The highest valency of an element is determined. Valence. Definition of valence

The level of knowledge about the structure of atoms and molecules in the 19th century did not allow explaining the reason why atoms form a certain number of bonds with other particles. But the ideas of scientists were ahead of their time, and valency is still being studied as one of the basic principles of chemistry.

From the history of the concept of "valency of chemical elements"

The outstanding English chemist of the 19th century, Edward Frankland, introduced the term "bond" into scientific use to describe the process of interaction of atoms with each other. The scientist noticed that some chemical elements form compounds with the same number of other atoms. For example, nitrogen adds three hydrogen atom in an ammonia molecule.

In May 1852, Frankland hypothesized that there was a specific number of chemical bonds that an atom could form with other tiny particles of matter. Frankland used the phrase "connecting force" to describe what would later be called valency. A British chemist has determined how many chemical bonds form the atoms of individual elements known in the middle 19th century. Frankland's work was an important contribution to modern structural chemistry.

Development of views

German chemist F.A. Kekule proved in 1857 that carbon is a tetrabasic one. In its simplest compound - methane - there are bonds with 4 hydrogen atoms. The scientist used the term "basicity" to denote the property of elements to attach a strictly defined amount of other particles. In Russia, data on were systematized by A. M. Butlerov (1861). The theory of chemical bonding received further development thanks to the doctrine of the periodic change in the properties of elements. Its author is another outstanding D. I. Mendeleev. He proved that the valency chemical elements in compounds, other properties are due to the position they occupy in the periodic system.

Graphical representation of valency and chemical bonding

The possibility of a visual representation of molecules is one of the undoubted advantages of the theory of valency. The first models appeared in the 1860s, and since 1864, circles with a chemical sign inside have been used. Between the symbols of atoms, a dash is indicated and the number of these lines is equal to the value of the valence. In the same years, the first ball-and-stick models were made (see photo on the left). In 1866, Kekule proposed a stereochemical drawing of a carbon atom in the form of a tetrahedron, which he included in his textbook Organic Chemistry.

The valency of chemical elements and the formation of bonds were studied by G. Lewis, who published his works in 1923 after the name of the negatively charged smallest particles that make up the shells of atoms. In his book, Lewis used dots around the four sides to represent valence electrons.

Valency for hydrogen and oxygen

Before the creation, the valency of chemical elements in compounds was usually compared with those atoms for which it is known. Hydrogen and oxygen were chosen as standards. Another chemical element attracted or replaced a certain number of H and O atoms.

In this way, properties were determined in compounds with monovalent hydrogen (the valency of the second element is indicated by a Roman numeral):

• HCl - chlorine (I):
• H 2 O - oxygen (II);
• NH 3 - nitrogen (III);
• CH 4 - carbon (IV).

In oxides K 2 O, CO, N 2 O 3, SiO 2, SO 3, the oxygen valency of metals and non-metals was determined by doubling the number of attached O atoms. The following values ​​were obtained: K (I), C (II), N (III) , Si (IV), S (VI).

How to determine the valency of chemical elements

There are regularities in the formation of a chemical bond involving shared electron pairs:

• The typical valency of hydrogen is I.
• The usual valency of oxygen is II.
• For non-metal elements, the lowest valence can be determined by formula 8 - the number of the group in which they are located in the periodic system. The highest, if possible, is determined by the group number.
• For elements of side subgroups, the maximum possible valence is the same as their group number in the periodic table.

The determination of the valence of chemical elements according to the formula of the compound is carried out using the following algorithm:

1. Write the known value for one of the elements above the chemical sign. For example, in Mn 2 O 7 the oxygen valency is II.
2. Calculate the total value, for which it is necessary to multiply the valence by the number of atoms of the same chemical element in the molecule: 2 * 7 \u003d 14.
3. Determine the valency of the second element for which it is unknown. Divide the value obtained in step 2 by the number of Mn atoms in the molecule.
4. 14: 2 = 7. in its highest oxide - VII.

Constant and variable valency

Valence values ​​for hydrogen and oxygen are different. For example, sulfur in the compound H 2 S is divalent, and in the formula SO 3 it is hexavalent. Carbon forms CO monoxide and CO 2 dioxide with oxygen. In the first compound, the valency of C is II, and in the second, IV. The same value in methane CH 4 .

Most elements exhibit not constant, but variable valency, for example, phosphorus, nitrogen, sulfur. The search for the main causes of this phenomenon led to the emergence of chemical bond theories, ideas about the valence shell of electrons, and molecular orbitals. Existence different values the same property was explained from the standpoint of the structure of atoms and molecules.

Modern ideas about valency

All atoms consist of a positive nucleus surrounded by negatively charged electrons. The outer shell that they form is unfinished. The completed structure is the most stable, containing 8 electrons (an octet). The emergence of a chemical bond due to common electron pairs leads to an energetically favorable state of atoms.

The rule for compound formation is to complete the shell by accepting electrons or donating unpaired ones, whichever process is easier. If an atom provides for the formation of a chemical bond negative particles that do not have a pair, then it forms as many bonds as it has unpaired electrons. By modern ideas, the valence of atoms of chemical elements is the ability to form a certain number of covalent bonds. For example, in a hydrogen sulfide molecule H 2 S, sulfur acquires valency II (-), since each atom takes part in the formation of two electron pairs. The "-" sign indicates the attraction of an electron pair to a more electronegative element. For a less electronegative value, “+” is added to the valence value.

With the donor-acceptor mechanism, electron pairs of one element and free valence orbitals of another element take part in the process.

The dependence of valence on the structure of the atom

Consider, using the example of carbon and oxygen, how the valence of chemical elements depends on the structure of the substance. The periodic table gives an idea of ​​the main characteristics of the carbon atom:

• chemical sign - C;
• element number - 6;
• core charge - +6;
• protons in the nucleus - 6;
• electrons - 6, including 4 external ones, of which 2 form a pair, 2 are unpaired.

If the carbon atom in CO monoxide forms two bonds, then only 6 negative particles come to its use. To acquire an octet, it is necessary that the pairs form 4 external negative particles. Carbon has valency IV (+) in dioxide and IV (-) in methane.

The serial number of oxygen is 8, the valence shell consists of six electrons, 2 of which do not form a pair and take part in chemical bonding and interaction with other atoms. The typical valency of oxygen is II (-).

Valency and oxidation state

In many cases it is more convenient to use the concept of "oxidation state". This is the name given to the charge an atom would acquire if all the bonding electrons were transferred to an element that has a higher electronegativity (EO) value. Oxidation number in simple matter equals zero. A “-” sign is added to the oxidation state of a more EO element, a “+” sign is added to a less electronegative one. For example, for metals of the main subgroups, oxidation states and ion charges are typical, equal to the group number with a “+” sign. In most cases, the valency and oxidation state of atoms in the same compound are numerically the same. Only when interacting with more electronegative atoms, the oxidation state is positive, with elements in which the EO is lower, it is negative. The concept of "valency" is often applied only to substances of a molecular structure.

Often people hear the word "valency" without fully understanding what it is. So what is valency? Valence is one of the terms used in chemical structure. Valence, in fact, determines the ability of an atom to form chemical bonds. Quantitatively, valency is the number of bonds in which an atom participates.

What is the valency of an element

Valence is an indicator of the ability of an atom to attach other atoms, forming chemical bonds with them, inside the molecule. The number of bonds of an atom is equal to the number of its unpaired electrons. These bonds are called covalent.

An unpaired electron is a free electron on the outer shell of an atom that pairs with outer electron another atom. Each pair of such electrons is called "electronic", and each of the electrons is called valence. So the definition of the word "valency" is the number of electron pairs with which one atom is connected to another atom.

Valency can be schematically depicted in structural chemical formulas. When this is not necessary, simple formulas are used where the valency is not indicated.

Maximum valency of chemical elements from one group periodic system Mendeleev is equal to the serial number of this group. Atoms of the same element can have different valencies in different chemical compounds. The polarity of the covalent bonds that form is not taken into account. This is why valence has no sign. Also, valency cannot be negative and equal to zero.

Sometimes the concept of "valency" is equated with the concept of "oxidation state", but this is not so, although sometimes these indicators do coincide. Oxidation state is a formal term that refers to the possible charge that an atom would receive if its electron pairs were transferred to more electrically negative atoms. Here the oxidation state can have some kind of sign and is expressed in units of charge. This term is common in inorganic chemistry, because in inorganic compounds it is difficult to judge valency. And, conversely, in organic chemistry, valency is used, because molecular structure contains most of the organic compounds.

Now you know what the valency of chemical elements is!

Lesson goals.

Didactic:

• based on the knowledge of students, repeat the concept of “chemical formula”;
• to promote the formation of the concept of “valence” among students and the ability to determine the valence of atoms of elements according to the formulas of substances;
• to focus the attention of schoolchildren on the possibility of integrating chemistry and mathematics courses.

Developing:

• continue the formation of skills to formulate definitions;
• explain the meaning of the concepts studied and explain the sequence of actions in determining valence according to the formula of a substance;
• promote enrichment vocabulary, the development of emotions, creativity;
• develop the ability to highlight the main, essential, compare, generalize, develop diction, speech.

Educational:

• foster a sense of camaraderie, the ability to work collectively;
• to increase the level of aesthetic education of students;
• orient students towards healthy lifestyle life.

Planned learning outcomes:

1. 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,
2. To 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.

Concepts introduced in the lesson for the first time: valency, constant and variable valency.

Organizational forms: conversation, individual tasks, independent work.

Means of education: algorithm for determining valency.

Demonstration equipment: ball-and-stick models of molecules of hydrogen chloride, water, ammonia, methane.

Equipment for students: on each table “Algorithm for determining valency”.

Advance task: individual task - to prepare a report on the topic “Evolution of the concept of “valence”.

During the classes

I. Approximate and motivational stage.

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

Exercise: What is written here? (Teacher's demonstration of formulas printed on separate sheets).

2. Individual work on the cards of three students on the topic “Relative molecular weight”. (Do the solution on the board.) Teacher check.

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

Reference data:

• Ar (Na) = 23
• Ar (Cl) = 35.5
• Ar (K) = 39
• Ar (O) = 16

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

Reference data:

• Ar (Cu) = 64
• Ar (O) = 16
• Ar(S)=3 2

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

Reference data:

• Ar (C) = 12
• Ar (H) = 1
• Ar (N) = 14
• Ar (O) = 16

3. Independent work students in notebooks.

The task is of an information-computational nature (the condition is written in the handout).

The effectiveness of toothpastes in the prevention of caries can be compared by the content of active fluoride in them, which can interact with tooth enamel. Toothpaste“Crest” (made in the USA) contains, as indicated on the package, SnF 2, and the toothpaste “FM extra DENT” (made in Bulgaria) contains NaF. Calculate which of these two pastes is more potent for caries prevention.

Examination: one student reads the solution orally.

II. Operational and executive stage.

1. Teacher's explanation. Formulation of the problem.

The concept of valency.

- Until now, we have used ready-made formulas given in the textbook. Chemical formulas can be derived based on data on the composition of substances. But most often, when compiling chemical formulas, the laws that the elements obey, connecting with each other, are taken into account.

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

Interview with students:

What do molecules have in common?

Suggested answer: The presence of hydrogen atoms.

– How do they differ from each other?

Suggested answer:

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

Demonstration of ball-and-stick models.

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

(Listen to student responses.)

Conclusion: Atoms have a different ability to hold a certain number of other atoms in compounds. This is called valence. The word "valence" comes from lat. valentia - strength.

Notebook entry:

Valency 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:

The valency of the hydrogen atom is taken as unity, and that of oxygen is II.

2. Evolution of the concept of “valence” (student post).

- 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 the largest 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 associated with 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 tetravalence of carbon in organic compounds. The concept of valency formed an important part of the theory of the chemical structure of A.M. Butlerov in 1861

Periodic law D.I. Mendeleev in 1869 revealed the dependence of the valency of an element on its position in the 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 .

3. Determination of the valence of atoms of elements in compounds.

Rule for determining valency: the number of valency units of all atoms of one element is equal to the number of valence units of all atoms of another element.

Algorithm for determining valency.

4. Exercise: determine the valency of elements in substances ( training apparatus: Pupils go to the blackboard in a chain). Assignment 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. Evaluative-reflexive stage.

Primary test of mastery of knowledge.

Within three minutes, you must complete one of the three tasks of your choice. Choose only the task that you can handle. Assignment in handout.

• 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.

Selective check. Student Advisor for ready template checks 4 notebooks of students.

Work on mistakes. The answers are on the back of the board.

IV. Summing up the lesson.

Interview with students:

• What problem did we pose at the beginning of the lesson?
• What conclusion did we come to?
• Define "valence".
• What is the valence of a hydrogen atom? Oxygen?
• How to determine the valence of an atom in a compound?

Evaluation of the work of students as a whole and individual students.

Homework: § 4, pp. 23–25, ex. on page 25.

- Thank you for the lesson. Goodbye.

One of the important topics in the study of school topics is the course on valency. This will be discussed in the article.

Valence - what is it?

Valency in chemistry means the property of the atoms of a chemical element to bind to itself the atoms of another element. Translated from Latin - strength. It is expressed in numbers. For example, the valence of hydrogen will always be equal to one. If we take the formula of water - H2O, it can be represented as H - O - H. One oxygen atom was able to bind two hydrogen atoms to itself. This means that the number of bonds that oxygen creates is two. And the valency of this element will be equal to two.

In turn, hydrogen will be divalent. Its atom can only be connected to one atom of a chemical element. In this case, oxygen. More precisely, atoms, depending on the valency of the element, form pairs of electrons. How many such pairs are formed - such will be the valence. The numeric value is called an index. Oxygen has an index of 2.

How to determine the valency of chemical elements according to the table of Dmitry Mendeleev

Looking at the periodic table of elements, you can see the vertical rows. They are called groups of elements. Valency also depends on the group. The elements of the first group have the first valence. The second is the second. Third - third. And so on.

There are also elements with a constant valence index. For example, hydrogen, halogen group, silver and so on. They must be learned.

How to determine the valence of chemical elements by formulas?

Sometimes it is difficult to determine the valence from the periodic table. Then you need to look at the specific chemical formula. Take the oxide FeO. Here, iron, like oxygen, has a valency index of two. But in Fe2O3 oxide it is different. Iron will be trivalent.

Must always be remembered different ways definitions of valency and do not forget them. Know its constant numerical values. Which elements have them. And, of course, use the table of chemical elements. And also to study individual chemical formulas. It is better to represent them in a schematic form: H - O - H, for example. Then the connections are visible. And the number of dashes (dashes) will be the numerical value of the valency.

Valence- the ability of elements to attach other elements to themselves.

talking plain language, is a number indicating how many elements a certain atom can attach to itself.

The key point in chemistry is the correct recording of the formulas of compounds.

There are several rules that make it easier for us to correctly compose formulas.

1. The valency of all metals of the main subgroups is equal to the group number:

The figure shows an example of the main and secondary subgroups of group I.

2. The valence of oxygen is two

3. The valence of hydrogen is equal to one

4. Non-metals exhibit two types of valency:

• Inferior (8th group number)
• Higher (equal to group number)

A) In compounds with metals, non-metals exhibit a lower valence!

B) In binary compounds, the sum of the valency of one type of atom is equal to the sum of the valency of another type of atom!

The valency of aluminum is three (aluminum is a group III metal). The valency of oxygen is two. The sum of valence for two aluminum atoms is 6. The sum of valency for three oxygen atoms is also 6.

1) Determine the valencies of the elements in the compounds:

The valency of aluminum is III. In formula 1 atom => the total valency is also 3. Therefore, for all chlorine atoms, the valence will also be 3 (the rule of binary compounds). 3:3=1. The valency of chlorine is 1.

The valency of oxygen is 2. In the compound there are 3 oxygen atoms => the total valency is 6. For two atoms the total valency is 6 => for one iron atom - 3 (6: 2 = 3)

2) Write the formulas of the compound consisting of:

sodium and oxygen

The valency of oxygen is II.

Sodium metal of the first group of the main subgroup => its valency is I.