How to determine what valence. Valence capabilities of atoms of chemical elements

Objectives lesson.

Didactic:

• relying on the knowledge of students, repeat the concepts of "chemical formula";
• promote the formation of students' valence and the ability to determine the valence of atoms of elements by the formulas of substances;
• accent schoolchildren's attention to the possibility of integrating chemistry courses, mathematics.

Developing:

• continue the formation of skills to formulate definitions;
• clarify the meaning of the studied concepts and explain the sequence of actions in determining valence according to the substance formula;
• contribute to the enrichment of the vocabulary, the development of emotions, creative abilities;
• develop the ability to allocate the main thing, substantial, compare, summarize, develop diction, speech.

Educational:

• bring up a sense of partnership, the ability to work collectively;
• increase the level of aesthetic education of students;
• orient students on a healthy lifestyle.

Planned learning outcomes:

1. Students should be able to formulate the definition of "valence", know the valence of hydrogen and oxygen atoms in compounds, to determine the valence of atoms of other elements in binary compounds,
2. To be able to explain the meaning of the concept of "valence" and the sequence of actions in determining the valence of atoms of elements according to the formulas of substances.

The concepts first introduced in the lesson: Valence, constant and variable valence.

Organizational forms:conversation, individual tasks, independent work.

Means of education:algorithm for definition of valence.

Demonstration equipment:schemery models of chloride molecules, water, ammonia, methane.

Equipment for students: On each table "Algorithm for the definition of valence".

Advanced task:an individual task is to prepare a message on the topic "Evolution of the concept of" valence ".

During the classes

I. Approximately motivational stage.

1. Frontal conversation with students on the topic "Chemical formula".

The task: What is written here? (Demonstration of the teacher of formulas printed on separate sheets).

2. Individual work on the cards of three students on the topic "relative molecular weight". (Perform a solution on the board).Check teacher.

Card number 1. Calculate the relative molecular weight of these substances: NaCl, K 2 O.

Reference data:

• Ar (Na) \u003d 23
• Ar (Cl) \u003d 35.5
• Ar (k) \u003d 39
• Ar (O) \u003d 16

Card number 2. Calculate the relative molecular weight of these substances: Cuo, SO 2.

Reference data:

• Ar (Cu) \u003d 64
• Ar (O) \u003d 16
• Ar (S) \u003d 3 2

Card number 3. Calculate the relative molecular weight of these substances: CH 4, NO.

Reference data:

• Ar (C) \u003d 12
• Ar (H) \u003d 1
• AR (N) \u003d 14
• Ar (O) \u003d 16

3. Independent work of students in notebooks.

The problem of information and computational nature (the condition is recorded in handouts).

The effectiveness of the dental paste in the prevention of caries can be compared to the content of an active fluorine in them, which can interact with dental enamel. The toothpaste "Crest" (US production) contains, as indicated on the package, SNF 2, and the toothpaste "FM Extra Dent" (production of Bulgaria) contains NAF. Calculate which of these two pastes a more potent means for the prevention of caries.

Check:one student verbally reads the decision.

II. Operational executive stage.

1. Explanation of the teacher. Formulation of the problem.

The concept of valence.

- So far we used ready-made formulas given in the textbook. Chemical formulas can be displayed on the basis of data on the composition of substances. But most often, in the preparation of chemical formulas, regularities are taken into account, which are subject to elements, connecting between themselves.

The task: compare the qualitative and quantitative composition in molecules: HCl, H 2 O, NH 3, CH 4.

Conversation with students:

- What is common in the composition of molecules?

Estimated answer:The presence of hydrogen atoms.

- What do they differ from each other?

Estimated answer:

• HCl - one chlorine atom holds one hydrogen atom,
• H 2 O - one oxygen atom holds two hydrogen atoms,
• NH 3 is one nitrogen atom holds three hydrogen atoms,
• CH 4 - one carbon atom holds four hydrogen atoms.

Demonstration of rowing models.

Problem: Why do different atoms hold different number of hydrogen atoms?

(Listen to the options for students' responses).

Output: Atoms have different ability to keep a certain number of other atoms in the compounds. This is called valence. The word "valence" comes from the lat. Valentia - power.

Record in notebook:

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

Valence is denoted by Roman numbers.

Entries on the board and in notebooks:

The valence of the hydrogen atom is adopted per unit, and at oxygen - II.

2. Evolution of the concept of "valence" (student message).

- At the beginning of the XIX century, J. Dalton was formulated by the law of multiple relationship, from which he shoulded that each atom of one element can be connected with one, two, three, etc. atoms of another element (as, for example, in the compounds considered by us atoms with hydrogen).

In the middle of the XIX century, when the exact relative weights of atoms were identified (I.Y. Burtsellius, etc.), it became clear that the largest number of atoms with which this atom could be connected, does not exceed a certain amount depending on its nature. This ability to bind or replace a certain number of other atoms and was named E.Frankeland in 1853 "Valence".

Since at that time, a compound was not known for hydrogen, where it would be associated with more than one atom of any other element, the hydrogen atom was chosen as a standard with valence equal to 1.

At the end of the 50s. XIX VEK A.S. Cooper and A. Kekul postulated the principle of permanent calibration of carbon in organic compounds. The ideas about valence amounted to an important part of the theory of the chemical structure A.M. Butlerova in 1861

Periodic law D.I. Mendeleev in 1869 revealed the dependence of the valence of the element from its position in the periodic system.

The contribution to the evolution of the concept of "valence" in different years was made by V. Kossel, A.Verner, Lyuis.

Starting from the 30s. The XX century of the presentation of nature and the nature of valence was constantly expanding and deepened. Significant progress was achieved in 1927, when V.Gateler and F. London performed the first quantitative quantum-chemical calculation of hydrogen H 2 molecule.

3. Determination of valence of atoms of elements in connections.

The rule of definition of valence: The number of units of valencies of all atoms of one element is equal to the number of units of valence of all atoms of another element.

Algorithm for definition of valence.

4. The exercise: determine the valence of elements in substances ( training apparatus: Pupils chain go to the board). Task in handout.

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.

III. Evaluation reflexive stage.

Primary verification of knowledge learning.

Within three minutes, you must complete one of the three jobs. Choose only the task you can handle. Task in handout.

• Reproductive level ("3"). Determine the valence of atoms of chemical elements according to the formulas of the compounds: NH 3, AU 2 O 3, SIH 4, Cuo.
• Applied level ("4"). From the given row, write out only those formulas in which metals atoms are divalent: MNO, Fe 2 O 3, Cro 3, Cuo, K 2 O, SAH 2.
• Creative level ("5"). Find regularity in the sequence of formulas: N 2 O, NO, N 2 O 3 and sall valence over each element.

Check selective. A consultant from among students on the finished template checks 4 student notebooks.

Work on errors. Answers on the back of the board.

IV. Summing up the lesson.

Conversation with students:

• What problem did we put at the beginning of the lesson?
• What conclusion did we come to?
• Give the definition of "valence".
• What is the valence of the hydrogen atom? Oxygen?
• How to determine the valence of an atom in the connection?

Assessment of student in general and individual students.

Homework: § 4, p. 23-25, UPR. on page 25.

- Thank you for the lesson. Bye.

There are elements whose valence is always constant, and they are quite a bit. But all other elements exhibit valence variable.

More lessons on the site

One atom of another monovalent element is connected to one atom of a monovalent element.(HCL) . Two atoms are combined with a bivalent element atom.(H 2 O) or one bivalent atom(CAO) . So, the valence of the element can be represented as a number that shows how many atoms of the monovalent element can be connected to an atom of this element. The element shaft is the number of connections that forms an atom:

Na - monovalent (one connection)

H - monovalent (one connection)

O - bivalent (two bonds at each atom)

S is hexavalent (forms six connections with neighboring atoms)

Rules for determining valence
Elements in connections

1. Val. hydrogen Accept for I.(Unit). Then, in accordance with the formula of water H 2 O to one atom of oxygen, two hydrogen atoms are attached.

2. Oxygen In its compounds always exhibits valence II.. Therefore, carbon in compounding CO 2 (carbon dioxide) has valence IV.

3. Higher Val. equal group number .

4. Low valence equal to the difference between the number 8 (number of groups in the table) and the number of the group in which this element is located, i.e. 8 — N. Groups .

5. Metals located in "A" subgroups shaft are equal to the group number.

6. Nemetals are mainly manifested by two valence: the highest and lowest.

Figuratively speaking, the shaft is the number of "hands", which the atom clings for other atoms. Naturally, there are no "hands" at atoms; Their role is played by T. N. Valence electrons.

You can say otherwise: - This is the ability of the atom of this element to attach a certain number of other atoms.

It is necessary to clearly assimilate the following principles:

There are elements with constant valence (their relatively little) and elements with variable valence (koi most).

Elements with constant valence must be remembered.

In the lessons of chemistry you have already met with the concept of valence of chemical elements. We collected in one place all useful information on this issue. Use it when you prepare for GIA and EGE.

Valence and chemical analysis

Valence - The ability of the atoms of chemical elements to enter into chemical compounds with atoms of other elements. In other words, this is the ability of an atom to form a certain number of chemical bonds with other atoms.

With Latin, the word "valence" is translated as "force, ability". Very faithful name, right?

The concept of "valence" is one of the main in chemistry. It was introduced even before the scientist became known to the structure of the atom (in the distant 1853). Therefore, as the structure of the atom has survived some changes.

So, from the point of view of the electronic theory, the valence is directly related to the number of external electrons of an element atom. This means that under the "valence" implies the number of electronic pairs that the atom is associated with other atoms.

Knowing this, scientists were able to describe the nature of the chemical bond. It lies in the fact that the pair of atoms of the substance divides among themselves a couple of valence electrons.

You ask how the 19th century chemists were able to describe the valence even when they thought that the smallest of the particle atom was not? It is impossible to say that it was so simple - they relied on a chemical analysis.

By chemical analysis, scientists of the past were determined by the composition of the chemical compound: how many atoms of various elements are contained in the molecule of the substance under consideration. To do this, it was necessary to determine what exactness of each element in the sample of pure (without impurities) of the substance.

True, this method is not without flaws. Because it is possible to identify the valence of the element in a similar way only in its simple compound with always monovalent hydrogen (hydride) or always bivalent oxygen (oxide). For example, nitrogen valence in NH 3 - III, since one hydrogen atom is associated with three nitrogen atoms. And carbon valence in methane (CH 4), on the same principle - IV.

This method for determining valence is suitable only for simple substances. But in acids in such a way we can only determine the valence of compounds like acid residues, but not all elements (except for the valence of hydrogen) separately.

As you have already paid attention, the valence of Roman numbers is indicated.

Valence and acid

Since the valence of hydrogen remains unchanged and well known for you, you can easily determine the valence of the acid residue. So, for example, in H 2 SO 3, the valence SO 3 - I, in HCLO 3 Valuation of CLO 3 - I.

An analogously, if the valence of the acid residue is known, it is easy to burn the proper formula of the acid: NO 2 (I) - HNO 2, S 4 O 6 (II) - H 2 S 4 O 6.

Valency and formula

The concept of valence makes sense only for the substances of molecular nature and is not too suitable for describing chemical bonds in compounds of cluster, ionic, crystalline nature, etc.

Indices in molecular formulas of substances reflect the number of elements atoms that are included in their composition. Proper indexes helps the knowledge of the valence of elements. In the same way, looking at the molecular formula and indexes, you can call the valence of part of the elements.

You perform such tasks in chemistry lessons at school. For example, having a chemical formula of a substance in which the valence of one of the elements is known, the valence of another element can be easily determined.

For this you only need to remember that in the molecular nature substance, the number of valencies of both elements are equal. Therefore, use the smallest common multiple (corresponds to the number of free valences required for the connection) to determine the valence of the element unknown to you.

To be clear, we take the formula of iron oxide Fe 2 O 3. Here, two iron atoms with valence III and 3 oxygen atoms with valence II are involved in the formation of chemical bond. The smallest common multiple for them is 6.

• Example: you have formulas Mn 2 O 7. You know the valence of oxygen, it is easy to calculate that the smallest of the total multiple - 14, from where the valence MN - VII.

In the same way, you can enroll on the contrary: to record the correct chemical formula of substance, knowing the valence of elements included in it.

• Example: To correctly burn phosphorus oxide formula, we take into account the valence of oxygen (II) and phosphorus (V). It means that the smallest common is more multiple for P and O - 10. Therefore, the formula has the following form: P 2 O 5.

Well knowing the properties of the elements that they exhibit in various compounds, their valence can be determined even by appearance of such compounds.

For example: copper oxides have red (Cu 2 O) and black (Cuo) painting. Copper hydroxides are painted in yellow (Cuon) and blue (Cu (O) 2) colors.

And so that covalent bonds in substances have become more visual and understandable for you, write their structural formulas. Dashs between elements are depicted between their communication atoms (valence):

Characteristics of valence

Today, the definition of valence of elements is based on knowledge of the structure of external electronic shells of their atoms.

Valence may be:

• constant (metals of the main subgroups);
• variable (non-metals and metals of sidegroups):
• higher valence;
• lowest valence.

Permanent in various chemical compounds remains:

• valence of hydrogen, sodium, potassium, fluorine (I);
• valuation of oxygen, magnesium, calcium, zinc (II);
• aluminum valence (III).

But the valence of iron and copper, bromine and chlorine, as well as many other elements changes when they form various chemicals.

Valence and electronic theory

In the framework of the electronic theory, the valence of the atom is determined on the basis of the number of unpaired electrons, which are involved in the formation of electronic pairs with electrons of other atoms.

Only electrons are involved in the formation of chemical connections, which are on the outer shell of the atom. Therefore, the maximum valence of the chemical element is the number of electrons in the outer electron shell of its atom.

The concept of valence is closely connected with the periodic law, open by D. I. Mendeleev. If you carefully look at the Mendeleev table, you can easily notice: the position of the element in the transition system and its valence is not equivous. The highest valence of elements that belong to the same group, corresponds to the procedure number of the group in the periodic system.

Low valence You will learn when from the number of groups in the Mendeleev table (eight), take away the number of the group of the element that interests you.

For example, the valence of many metals coincides with the number numbers in the table of periodic elements to which they relate.

Chemical element valence table

In brackets are given those valences that have elements possessing are rarely.

Valence and degree of oxidation

So, speaking of the degree of oxidation, they mean that an atom in the substance is ionic (which is important) of nature has a conditional charge. And if the valence is a neutral characteristic, the degree of oxidation may be negative, positive or equal to zero.

Interestingly, for the atom of the same element, depending on the elements with which it forms a chemical compound, the valence and the degree of oxidation may coincide (H 2 O, CH 4, etc.) and vary (H 2 O 2, HNO 3 ).

Conclusion

Digging your knowledge about the structure of atoms, you will find out deeper and more details and valence. This characteristic of chemical elements is not exhaustive. But she has a great applied value. What you have changed more than once, solving the tasks and conducting chemical experiments in the lessons.

This article is created to help you systematize your knowledge of valence. And also recall how it can be determined and where the valence finds the use.

We hope this material will be useful for you when preparing homework and self-preparation to the control and exams.

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Considering the formulas of various compounds, it is not difficult to notice that number of atoms The same element in the molecules of various substances is not equally. For example, HCl, NH 4 Cl, H 2 S, H 3 PO 4, etc. The number of hydrogen atoms in these compounds varies from 1 to 4. This is characteristic not only for hydrogen.

How to guess which index put next to the designation of the chemical element?How are the formulas of the substance are compiled? It is easy to do when you know the valence of elements that are part of the molecule of this substance.

– This property of the atom of this element is to attach, hold or replace in chemical reactions a certain number of atoms of another element. The valence of the hydrogen atom is adopted per unit of valence. Therefore, sometimes the definition of valence is formulated as: valence – This property of the atom of this element is to attach or replace a certain number of hydrogen atoms.

If one hydrogen atom is attached to one atom of this element, the element is monovalent, if two – two-duty I.etc. Hydrogen compounds are known not for all elements, but almost all elements form compounds with oxygen O. oxygen is considered constantly bivalent.

Permanent valence:

I. – H, na, li, k, rb, cs
II. – O, BE, MG, CA, SR, BA, RA, ZN, CD
III – B, AL, GA, IN

But what to do if the element does not connect with hydrogen? Then the valence of the desired element is determined by the valence of the known element. Most often, it is found using oxygen valence, because in the compounds its valence is always 2. For example, It will not be difficult to find the valence of elements in the following compounds: Na 2 O (Na valence – 1, O. – 2), Al 2 O 3 (Al valence – 3, O. – 2).

The chemical formula of this substance can be made, only knowing the valence of elements. For example, make formulas such compounds such as CaO, Bao, CO, simply, because the number of atoms in molecules is equally, since the valence of elements is equal.

And if valence is different? When do we act in this case? It is necessary to remember the following rule: in the formula of any chemical compound, the product of the valence of one element by the number of its atoms in the molecule is equal to the product of the valence to the number of atoms of another element. For example, if it is known that the mn valence in the compound is 7, and o – 2, then the compound formula will look like Mn 2 O 7.

How did we get the formula?

Consider the algorithm for the compilation of valence formulas for consisting of two chemical elements.

There is a rule that the number of valencies in one chemical element is equal to the number of valences in another. Consider on the example of the formation of a molecule consisting of manganese and oxygen.
We will be in accordance with the algorithm:

1. Record near the symbols of chemical elements:

2. We put on the chemical elements of their valence numbers (the valence of the chemical element can be found in the table of the periodic Mendeleva system, manganese – 7, at oxygen – 2.

3. We find the smallest common multiple (the smallest number that is divided without a balance of 7 and 2). This is a number 14. We divide it on the valence of elements 14: 7 \u003d 2, 14: 2 \u003d 7, 2 and 7, they are indexes, respectively, phosphorus and oxygen. We substitute indexes.

Knowing the valence of one chemical element, following the rule: the valence of one element × the number of its atoms in the molecule \u003d the valence of another element × the number of atoms of this (other) element, one can determine the valence of the other.

Mn 2 O 7 (7 · 2 \u003d 2 · 7).

The concept of valence was introduced into chemistry before the structure of the atom was known. It is now established that this property of the element is associated with the number of external electrons. For many elements, the maximum valence follows from the position of these elements in the periodic system.

Have questions? Want to know more about valence?
To get the help of a tutor -.

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There are several definitions of the concept of "valence". Most often, this term is the ability of one element atoms to attach a certain number of atoms of other elements. Often, those who are just starting to study chemistry, the question arises: how to determine the valence of the element?. Make it easy, knowing several rules.

Valence constant and variables

Consider compounds HF, H2S and CAH2. In each of these examples, one hydrogen atom attaches to itself only one atom of another chemical element, which means its valence is one. The value of valence is recorded above the symbol of the chemical element of Roman numbers.

In the given example, the fluorine atom is associated with only one monovalent H atom, it means it is also the valence of it is also equal to 1. The sulfur atom in H2S is already attached to the two H atoms, so it is bivalent in this connection. With two hydrogen atoms, calcium in its hydride Cah2, which means its valence is two.

Oxygen in the overwhelming majority of its compounds is bivalent, that is, forms two chemical bonds with other atoms.

The sulfur atom in the first case joins two oxygen atoms to itself, that is, the total forms 4 chemical bonds (one oxygen forms two bonds, then sulfur - twice 2), that is, its valence is equal to 4.

In the compound SO3, the sulfur already joins the three atoms O, therefore the valence of it is equal to 6 (three times forms two bonds with each oxygen atom). An calcium atom also attaches only one oxygen atom, forming two bonds with it, which means that its valence is the same as in o, that is, equal to 2.

Note that the H atom is monovalent in any connection. Always (except ion hydroxonium H3O (+)) is 2 oxygen valence. Two chemical bonds both with hydrogen and oxygen forms calcium. These are elements with permanent valence. In addition to the already mentioned, permanent valence have:

• Li, Na, k, F are monovalent;
• BE, MG, CA, Zn, CD - have valence, equal to II;
• B, Al and Ga are trivalent.

The sulfur atom, in contrast to the considered cases, in compound with hydrogen, has a valence equal to II, and with oxygen can be four- and hexavalent. About the atoms of such elements say they have a variable valence. At the same time, its maximum value in most cases coincides with the number of the group in which the element in the periodic system is located (rule 1).

From this rule there are many exceptions. So, the element 1 of the group copper, exhibits valence and I, and II. Iron, cobalt, nickel, nitrogen, fluorine, on the contrary, have a maximum valence, smaller than the group number. So, for Fe, CO, Ni, it is II and III, for N - IV, and for fluorine - I.

The minimum value of valence always corresponds to the difference between Number 8 and the group number (rule 2).

To unambiguously determine what the valence of elements in which it is variable, only by the formula of a certain substance.

Determination of valence in binary connection

Consider how to determine the valence of the element in the binary (of two elements) of the connection. Here are two options: in conjunction, the valence of atoms of one element is known exactly or both particles with a variable valence.

The case is the first:

Second case:

Determination of valence according to the formula of a three-element particle.

Not all chemicals consist of diatomic molecules. How to determine the valence of the element in the three-element particle? Consider this issue on the example of the formulas of two K2CR2O7 compound.

If instead of potassium in the formula will be present an iron, or another element with a variable valence, we will need to know what the valence of the acid residue. For example, it is necessary to calculate the valence of atoms of all elements in the compound with FESO4 formula.

It should be noted that the term "valence" often use in organic chemistry. In compiling the formulas of inorganic compounds, the concept of "oxidation" is more often used.