Determination of valence in chemical compounds. Valence

Until now, you have used the chemical formulas of substances given in the textbook, or those that the teacher told you. How to correctly compose chemical formulas?

Chemical formulas of substances are compiled based on knowledge of the qualitative and quantitative composition of the substance. There are a huge number of substances; naturally, it is impossible to remember all the formulas. This is not necessary! It is important to know a certain pattern according to which atoms are able to combine with each other to form new chemical compounds. This ability is called valency.

Valence– the property of atoms of elements to attach a certain number of atoms of other elements

Let's consider models of molecules of some substances, such as water, methane and carbon dioxide.

It can be seen that in a water molecule an oxygen atom attaches two hydrogen atom. Therefore, its valence is two. In a methane molecule, a carbon atom attaches four hydrogen atoms, its valency in this substance is four. The valence of hydrogen in both cases is equal to one. Carbon exhibits the same valence in carbon dioxide, but unlike methane, the carbon atom attaches two oxygen atoms, since the valency of oxygen is two.

There are elements whose valence does not change in compounds. Such elements are said to have constant valence. If the valence of an element can be different, these are elements with variable valence. Valence of some chemical elements is given in Table 2. Valency is usually denoted by Roman numerals.

Table 2. Valency of some chemical elements

Element symbol	Valence	Element symbol	Valence
H, Li, Na, K, F, Ag	I	C, Si, Sn, Pb	II, IV
Be, Mg, Ca, Ba, Zn, O	II	N	I, II, III, IV
Al,B	III	P, As, Sb	III, V
S	II, IV, VI	Cl	I, II, III, IV, V, VII
Br,I	I, III, V	Ti	II, III, IV

It is worth noting that the highest valence of an element numerically coincides with serial number group of the Periodic System in which it is located. For example, carbon is in group IV, its highest valency is IV.

There are three exceptions:

nitrogen– is in group V, but its highest valency is IV;
oxygen– is in group VI, but its highest valency is II;
fluorine– is in group VII, but its highest valence is I.

Based on the fact that all elements are located in eight groups of the Periodic System, valence can take values from I to VIII.

Drawing up formulas of substances using valence

To compile formulas of substances using valence, we will use a certain algorithm:

Determination of valency using the formula of a substance

To determine the valency of elements using the formula of a substance, the reverse procedure is necessary. Let's also consider it using the algorithm:

When studying this section, we considered complex substances that contain only two types of atoms of chemical elements. Formulas more complex substances are compiled differently.

Binary compounds – compounds that contain two types of atoms of elements

To determine the order of the sequence of compounds of atoms, structural (graphical) formulas of substances are used. In such formulas, the valencies of elements are indicated by valence strokes (dashes). For example, a water molecule can be represented as

N─O─N

The graphical formula depicts only the order of connection of atoms, but not the structure of molecules. In space, such molecules may look different. Thus, a water molecule has the angular structural formula:

Valence– the ability of atoms of elements to attach a certain number of atoms of other chemical elements
There are elements with constant and variable valence
The highest valence of a chemical element coincides with its group number in the Periodic Table of Chemical Elements D.I. Mendeleev. Exceptions: nitrogen, oxygen, fluorine
Binary compounds– compounds that contain two types of atoms of chemical elements
Graphic formulas reflect the order of bonds of atoms in a molecule using valence strokes
The structural formula reflects the actual shape of the molecule in space

In order to determine the valence of a particular substance, you need to look at Mendeleev’s periodic table of chemical elements; the designations in Roman numerals will be the valences of certain substances in this table. For example, BUT, hydrogen (H) will always be monovalent, and oxygen (O) will always be divalent. Here is a cheat sheet below that I think will help you)

First of all, it is worth noting that chemical elements can have both constant and variable valency. As for constant valency, you simply need to memorize such elements

Are considered monovalent alkali metals, hydrogen, as well as halogens;

But boron and aluminum are trivalent.

So, now let's go through the periodic table to determine valence. The highest valence for an element is always equated to its group number

The lowest valence is determined by subtracting the group number from 8. Non-metals are endowed with a lower valence to a greater extent.

Chemical elements can be of constant or variable valence. Elements with constant valence must be learned. Always

monovalent hydrogen, halogens, alkali metals
divalent oxygen, alkaline earth metals.
trivalent aluminum (Al) and boron (B).

Valency can be determined using the periodic table. The highest valence of an element is always equal to the number of the group in which it is found.

Nonmetals most often have the lowest variable valency. To find out the lowest valency, the group number is subtracted from 8 - the result will be the desired value. For example, sulfur is in group 6 and its highest valency is VI, the lowest valency will be II (86 = 2).

According to school definition Valency is the ability of a chemical element to form a certain number of chemical bonds with other atoms.

As is known, valency can be constant (when a chemical element always forms the same number of bonds with other atoms) and variable (when, depending on a particular substance, the valency of the same element changes).

The periodic system of chemical elements by D.I. Mendeleev will help us determine valence.

The following rules apply:

1) Maximum The valence of a chemical element is equal to the group number. For example, chlorine is in the 7th group, which means it has a maximum valence of 7. Sulfur: it is in the 6th group, which means it has a maximum valency of 6.

2) Minimum valence for non-metals equals 8 minus the group number. For example, the minimum valency of the same chlorine is 8 7, that is, 1.

Alas, there are exceptions to both rules.

For example, copper is in group 1, but the maximum valency of copper is not 1, but 2.

Oxygen is in group 6, but its valency is almost always 2, and not 6 at all.

It is useful to remember the following rules:

3) All alkaline metals (metals of group I, the main subgroup) always have valence 1. For example, the valency of sodium is always 1 because it is an alkali metal.

4) All alkaline earth metals (metals of group II, the main subgroup) always have valence 2. For example, the valency of magnesium is always 2 because it is an alkaline earth metal.

5) Aluminum always has a valency of 3.

6) Hydrogen always has a valence of 1.

7) Oxygen almost always has a valence of 2.

8) Carbon almost always has a valence of 4.

It should be remembered that in different sources definitions of valence may vary.

More or less accurately, valence can be defined as the number of shared electron pairs through which a given atom is connected to others.

According to this definition, the valence of nitrogen in HNO3 is 4, not 5. Nitrogen cannot be pentavalent, because in this case there would be 10 electrons circling the nitrogen atom. But this cannot happen, because the maximum number of electrons is 8.

The valence of any chemical element is its property, or rather the property of its atoms (atoms of this element) to hold a certain number of atoms, but of another chemical element.

There are chemical elements with both constant and variable valence, which changes depending on which element it (this element) is in combination with or enters into.

Valencies of some chemical elements:

Let's now move on to how the valency of an element is determined from the table.

So, valence can be determined by periodic table:

the highest valency corresponds to (equal to) the group number;
the lowest valence is determined by the formula: group number - 8.

From school course in chemistry we know that all chemical elements can have a constant or variable valence. Elements that have a constant valency just need to be remembered (for example, hydrogen, oxygen, alkali metals and other elements). Valency can be easily determined from the periodic table, which is in any chemistry textbook. The highest valency corresponds to its number of the group in which it is located.

The valence of any element can be determined by the periodic table itself, by the group number.

At least this can be done in the case of metals, because their valence is equal to the group number.

The story with non-metals is a little different: their highest valence (in compounds with oxygen) is also equal to the group number, but the lowest valency (in compounds with hydrogen and metals) must be determined using the following formula: 8 - group number.

The more you work with chemical elements, the better you remember their valency. To get started, this cheat sheet will suffice:

Those elements whose valence is not constant are highlighted in pink.

Valency is the ability of atoms of some chemical elements to attach to themselves atoms of other elements. To successfully write formulas, the right decision tasks you need to know well how to determine valence. First you need to learn all the elements with constant valency. Here they are: 1. Hydrogen, halogens, alkali metals (always monovalent); 2. Oxygen and alkaline earth metals (divalent); 3. B and Al (trivalent). To determine valency using the periodic table, you need to find out which group the chemical element is in and determine whether it is in the main group or a secondary group.

An element can have one or more valencies.

The maximum valency of an element is equal to the number of valence electrons. We can determine valency by knowing the location of the element in periodic table. The maximum valency number is equal to the number of the group in which the required element is located.

Valence is indicated by a Roman numeral and is typically written in the upper right corner of the element symbol.

Some elements may have different valencies in different compounds.

For example, sulfur has the following valencies:

II in H2S compound
IV in SO2 compound
VI in SO3 compound

The rules for determining valence are not as easy to use, so they need to be remembered.

Determining valency using the periodic table is simple. As a rule, it corresponds to the number of the group in which the element is located. But there are elements that can have different valencies in different compounds. In this case we're talking about about constant and variable valence. The variable can be maximum, equal to the group number, or it can be minimum or intermediate.

But it is much more interesting to determine the valency in compounds. There are a number of rules for this. First of all, it is easy to determine the valency of elements if one element in a compound has a constant valence, for example, oxygen or hydrogen. On the left is a reducing agent, that is, an element with a positive valence, on the right is an oxidizing agent, that is, an element with a negative valence. The index of an element with a constant valence is multiplied by this valence and divided by the index of an element with an unknown valence.

Example: silicon oxides. The valence of oxygen is -2. Let's find the valency of silicon.

SiO 1*2/1=2 The valence of silicon in monoxide is +2.

SiO2 2*2/1=4 The valence of silicon in dioxide is +4.

There are several definitions of the concept of “valency”. Most often, this term refers to the ability of atoms of one element to attach a certain number of atoms of other elements. Often those who are just starting to study chemistry have a question: How to determine the valency of an element? This is easy to do if you know a few rules.

Valences constant and variable

Let's consider the 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 equal to one. The valency value is written above the symbol of the chemical element in Roman numerals.

In the above example, the fluorine atom is bonded to only one monovalent H atom, which means its valence is also equal to 1. The sulfur atom in H2S already attaches two H atoms to itself, so it is in this connection bivalent. Calcium in its hydride CaH2 is also bound to two hydrogen atoms, which means its valency is two.

Oxygen in the vast majority of its compounds is divalent, that is, it forms two chemical bonds with other atoms.

In the first case, the sulfur atom attaches two oxygen atoms to itself, that is, it forms 4 chemical bonds in total (one oxygen forms two bonds, which means sulfur - two times 2), that is, its valency is 4.

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

Note that the H atom is monovalent in any compound. The valence of oxygen is always (except for the hydronium ion H3O(+)) equal to 2. Calcium forms two chemical bonds with both hydrogen and oxygen. These are elements with constant valence. In addition to those already mentioned, the following have constant valence:

Li, Na, K, F - monovalent;
Be, Mg, Ca, Zn, Cd - have a valence of II;
B, Al and Ga are trivalent.

The sulfur atom, in contrast to the cases considered, in combination with hydrogen has a valence of II, and with oxygen it can be tetra- or hexavalent. Atoms of such elements are said to have variable valence. Moreover, its maximum value in most cases coincides with the number of the group in which the element is located in Periodic table(rule 1).

There are many exceptions to this rule. Thus, element 1 of group copper exhibits valencies of both I and II. Iron, cobalt, nickel, nitrogen, fluorine, on the contrary, have a maximum valency less than the group number. So, for Fe, Co, Ni these are II and III, for N - IV, and for fluorine - I.

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

It is possible to unambiguously determine what the valence of elements for which it is variable is only by the formula of a certain substance.

Determination of valence in a binary compound

Let's consider how to determine the valency of an element in a binary (of two elements) compound. Here two options are possible: in a compound, the valence of the atoms of one element is known exactly, or both particles have a variable valence.

Case one:

Case two:

Determination of valency using the three-element particle formula.

Not everyone chemical substances consist of diatomic molecules. How to determine the valence of an element in a three-element particle? Let's consider this question using the example of the formulas of two compounds K2Cr2O7.

If, instead of potassium, the formula contains iron, or another element with variable valence, we will need to know what the valence of the acid residue is. For example, you need to calculate the valences of the atoms of all elements in combination with the formula FeSO4.

It should be noted that the term “valence” is more often used in organic chemistry. When compiling formulas for inorganic compounds, the concept of “oxidation state” is often used.

There are elements whose valence is always constant, and there are very few of them. But all other elements exhibit variable valence.

More lessons on the site

One atom of another monovalent element combines with one atom of a monovalent element(HCl) . An atom of a divalent element combines with two atoms of a monovalent element.(H2O) or one divalent atom(CaO) . This means that the valence of an element can be represented as a number that shows how many atoms of a monovalent element an atom can combine with of this element. The shaft of an element is the number of bonds that an atom forms:

Na – monovalent (one bond)

H – monovalent (one bond)

O – divalent (two bonds per atom)

S – hexavalent (forms six bonds with neighboring atoms)

Rules for determining valency
elements in connections

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

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

3. Supreme shaft equal to group number .

4. Lowest valency 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, i.e. 8 — N groups .

5. For metals in the “A” subgroups, the shaft is equal to the group number.

6. Nonmetals generally exhibit two valences: higher and lower.

Figuratively speaking, a shaft is the number of “arms” with which an atom clings to other atoms. Naturally, atoms do not have any “hands”; their role is played by the so-called. valence electrons.

You can say it differently: is the ability of an atom of a given element to attach a certain number of other atoms.

The following principles must be clearly understood:

There are elements with constant valence (of which there are relatively few) and elements with variable valence (of which the majority are).

Elements with constant valence must be remembered.

Lesson objectives.

Didactic:

Based on the students’ knowledge, repeat the concepts “ chemical formula”;
promote the formation in students of the concept of “valence” and the ability to determine the valence of atoms of elements using the formulas of substances;
to focus schoolchildren’s attention on the possibility of integrating chemistry and mathematics courses.

Educational:

continue to develop the skills to formulate definitions;
explain the meaning of the concepts studied and explain the sequence of actions when determining valency using the formula of a substance;
promote enrichment vocabulary, development of emotions, creative abilities;
develop the ability to highlight the main, essential, compare, generalize, develop diction and speech.

Educational:

foster a sense of camaraderie and the ability to work collectively;
increase the level of aesthetic education of students;
guide students towards healthy image life.

Planned learning outcomes:

Students should be able to formulate the definition of “valency”, know the valence of hydrogen and oxygen atoms in compounds, and use it to determine the valence of atoms of other elements in binary compounds,
Be able to explain the meaning of the concept of “valency” and the sequence of actions when determining the valence of atoms of elements using the formulas of substances.

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

Organizational forms: conversation, individual assignments, independent work.

Means of education: algorithm for determining valence.

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

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

Leading task: individual task – prepare a report on the topic “Evolution of the concept of “valency”.

During the classes

I. Orientative-motivational stage.

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

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

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

Card No. 1. Calculate 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 No. 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 and computational nature (the condition is written in the handout).

The effectiveness of toothpastes in preventing caries can be compared by the content of active fluoride in them, which can interact with tooth enamel. “Crest” toothpaste (made in the USA) contains, as indicated on the packaging, SnF 2, and “FM extra DENT” toothpaste (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 valence.

– 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 drawing up chemical formulas, the patterns that the elements obey when 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.

Conversation with students:

– What do the molecules have in common?

Suggested answer: 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 quantity hydrogen atoms?

(We listen to the students' answers.)

Conclusion: Atoms have different abilities to hold a certain number of other atoms in compounds. This is called valence. The word “valence” comes from the Latin. valentia – strength.

Notebook entry:

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

Valence is indicated by Roman numerals.

Notes on the board and in notebooks:

I II
H2O

I III
H3N

I IV
H4C

The valency of the hydrogen atom is taken to be one, and that of oxygen is II.

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

- IN early XIX century, J. Dalton formulated the law of multiple relations, 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 that we considered).

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 greatest 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 “valence” by E. Frankland in 1853.

Since at that time there were no compounds known for hydrogen in which it was bonded to more than one atom of any other element, the hydrogen atom was chosen as the standard, having a valence of 1.

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

Contributions to the evolution of the concept of “valence” over the years were made by V. Kossel, A. Werner, and G. Lewis.

Since the 30s. In the 20th century, ideas about the nature and character of valence constantly expanded and deepened. 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 valence: 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.

Valency determination algorithm.

Valency determination algorithm	Example
1. Write down the formula of the substance.	H2S, Cu2O
2. Designate the known valency of the element	I H2S,
3. Find the number of valence units of atoms of a known element by multiplying the valency of the element by the number of its atoms	2 I H2S	2 II Cu2O
4. Divide the number of valency units of the atoms by the number of atoms of the other element.	2 The resulting answer is the desired valency H2S	2 The resulting answer is the desired valency Cu2O
I II	5. Do a check, that is, count the number of valence units of each element I II (2=2)	H2S Cu2O (2=2)

4. I II Exercise: determine the valence of elements in substances ( training apparatus

: students come to the board in a line). The task is in the 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-reflective stage.

Primary test of knowledge acquisition.

Within three minutes you must complete one of three tasks of your choice. Choose only the task that you can handle. The task is in the handout. Reproductive level (“3”).
Determine the valence of atoms of chemical elements using the formulas of compounds: NH 3, Au 2 O 3, SiH 4, CuO. Application layer (“4”).
From the given series, write down only those formulas in which the metal atoms are divalent: MnO, Fe 2 O 3, CrO 3, CuO, K 2 O, CaH 2. (“5”). Creative level

Find a pattern in the sequence of formulas: N 2 O, NO, N 2 O 3 and put the valencies above each element.. Random check Student consultant ready-made template

checks 4 student notebooks. Work on mistakes.