Compose the chemical formulas of the compounds according to the valency of the elements. Drawing up chemical formulas. Student's message about valency

Valence.
Drafting chemical formulas
by valence

8th grade

Lesson type. Combined.

Teaching methods. Partially search, reproductive, programmed survey, conversation with lecture elements.

Epigraph to the lesson.“Every substance, from the simplest to the most complex, has three different but interrelated aspects: properties, composition, structure…” (B.M. Kedrov).

Goals. Didactic: consider the concept of "valency" as the atomicity of an element, introduce students to various types of valency (higher and lower, variable and constant).

Psychological: arouse interest in the subject, develop the ability to reason logically, competently express one's thoughts.

Educational: to develop the ability to work collectively, evaluate the answers of their comrades.

Equipment. Models water molecules, carbon dioxide, kits for building models of molecules of various substances, individual cards for checking homework and independent work of students in a group, anagram tablets for chemical warm-up, a scale for determining emotional state student.

DURING THE CLASSES

Orientation-motivational stage

Psychological warm-up

The purpose of the warm-up is to determine the emotional state of the students. Each student has a plate with six faces glued on the inside cover of the notebook - a scale for determining the emotional state (Fig.). Each student puts a tick under the face, whose expression reflects his mood.

Teacher. It would be great if by the end of the lesson everyone could move the tick at least one cell to the left.

To do this, you need to think about the questions: can a person fall in love with a subject that is not very interesting to him? What do I need to do?

Chemical warm-up

The warm-up is prepared and conducted by the students.

Student. Anagrams are words in which the order of the letters has been reversed. Try to solve some of the chemical anagrams. Rearrange the letters in each word and get the name of the chemical element. Pay attention to the hint.

"Odovrod" - this element has the smallest relative atomic mass.

"Mailinuy" - this element is called "winged" metal.

"Tyurt" - contained in a medical thermometer.

"Tsalky" - without it, our bones would be fragile and fragile.

"Rosfof" - a substance consisting of atoms of this element, was smeared with the hair of the Baskervilles' dog.

Teacher. If you can easily guess the anagram words, say to yourself: "I'm done!"

Chemical signs and chemical formulas
(Homework check)

Individual work at the card board.

Digital dictation

Students control the implementation of the dictation by the method of mutual verification.

The task. Put 1 next to correct statements and 0 to wrong ones.

1. A chemical element is a certain kind of atoms.

2. In each cell of the table of D.I. Mendeleev, in addition to the designation and name of the element, two numbers are written: the upper one is the relative atomic mass of the element, the lower one is its serial number.

3. The chemical element gallium was named after France.

4. In the table of D.I. Mendeleev, the elements are arranged, as a rule, in descending order of their atomic masses.

5. The values of the relative atomic mass and the mass of the atom, expressed in a. e. m., never coincide numerically.

6. Simple substances are called substances consisting of atoms of one element.

8. The mass fraction of an element shows what part (share) is the mass of this element from the total mass of the substance.

9. The relative molecular weight of water H 2 O is 20.

10. The mass fraction of calcium in calcium oxide CaO is 71%.

Correct answers: 1 - 1, 2 - 0, 3 - 1, 4 - 0, 5 - 0, 6 - 1, 7 - 0, 8 - 1, 9 - 0, 10 - 1.

Operational and executive stage

Teacher. You know that the chemical formulas of substances show the quantitative ratios in which atoms are connected to each other, you also learned how to calculate the mass fraction of an element from the chemical formula of a substance. For example, in water H 2 O There are two hydrogen atoms per oxygen atom, or 11% H and 89% ABOUT. In carbon dioxide CO 2 There are two oxygen atoms per carbon atom.(demonstration of models of molecules of these substances.)

Valence

Teacher. Valence is the ability of atoms to attach a certain number of other atoms to themselves.

One atom of another monovalent element combines with one atom of a monovalent element (HF, NaCl) . Two monovalent atoms combine with an atom of a divalent element(H2O) or one divalent atom(CaO) . Means, valency element and can be represented as a number that shows how many atoms of a monovalent element an atom of this element can combine with.

Rules for determining valency
elements in connections

The valency of hydrogen is taken as I (one). Then, in accordance with the formula of water H 2 O, two hydrogen atoms are attached to one oxygen atom.

Oxygen in its compounds always exhibits valency II. Therefore, carbon in the CO 2 compound ( carbon dioxide) has valency IV.

Teacher.How to determine the valency of an element based on the table of D.I. Mendeleev?

For metals in groups a, the valence is equal to the group number.

In non-metals, two valences mainly appear: higher and lower (scheme).

The highest valency is equal to the group number.

The lowest valence is equal to the difference between the number 8 (the number of groups in the table) and the number of the group in which this element is located.

Teacher.For example: sulfur has a higher valence VI and a lower one (8 - 6) equal to II; phosphorus exhibits valencies V and III.

Valency can be constant (for elements of the main subgroups of the D.I. Mendeleev table) or variable (for elements of secondary subgroups in the table), but you will get to know this phenomenon a little later, and if you are interested, then read the 9th grade textbook.

The valency of elements must be known in order to compose the chemical formulas of compounds. To do this, it is convenient to use the following table.

table

Algorithm for compiling the formula of the compound P and O

Sequencing	Formulating phosphorus oxide
1. Write element symbols
2. Determine the valencies of the elements
3. Find the least common multiple numerical values valencies
4. Find the ratios between the atoms of the elements by dividing the found smallest multiple by the corresponding valencies of the elements	10: 5 = 2, 10: 2 = 5;
5. Write indexes at element symbols
6. Formula of the compound (oxide)

Teacher. Remember two more rules for compiling chemical formulas of non-metal compounds with each other.

1) The lower valence is shown by the element that is in the D.I. Mendeleev table to the right and above, and the highest valence is the element located to the left and below. (Demonstration of the table of D.I. Mendeleev.)

For example, in combination with oxygen, sulfur exhibits a higher valency VI, and oxygen a lower II. So the formula for sulfur oxide would be SO 3.

In the combination of silicon with carbon, the first exhibits a higher valence IV, and the second - a lower IV. So the formula is SiC. It is silicon carbide, the basis of refractory and abrasive materials.

2) In the formulas of compounds, the non-metal atom, which exhibits the lowest valence, always comes in second place, and the name of such a compound ends in "id".

For example, CaO - calcium oxide, NaCl - sodium chloride, PbS - lead sulfide.

Now you yourself can write the formulas of any compounds of metals with non-metals.

Independent work

The text of the work is written in advance on the board. Two students solve the problem on the back of the board, the rest in notebooks.

Exercise 1. Check if the formulas of the following compounds are written correctly: Na 2 S, KBr, Al 2 O 3,
Mg3N2, MgO.

Task 2. Write the formulas for compounds of metals with non-metals: calcium with oxygen, aluminum with chlorine, sodium with phosphorus. Name these compounds.

After completing the work, students exchange notebooks, a mutual check takes place. The teacher can selectively check some notebooks, praise those students who completed the fastest and made the fewest mistakes.

Consolidation of the studied material

Conversation with students on questions

1) What is valence?

2) Why is valency sometimes called the atomicity of an element?

3) What are the valences of hydrogen and oxygen?

4) What two valency values can non-metals exhibit?

5) How to determine the lower and higher valencies of non-metals?

6) How to find the least common multiple between the numerical values of the valencies?

7) Can atoms in a compound have free valencies?

8) Which of the two non-metals in the chemical formula of their compound takes 1st place, and which -
2nd? Explain using NO 2 oxide as an example, using the table of D.I. Mendeleev.

Creative work in groups

The task. Using molecular modeling kits for various substances, create formulas and molecular models for the following compounds:

1st group - copper and oxygen,

2nd group - zinc and chlorine,

3rd group - potassium and iodine,

4th group - magnesium and sulfur.

After the end of the work, one student from the group reports on the completed task and, together with the class, provides an analysis of errors.

Homework assignment. According to the textbook "Chemistry-8" L.S. Guzeya: § 3.1, assignments No. 3, 4, 5, p. 51. Those who wish can prepare reports about French scientist J.L. Prouste and English scientist J. Dalton.

Reflective-evaluative stage and summing up the lesson

Announce grades for the lesson to the students who answered, thank everyone for their work in the lesson. To assess the emotional state on a scale (see Fig.). The teacher once again reminds the questions that need to be considered for effective work in the next lesson.

Literature

Guzey L.S., Sorokin V.V., Surovtseva R.P. Chemistry-8, M.: Bustard, 2000; Tyldsepp A.A., Kork V.A. We are studying chemistry. M.: Enlightenment, 1988; Bukreeva R.V., Bykanova T.A. Lessons of new technologies in chemistry. Voronezh, 1997.

Chemistry lesson in grade 8 No. 14

Topic: Drawing up formulas of compounds by valency.

The purpose of the lesson: to teach how to apply the acquired knowledge to carry out calculations on compiling formulas for the valence of a substance.

Planned learning outcomes:

Students should be able to formulate the definition of “valency”, know the valency of hydrogen and oxygen atoms in compounds, determine the valence of atoms of other elements in binary compounds from it,

Be able to explain the meaning of the concept of “valence” and the sequence of actions in determining the valency of the atoms of elements according to the formulas of substances.

Organizational forms: conversation, individual tasks, independent work.

Means of education: algorithm for determining valence by the formulas of substances and compiling chemical formulas of compounds by the valency of the element

Demonstration equipment: presentation

Equipment for students: "Algorithm for compiling chemical formulas by valence".

During the classes

I. Approximate and motivational stage.

.Teacher. Today we will make a trip to the country of Himland, where more than a hundred thousand different chemical substances. There will be many obstacles on the way, so you need to use all your mental capacity knowledge gained in previous lessons. Before heading out, let's warm up.

Chemical warm-up

Tasks on the topics covered

1. Find "extra substance", i.e. one that does not form a homogeneous group with the rest: H 2 SO 2 Na Br 2 Al 2 O 3 N 2 O 5 (N a is an atom among molecules)

2..Find "extra substance", i.e. one that does not form a homogeneous group with the rest: O 2 N 2 Ca O Mg Li Na (Ca O is a complex substance, among simple ones)

Teacher. Now we need to pack for the trip.

3. .Choose the names of bodies: gold, paraffin, package, glass, mug, pencil, flashlight, water, candle.

4. Choose pure substances: air, Mercury, sea water, salt, silver, hydrogen, oil, iron, fog.

We have rocks on the way. We need to build a bridge.

5. Match the chemical symbol of an element with its name

Now we have a river on our way. In order to pass it, you must complete the following task.

6. How do you pronounce the formulas of the following substances? CuO, ZnO, P 2 O 5, NO 2, SO 3, Al 2 O 3, H 2 O, SO 2.

Well done, we have to go through the forest

Whiteboard work

1. Calculate relative molecular weight of these substances: NaI, CuO

2. Calculate the relative molecular weight of these substances:Al 2 O 3 , MgO

( The class performs in a notebook)

We have completed all the tasks. Well done!

II. Operational and executive stage.

Updating of basic knowledge

The last stage is a high mountain. Before us is not an easy task, we need a lot of effort in order to rise, our knowledge will help us.Remember how the elements are connected to each other?

Define valency.

Valency is the property of atoms to hold a certain number of other atoms in a compound.

How is valency defined? (Valency is indicated by Roman numerals)

What are the types of valency? (constant and variable)

Before the difficult uphill climb, let's do one more warm-up.

The exercise: determine the valence of elements in substances (Snowball, each determines the valency and passes the move to the other)Al 2 O 3 , ZnO, SO 3 , K 2 O, KI, CO 2 , Na 2 O, P 2 O 5

Tell me, is it necessary to know the valency of chemical elements? (to make chemical formulas of compounds).

Topic message

Let's write the topic of the lesson in a notebook. What is our goal?

So, now we need to go through our mountain safe and sound, for this we need to learn the rules.

Algorithm for compiling a chemical formula by the valency of an element.

Sequencing

Formulating phosphorus oxide

1. Write element symbols

R O

2. Determine the valencies of the elements

VII
PO

3. Find the least common multiple of the numerical values of the valencies

5 2 = 10

4. Find the ratios between the atoms of the elements by dividing the found smallest multiple by the corresponding valencies of the elements

10: 5 = 2, 10: 2 = 5;

P:O=2:5

We have already learned the rules, we will begin the ascent.

The exercise: write chemical formulas according to the valency of the elements. Task in the workbook (Working with the textbook)

III. Evaluative-reflexive stage.

We safely reached the place, we can rest.

Primary test of mastery of knowledge.

During three minutes You must complete one of three optional tasks. Choose only the task that you can handle.

Make chemical formulas according to the valency of elements A LCl, KO, CuO ( II), sns( IV)
Make formulas for oxides:N ( IV), Mn ( VII) , S ( VI), Na ( I) , Cr ( III).

Correct the mistakes made in some formulas: NO 3, BO 3, MgO 2 , AlI 2 , NaS

IV. Summing up the lesson

Today we made a trip, tell me, did you like it? What knowledge did you bring from the trip?

Homework: Textbook paragraph.

Task cards

Compose formulas of substances

"3" NaO, CO(IV), BO(III)

"4"SnO(IV), FeO(III), MnO(VI)

Atomic-molecular doctrine. Atoms. Molecules. Molecular and non-molecular structure of matter. Relative atomic and molecular mass. The law of conservation of mass, its significance in chemistry. A mole is a unit of quantity of a substance. Molar mass. Avogadro's law and the molar volume of a gas. Relative density of matter.

The subject of chemistry. Phenomena chemical and physical.

Chemistry is the science of substances and their transformations. It studies the composition and structure of substances, the dependence of their properties on the structure, the conditions and methods for the transformation of one substance into another.

Chemistry is of great practical importance. Many millennia ago, man used chemical phenomena in the smelting of metals from ores, obtaining alloys, melting glass, etc. Back in 1751. M.V. Lomonosov, in his famous “Sermon on the Benefits of Chemistry,” wrote: “Chemistry spreads its hands wide in human affairs. Wherever we look, wherever we look back - everywhere the successes of its application turn before our eyes.

In our time, the role of chemistry in the life of society is immeasurable. Chemical Knowledge have now reached such a level of development that, on their basis, ideas about the nature and mechanism of a number of important natural and technological processes are radically changing. Chemistry has helped us to discover and use not only previously unknown properties of substances and materials, but also to create new substances and materials that do not exist in nature.

A substance is a type of matter that has constant physical properties under certain conditions. However, as conditions change, the properties of matter change.

Any changes that occur with matter are called phenomena. Phenomena are both physical and chemical.

Physical phenomena are called such phenomena that lead to a change in the shape, state of aggregation, temperature of a substance without changing its composition. The chemical composition of a substance does not change as a result of a physical phenomenon. For example, water can be turned into ice, into steam, but it chemical composition while remaining the same.

Chemical phenomena are called such phenomena in which a radical change in the composition and properties of substances occurs. As a result of chemical phenomena, some substances are transformed into others, i.e. the composition of molecules changes, molecules of another substance are formed. However, atoms in chemical reactions remain unchanged. An example is the decomposition of limestone:

CaCO3 → CaO + CO2

Chemical phenomena are otherwise called chemical reactions. Characteristic features chemical phenomena (reactions): release of heat, gas, precipitation, color change, odor. At physical phenomena this cannot be observed.

The theoretical basis of modern chemistry is the atomic and molecular theory.

Atoms are the smallest chemical particles that are the limit of the chemical decomposition of any substance.

A chemical element is a type of atom with the same positive nuclear charge.

Consequently, An atom is the smallest particle of a chemical element that retains all of its chemical properties. Currently, 110 elements are known, of which 92 are found in nature.

Depending on the nature of the particles from which the substance is built, substances with a molecular and non-molecular structure are distinguished. Almost all organic matter(i.e., the vast majority of known substances) are made up of molecules. Among inorganic compounds about 5% have a molecular structure. Thus, the most typical form of the existence of matter is a molecule.

A molecule is the smallest particle of a substance that can exist independently and retains its basic chemical properties.

Under normal conditions, substances with a molecular structure can be in solid, liquid or gaseous state. Substances with a non-molecular structure are found only in the solid state, predominantly in crystalline form. The carriers of the chemical properties of such substances are not molecules, but combinations of atoms or ions that form this substance.

The symbolic record of the simplest numerical ratio in which atoms of different elements form a chemical compound is called formula. Therefore, the formula expresses a certain (qualitative and quantitative) composition of the compound. So, SO2, N2, CO are formulas of substances having a molecular structure. Their composition is always strictly constant. NaCl, AlF3, ZnS - formulas of substances that do not have a molecular structure under normal conditions. The composition of such substances is not always constant and often depends on the conditions for their preparation. The deviation from the integer ratio can be expressed by writing the formula: Fe0.9S, TiO0.7, ZrN0.69. Substances with constant composition are called daltonids, substances with variable composition berthollids.

The masses of atoms of chemical elements are extremely small. In chemistry, they do not use their absolute values, but relative ones.

The relative atomic mass of the chemical Ar element is the value, equal to the ratio the average mass of atoms of a given element (taking into account the percentage of its isotopes in nature) to 1/12 of the mass of carbon isotopes - 12. 1/12 of the mass of an atom of the isotope carbon 12 is taken as atomic mass unit(a.e.m.), international designation - u.

Relative atomic mass is a dimensionless quantity.

The relative molecular weight Mr of a substance is the ratio of the mass of its molecule to 1/12 of the mass of an atom of the carbon isotope 12.

Since the majority inorganic substances under normal conditions do not have molecular structure, in this case we can talk about formula weight F, meaning by it the sum of the atomic masses of all elements included in the compound, taking into account the number of atoms of each element in the formula.

The unit of measurement of the amount of substance n (ν) in the International System of Units is mole.

Mole - the amount of a substance containing as many structural elementary units (atoms, ions, molecules, electrons, equivalents, etc.) as there are atoms in 0.012 kg of the carbon 12 isotope.

The number of NA atoms in 0.012 kg of carbon (i.e., in 1 mol) is easy to determine, knowing the mass of the carbon atom. The exact value of this quantity is 6.02 10²³. This value is called constant Avogadro and is one of the most important universal constants. It is equal to the number structural units in 1 mole of any substance.

The mass of 1 mole of substance X is called molar mass M(X) and represents the ratio of the mass m of this substance to its quantity n.

Law of conservation of mass: "The mass of substances that entered into chemical reaction, is equal to the mass of substances formed as a result of the reaction, taking into account the mass corresponding to the thermal effect of the reaction. It was formulated by the great Russian scientist M.V. Lomonosov in 1748. and confirmed experimentally by himself in 1756. and independently of him by the French chemist A.L. Lavoisier in 1789.

The law of constancy of the composition of matter: "Any complex substance of molecular structure, regardless of the method of preparation, has a constant quantitative composition."

Avogadro's law: "Equal volumes of different gases under the same conditions contain the same number of molecules."

Consequences: 1. If the number of molecules of some gases is equal, then at n.o. they occupy equal volumes. If the number of molecules is 6.02 10²³, then the volume of the gas is 22.4 liters. This volume is called molar volume.

1. The absolute density of a gas is equal to the ratio of its molecular weight to the molar volume

2. The relative density of a gas (X) over another gas (Y) is equal to the ratio of the molar mass of the gas (X) to the molar mass of the gas (Y).

Claperon equation:

Boyle-Mariotte equation:

Charles-Gay-Lusac equation:

Chemical element. Simple and complex substances. Chemical formulas.

A type of atom with the same chemical properties is called an element. Atoms of the same element can differ only in mass. Chemical properties they have the same. There are varieties of atoms of the same element, called isotopes.

The concept of "chemical element" equally refers to the atoms of a given element, both in free form and included in compounds.

Molecules are formed from atoms. Depending on whether the molecule consists of atoms of the same element or of atoms of different elements, all substances are divided into simple and complex.

Simple substances are those whose molecules consist of atoms of one element. molecules simple substances may consist of one, two or more atoms of one element. At present, the fact of the existence of the same element in a free state in the form of a number of different forms is indisputable, i.e. in the form of several simple substances.

The existence of an element in the form of several simple substances is called allotropy. Simple substances formed by the same element are called allotropic modifications this element. These modifications differ both in the number and arrangement of the same atoms in the molecule.

Complex substances or chemical compounds are those whose molecules consist of atoms of two or more elements. Atoms that enter into a chemical compound do not remain unchanged. They influence each other mutually. Atoms in different molecules are in different states.

A chemical formula is a representation of the composition of a substance through chemical signs. Chemical formulas denote the molecule of a substance, its qualitative and quantitative composition.

An important concept in chemistry is valence . Valency is the ability of an atom to combine with other atoms by a certain number of chemical bonds. The numerical value of valence is determined by the total number of atomic orbitals involved in the formation of a chemical bond:

↓

	↓	↓

CO: C≡O C: 1s 2 2s 2 2p 2 valency: 3

O: 1s 2 2s 2 2p 4

All of the above applies to compounds with a covalent bond. If the element forms ionic bonds, then its valency is called stoichiometric. She says nothing about the number of connections. The highest valency is equal to the number of the group in which the element is located, however, N, O and F have the highest valency - 4. This is due to the fact that the atoms of these elements cannot go into an excited state due to the lack of vacant orbitals on the wind turbine.

DEFINITION

Under valence the property of an atom of a given element to attach or replace a certain number of atoms of another element is implied. Therefore, the measure of valence can be the number of chemical bonds formed by a given atom with other atoms.

Thus, at present, the valency of a chemical element is usually understood as its ability (in a narrower sense, a measure of its ability) to form chemical bonds. In the representation of the method of valence bonds, the numerical value of valency corresponds to the number covalent bonds that form an atom.

Compilation of chemical formulas by valency

Using chemical signs for the valence of elements, you can make a formula complex substance. For this you need to know:

- chemical signs of the elements that make up a complex substance;

- valency of elements;

- be able to find the least common multiple for the valencies of elements;

— to define indices for atoms of elements.

Consider the rules for compiling chemical formulas of complex compounds by valency using the example of inorganic substances of different classes:

a) oxides

Suppose we need to derive the formula for iron oxide (III). To do this, you need to perform the following steps:

- we depict the chemical signs of the elements that make up the complex substance:

- put a valence over the sign of each element with a Roman numeral:

- find the least common multiple of the numbers of valency units:

- we divide the least common multiple by the number of valency units of each element separately (the resulting private ones will be indices in the formula):

b) bases, salts and acids

When formulating bases and salts, the same actions are used as when formulating oxides. The only difference is that instead of the oxygen atom there will be a hydroxo group (OH) or acid residues (SO 4, SO 3, CO 3, NO 3, PO 4, SiO 3, S, Cl, etc.).

Suppose we need to derive the formula for calcium hydroxide:

common multiple II × I = 2

2 / 2 \u003d 1 (one is not set);

2 / 1 = 2 (OH should be in brackets);

Examples of problem solving

EXAMPLE 1

The task

Determine the valency of the elements in the following compounds: a) Mg 3 P 2; b) Al 2 S 3; c) Na 2 O; d) AgCl; e) FeCl 3 .

Solution

Determining the valency of elements in a chemical compound should begin with an indication of the valence of a known element. In option “a”, this is magnesium, since the elements of group IIA have a constant valence value equal to the group number, i.e. II. We write down the formula of the substance and indicate in Roman numerals the valency of the known element:

We find the smallest multiple of the numbers of units of valencies. To do this, we multiply the magnesium valency value by the number of atoms of this element in compound (3):

To determine the valency of phosphorus, we divide the smallest multiple of the number of valency units by the number of phosphorus atoms in compound (2):

This means that the valency of phosphorus is III:

Mg II 3 P III 2 .

Similarly, we determine the valencies of elements in other compounds: b) Al III 2 S II 3; c) Na I 2 O II; d) Ag I Cl I; e) Fe III Cl I 3 .

Answer

a) Mg II 3 P III 2; b) Al III 2 S II 3; c) Na I 2 O II; d) Ag I Cl I; e) Fe III Cl I 3

EXAMPLE 2

The task

Determine the valency of the elements in the following compounds: a) CuO; b) Au 2 O; c) PbO 2 ; d) Li 3 N; e) AlF 3 .

Solution

Determining the valency of elements in a chemical compound should begin with an indication of the valence of a known element. In option "a" - this is oxygen, since its valence is always equal to II:

Put the resulting value to the right of chemical symbol this element, denoted by Arabic numerals:

Now we divide total number valency units per number of atoms (index) of an element for which valence is known:

We put the resulting quotient (2) with a Roman numeral over the desired element as its valence:

This means that the valency of the elements in the CuO compound is: for copper - II and for oxygen - II.

Similarly, we determine the valencies of elements in other compounds: b) Au I 2 O II; c) Pb IV O II 2 ; d) Li I 3 N II; e) Al III F I 3 .

Answer

a) Cu II O II; b) Au I 2 O II; c) Pb IV O II 2 ; d) Li I 3 N II; e) Al III F I 3

Lesson topic: Drawing up chemical formulas of binary compounds by valency.

"At scientific study subjects two main or ultimate goals: foresight and benefit "

D. I. Mendeleev

Goals:

Educational: consider the concept of "valency" as the atomicity of an element, teach students to determine valency in binary compounds, introduce students to different types of valence, repeat the concepts of a multiple of a given number, the least common multiple of several numbers, repeat the rule for finding LCM of several numbers and applying this rule; draw students' attention to the integration of chemistry and mathematics courses.

Developing: develop cognitive interest students, develop the ability to reason logically, apply previously acquired knowledge, and competently express their thoughts.

Educational: to promote the education of interest in the subject, in the result of their work, to develop the ability to work in pairs, collectively, to evaluate the answers of their comrades.

Planned learning outcomes:

Students should know:

definition of the concept of “valence”;

valency of hydrogen and oxygen atoms in compounds.

Students should be able to:

determine the valency of atoms of other elements in binary compounds by the valency of hydrogen and oxygen atoms;

determine the valency of the atoms of elements according to the formulas of substances, using an algorithm for solving problems.

Basic concepts: valency, constant and variable valency, binary compounds, least common multiple.

Lesson type: combined.

Means of education: algorithm for determining valency.

Equipment: Periodic system of chemical elements of D. I. Mendeleev, ball-and-stick models of molecules, table “Algorithm for determining valency”.

During the classes

Organizational stage: Greeting students.

Updating of basic knowledge.

Frontal conversation chemistry teacher with students on the topic “Chemical formula”.

When studying chemistry, it is very important to learn how to compose chemical formulas.

What does a chemical formula express? (composition of a certain substance, taken in its pure form)

Chemical formula- this is a conventional designation of a substance, atom, molecule, ion using the symbols of the elements, numerical and auxiliary signs.

According to the chemical formula, we can determine:

type of substance,

Qualitative and quantitative composition,

relative molecular weight,

Mass fraction a chemical element in a given substance,

Valency of chemical elements.

All substances are made up of atoms. One of the main properties of atoms is the ability to form chemical bonds. Atoms of different elements can form a certain number of bonds characteristic of them.

Let's compare the qualitative and quantitative composition in molecules: HCl, H 2 O, NH 3, CH 4.

What do molecules have in common? (presence of hydrogen atoms)

How do these substances differ from each other? (these substances have a different number of hydrogen atoms)

A hydrogen atom cannot attach more than one atom of another chemical element, so the hydrogen valency is taken as one. And therefore, the valency of all other elements is compared with the valency of hydrogen.

Examples:

HCl - one chlorine atom is bonded to one hydrogen atom;

H 2 O - one oxygen atom binds two hydrogen atoms;

NH 3 - one nitrogen atom binds three hydrogen atoms;

CH 4 - one carbon atom binds four hydrogen atoms.

Why do different atoms hold different numbers of hydrogen atoms?

( Each atom forms a certain number of bonds with other atoms.

It's called valence.

Valence is the property of atoms to hold a certain number of other atoms in a compound.

Valency is indicated by Roman numerals.

Notes on the board and in notebooks:

I I
HCl

I II
H 2 O

I III
H 3 N

I IV
H 4 C

The valency of the hydrogen atom is taken as a unit - I, and for oxygen - II.

Student's message about valence.

IN early XIX century, J. Dalton formulated the law of multiple ratios, from which it followed that each atom of one element can combine with one, two, three, etc. atoms of another element (as, for example, in the compounds of atoms with hydrogen considered by us).

In the middle of the 19th century, when the exact relative weights of atoms were determined (I.Ya. Berzelius and others), it became clear that largest number number of atoms with which a given atom can combine does not exceed a certain value, depending on its nature. This ability to bind or replace a certain number of other atoms was called by E. Frankland in 1853 "valency".

Since at that time no compounds were known for hydrogen where it would be bonded to more than one atom of any other element, the hydrogen atom was chosen as the standard, having a valency of 1.

At the end of the 50s. XIX century A. S. Cooper and A. Kekule postulated the principle of constant tetravalent carbon in organic compounds. The concept of valency formed an important part of the theory chemical structure A. M. Butlerov in 1861

Periodic Law DI. Mendeleev in 1869 revealed the dependence of the valency of an element on its position in periodic system.

V. Kossel, A. Werner, G. Lewis contributed to the evolution of the concept of “valency” in different years.

Since the 30s. In the 20th century, ideas about the nature and nature of valency were constantly expanding and deepening. Significant progress was made in 1927, when W. Heitler and F. London performed the first quantitative quantum-chemical calculation of the hydrogen molecule H 2 .

Interview with students:What is valency?

The definition of valency in different sources sounds different. Let's think about which of these three definitions is more perfect and what are the shortcomings of the others.

1st row
"Valence chemical element - the ability of its atoms to combine with other atoms in certain ratios.

2nd row
"Valence - the ability of atoms of one element to attach a certain number of atoms of another element.

3rd row
"Valence - property of atoms, entering into chemical compounds, donate or accept a certain number of electrons, or combine electrons to form electron pairs common to two atoms.

Group discussion, we come to the conclusion that the 3rd definition most accurately reflects the essence of the definition of valence.

Presentation of new material:

The topic of our lesson: Drawing up formulas of binary compounds by valency.
new material it is easier to remember if it is strung on already acquired knowledge. Therefore, now we have to work with the extraction of this material from memory. This is where a math teacher can help you.

The teacher is talking to the students:

Teacher: List some numbers that are multiples of 12.

12, 24, 36, 48 ….

Teacher: List several numbers that are multiples of 18.

18, 36, 54, 72……

Teacher: name the numbers that are multiples of 12 and 18.

36 and 72 etc.

Teacher: What is the least common multiple of 12 and 18.

Teacher: Formulate the definition of the least common multiple of several numbers.

The least common multiple of several numbers is the smallest natural number, which is divisible by each of the given numbers.

Teacher: formulate a rule for finding the least common multiple of two or more numbers.

To find the LCM of two or more numbers, you need to decompose these numbers into prime factors, then, taking the decomposition of one of them, multiply it by the missing prime factors from the decompositions of other numbers.

The teacher suggests the following exercise:

№1 . Find A) NOC (48, 90); B) NOC (6, 15, 20)

Solution:

A) 48/2 90/2 B) 6/2 15/3 20/2

24/2 45/3 3/3 5/5 10/2

12/2 15/3 1/ 1/ 5\5

LCM (48.90) = 2 4 *3*3*5 LCM (6.15.20) = 2 3 * 5 * 3

NOK (48.90) = 720 NOK (6.15.20) = 60

Answer: A) 720; B) 60.

Teacher: What is the LCM of two coprime numbers?

The LCM of two coprime numbers is equal to their product.

№2. Find NOC (7.11)

Teacher: As you remember, there is another way to find the LCM of numbers - this is the selection method.

Find an NOC the following numbers by selection:

A) 10 and 2 B) 14 and 21 C) 20 and 15 D) 2; 3 and 5

Answers: A) 10 B) 42 C) 60 D) 30.

Teacher: Guys, we repeated with you how you can find the LCM of two or more numbers. And now you will get acquainted with how this knowledge is applied in chemistry in determining valency.

W. Goethe once said: "Just knowing is not everything, knowledge must be able to use."

Determination of the valency of elements by formulas in binary compounds.

Remember: in the formulas of binary compounds, the number of valence units of all atoms of one element is equal to the number of valence units of all atoms of another element.

Write down the formula of the substance. R 2 O 5

Roman numerals indicate the valency of one of the elements. P 2 O 5 (II)

Determine the valency of another chemical element. R 2 O 5

X * 2 \u003d II * 5 X \u003d V

Compilation of chemical formulas of binary compounds according to the valency of the elements.

Writing down symbols chemical elements that make up the formula, putting down the corresponding valency values above them:

CaO, B 2 O 3, CO 2,

We make a formula compounds by valence:

a) CaO: if the valencies of chemical elements are equal, then we do not set indices.

b) B 2 O 3: if the valency values are not divided by each other, put the valency values crosswise.

c) CO 2: if the valency of one element is divided by the valence of another, then divide the value of the greater valency by the value of the smaller one and put the resulting number in the form of an index next to the element with a lower valence.

The exercise: determine the valency of elements in substances: students go to the board in a chain. We project the task on the board.

SiH 4 , CrO 3 , H 2 S, CO 2 , CO, SO 3 , SO 2 , Fe 2 O 3 , FeO, HCl, HBr, Cl 2 O 5 , Cl 2 O 7 , PH 3 , K 2 O, Al 2 O 3 , P 2 O 5 , NO 2 , N 2 O 5 , Cr 2 O 3 , SiO 2 , B 2 O 3 , SiH 4 , Mn 2 O 7 , MnO, CuO, N 2 O 3 .

Evaluative-reflexive stage.

Primary test of mastery of knowledge. To complete this task, students receive an "Algorithm for determining valence" and tasks of three levels. Each student chooses their own level of assignment.

Reproductive level (“3”). Determine the valence of atoms of chemical elements according to the formulas of compounds: NH 3, Au 2 O 3, SiH 4, CuO.

Application layer (“4”). From the above series, write out only those formulas in which the metal atoms are divalent: MnO, Fe 2 O 3, CrO 3, CuO, K 2 O, CaH 2.

Creative level (“5”). Find a pattern in the sequence of formulas: N 2 O, NO, N 2 O 3 and put down the valencies over each element.

Algorithm for determining valence	Example
1. Write down the formula of the substance.
2. Designate the known valency of the element
3. Find the number of units of valence of atoms of a known element by multiplying the valency of the element by the number of its atoms
4. Divide the number of valency units of atoms by the number of atoms of another element. The received answer is the desired valency	2 I II H 2 S	2 I II Cu2O
5. Make a check, that is, count the number of units of valency of each element	I II H 2 S (2=2)	I II Cu2O (2=2)