chemical bonds.
A fairy tale about a polar covalent bond
In a certain kingdom, in a certain state called the “Periodic Table,” there lived a small electron. He had no friends. But one day another electronic device came to him in a village called “External Level”, exactly similar to the first one. They immediately became friends, always walked together and did not even notice how they ended up paired. These electrons are called covalent.
A fairy tale about ionic bonding
Two friends lived in the house of the periodic system of Mendeleev - the metal Na and the non-metal Cl. Each lived in his own apartment: Na - in apartment No. 11, and Cl - at No. 17.
And so the friends decided to join the circle, and there they were told: in order to enter this circle, they must complete the energy level. The friends got upset and trudged home. At home, they thought about how to complete the energy level. And suddenly Cl said:
- Come on, you give me one electron from your third level.
- That is, how will I give it? – Na asked.
- So, take it and give it to me. You will have two levels and all completed, and I will have three levels and also all completed. Then we will be accepted into the circle.
“Okay, take it,” Na said and gave away his electron.
When they came to the circle, the director of the circle asked: “How did you manage to do this?” They told him everything. The director said: “Well done, guys,” and accepted them into his circle. The director gave sodium a card with the sign “+1”, and chlorine – with the sign “-1”. And now he accepts everyone into the circle - metals and non-metals. And what Na and Cl did was what he called an ionic bond.
TEACHER. Do you have a good understanding of the types of chemical bonds? This knowledge will be useful when studying crystal lattices. The world of substances is large and diverse. They have a variety of properties. Distinguish between physical and chemical properties of substances. What properties do we classify as physical?
Student answers: state of aggregation, color, density, melting and boiling points, solubility in water, electrical conductivity.
TEACHER. Describe the physical properties of substances: O 2 , H 2 O, NaCl, graphite WITH.
Students fill out the table, which as a result takes on the following form.
Table
| Physical properties |
Substances | |||
|---|---|---|---|---|
| O 2 | H 2 O | NaCl | C | |
| State of aggregation | Gas | Liquid | Solid | Solid |
| Density, g/cm 3 | 1.429 (g/l) | 1,000 | 2,165 | 2,265 |
| Color | Colorless | Colorless | White | Black |
| t pl, °С | –218,8 | 0,0 | +801,0 | – |
| t kip, °С | –182,97 | +100 | +1465 | +3700 |
| Solubility in water | Slightly soluble | – | Let's dissolve | Insoluble |
| Electrical conductivity | Non-conductive | Weak | Conductor | Conductor |
TEACHER. Based on the physical properties of substances, their structure can be determined.
Transparency.
TEACHER.A crystal is a solid body whose particles (atoms, molecules, ions) are arranged in a certain, periodically repeating order (at nodes). When mentally connecting nodes with lines, a spatial framework is formed - a crystal lattice. There are four types of crystal lattices (Scheme 2, see p. 24 ).
Scheme 2
CRYSTAL LATTICES
TEACHER. What crystal lattices do O 2, H 2 O, NaCl, C ?
Students' answer. O 2 and H 2 O are molecular crystal lattices, NaCl is an ionic lattice,
C – atomic lattice.
Demonstration of crystal lattice models:
NaCl, C (graphite), Mg, CO 2.
TEACHER.Pay attention to the types of crystal lattices of simple substances depending on their position in the periodic table (p. 79 of the textbook).
What type of lattice is not found in simple substances?
Students' answer. Simple substances do not have ionic lattices.
 |
|
TEACHER. Substances with a molecular lattice are characterized by the phenomenon of sublimation or sublimation.
Demonstration experience.
Sublimation of benzoic acid or naphthalene. (Sublimation is the transformation (when heated) of a solid into a gas, bypassing the liquid phase, and then crystallizing again in the form of frost.)
TEACHER.Substances with a molecular structure obey the law of constancy of the composition of the substance; substances of molecular structure have a constant composition regardless of the method of their preparation. The law was discovered by J.L. Proust. He resolved the long dispute between K.L. Berthollet and J. Dalton in favor of the former.
For example, carbon dioxide or carbon monoxide (IV) CO2 - a complex substance of molecular structure. It consists of two elements: carbon and oxygen, and the molecule contains one carbon atom and two oxygen atoms. Relative molecular weight M r ( CO2 ) = 44, molar mass M( CO2 ) = 44 g/mol. Molar volume V M ( CO2 ) = 22.4 mol (n.s.). Number of molecules in 1 mole of substance N A ( CO2 ) = 6 10 23 molecules.
For substances with an ionic structure, Proust's law is not always satisfied.
Graphic dictation
“Types of chemical bonds and types of crystal lattices”
The signs “+” and “–” indicate whether this statement (1–20) is typical for the type of chemical bond of the specified option.
Option 1.
Ionic bond.
Option 2.
Covalent nonpolar bond.
Option 3.
Covalent polar bond.
Statements.
1. Bonds are formed between metal and non-metal atoms.
2. Bonds are formed between metal atoms.
3. Bonds are formed between non-metal atoms.
4. During the interaction of atoms, ions are formed.
5. The resulting molecules are polarized.
6. Bonding is established by pairing electrons without shifting shared electron pairs.
7. A bond is established by pairing electrons and shifting a common pair to one of the atoms.
8. During a chemical reaction, a complete transfer of valence electrons occurs from one atom of the reacting elements to another.
9. The oxidation state of atoms in a molecule is zero.
10. The oxidation states of atoms in a molecule are equal to the number of electrons given or received.
11. The oxidation states of atoms in a molecule are equal to the number of displaced common electron pairs.
12. Compounds with this type of bond form an ionic type crystal lattice.
13. Compounds with this type of chemical bond are characterized by crystalline lattices of the molecular type.
14. Compounds with this type of bond form atomic crystal lattices.
15. Compounds may be gaseous under normal conditions.
16. Compounds are solid under normal conditions.
17. Connections with this type of connection are usually refractory.
18. Substances with this type of bond can be liquid under normal conditions.
19. Substances with such a chemical bond have an odor.
20. Substances with such a chemical bond have a metallic luster.
Answers(self-esteem).
Option 1
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| + | – | – | + | + | – | – | + | – | + |
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| – | + | – | – | – | + | + | – | – | – |
Option 2
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| – | – | + | – | – | + | – | – | + | – |
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| – | – | + | – | + | + | – | + | + | – |
Option 3
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| – | – | + | – | + | – | + | – | – | – |
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| + | – | + | + | + | – | + | + | + | – |
Evaluation criteria: 1–2 errors – “5”, 3–4 errors – “4”, 5–6 errors – “3”.
Fixing the material
Silicon has an atomic crystal lattice. What are its physical properties?
What type of crystal lattice does Na 2 SO 4 have?
CO 2 oxide has low t pl, and quartz SiO 2 – very high (quartz melts at 1725 ° C). What crystal lattices should they have?
TEACHER. We've looked into the guts of things, haven't we? In conclusion, I would like to mention precious stones: diamond, sapphire, emerald, alexandrite, amethyst, pearl, opal, etc. Healing properties have long been attributed to precious stones. It was believed that the amethyst crystal protected against drunkenness and brought happy dreams. Emerald saves from storms. Diamond protects against diseases. Topaz brings happiness in November, and garnet in January.
Precious stones served as a measure of the wealth of princes and emperors. Foreign ambassadors who visited in the 17th century. in Russia, they wrote that they were seized by “quiet horror” at the sight of the luxurious outfits of the royal family, completely studded with precious stones.
On the head of Tsarina Irina Godunova there was a crown, “like a wall with battlements,” divided into 12 turrets, skillfully made of rubies, topazes, diamonds and “ramp pearls”, all around the crown was studded with huge amethysts and sapphires.
 |
It is known that the hat of Prince Potemkin of Tauride was so studded with diamonds and because of this it was so heavy that the owner could not wear it on his head; the adjutant carried the hat in his hands behind the prince. One of Empress Elizabeth's dresses was sewn with so many precious stones that she, unable to bear their weight, fainted at the ball. However, even earlier, a more annoying incident happened to the wife of Tsar Alexander Mikhailovich: she had to interrupt the wedding ceremony in order to take off her outfit strewn with gems.
The largest diamonds in the world are known each by their own name: “Orlov”, “Shah”, “Konkur”, “Regent”, etc.
Crystals are necessary - in watches, echo sounders, microphones; diamond – “worker” (in bearings, glass cutters, etc.).
“Stone now in the hands of man is not fun and luxury, but a wonderful material to which we have managed to return its place, a material among which it is more beautiful and more fun to live. It will not be a “precious stone” - its time has passed: it will be a gem that gives beauty to life. ...In him a person will see the embodiment of the unsurpassed colors and incorruptibility of nature itself, which an artist can only touch with the burning fire of inspiration,” wrote academician A.E. Fersman.
Crystals can be grown even at home. Try some creative crystal growing homework.
Homework
"Growing Crystals"
Equipment and reagents. Clean glasses, cardboard, pencil, thread; water, salt (NaCl, or CuSO 4, or KNO 3.)
Progress
First way.
Prepare a saturated solution of your chosen salt. To do this, pour salt into hot water in portions and stir until dissolved. As soon as the salt stops dissolving, the solution is saturated. Filter the solution through gauze. Pour this solution into a glass, put a pencil with a thread and a weight (a button, for example). After 2–3 days, the cargo should become covered with crystals.
Second way.
Cover the jar with the saturated solution with cardboard and wait until crystals fall to the bottom during slow cooling. Dry the crystals on a napkin, fasten a few of the most attractive ones on a thread, tie them to a pencil and lower them into a saturated solution freed from other crystals. Crystals can take 2-3 weeks to grow.
Most solids have crystal structure, in which the particles from which it is “built” are in a certain order, thereby creating crystal lattice. It is built from repeating identical structural units - unit cells, which communicates with neighboring cells, forming additional nodes. As a result, there are 14 different crystal lattices.
Types of crystal lattices.
Depending on the particles that stand at the lattice nodes, they are distinguished:
- metal crystal lattice;
- ionic crystal lattice;
- molecular crystal lattice;
- macromolecular (atomic) crystal lattice.
Metallic bond in crystal lattices.
Ionic crystals have increased fragility, because a shift in the crystal lattice (even a slight one) leads to the fact that like-charged ions begin to repel each other, and bonds break, cracks and splits form.
Molecular bonding of crystal lattices.
The main feature of the intermolecular bond is its “weakness” (van der Waals, hydrogen).
This is the structure of ice. Each water molecule is connected by hydrogen bonds to 4 molecules surrounding it, resulting in a tetrahedral structure.
Hydrogen bonding explains the high boiling point, melting point and low density;
Macromolecular connection of crystal lattices.
There are atoms at the nodes of a crystal lattice. These crystals are divided into 3 types:
- frame;
- chain;
- layered structures.
Frame structure diamond is one of the hardest substances in nature. The carbon atom forms 4 identical covalent bonds, which indicates the shape of a regular tetrahedron ( sp 3 - hybridization). Each atom has a lone pair of electrons, which can also bond with neighboring atoms. As a result, a three-dimensional lattice is formed, in the nodes of which there are only carbon atoms.
It takes a lot of energy to destroy such a structure; the melting point of such compounds is high (for diamond it is 3500°C).
Layered structures speak of the presence of covalent bonds within each layer and weak van der Waals bonds between the layers.
Let's look at an example: graphite. Each carbon atom is in sp 2 - hybridization. The 4th unpaired electron forms a van der Waals bond between the layers. Therefore, the 4th layer is very mobile:
The bonds are weak, so they are easy to break, which can be observed in a pencil - “writing property” - the 4th layer remains on the paper.
Graphite is an excellent conductor of electric current (electrons are able to move along the plane of the layer).
Chain structures have oxides (for example, SO 3 ), which crystallizes in the form of shiny needles, polymers, some amorphous substances, silicates (asbestos).