From the lesson materials you will learn that the constancy of the composition of a substance is explained by the presence of certain valence possibilities in the atoms of chemical elements; get acquainted with the concept of “valence of atoms of chemical elements”; learn to determine the valence of an element using the formula of a substance if the valence of another element is known.

Topic: Initial chemical ideas

Lesson: Valency of chemical elements

The composition of most substances is constant. For example, a water molecule always contains 2 hydrogen atoms and 1 oxygen atom - H 2 O. The question arises: why do substances have a constant composition?

Let's analyze the composition of the proposed substances: H 2 O, NaH, NH 3, CH 4, HCl. They all consist of atoms of two chemical elements, one of which is hydrogen. There can be 1,2,3,4 hydrogen atoms per atom of a chemical element. But in no substance will there be per hydrogen atom have to several atoms of another chemical element. Thus, a hydrogen atom can attach to itself minimal amount atoms of another element, or rather, only one.

The property of atoms of a chemical element to attach to themselves a certain number of atoms of other elements is called valence.

Some chemical elements have constant valence values ​​(for example, hydrogen(I) and oxygen(II)), others can exhibit several valence values ​​(for example, iron(II,III), sulfur(II,IV,VI), carbon(II, IV)), they are called elements with variable valency. The valence values ​​of some chemical elements are given in the textbook.

Knowing the valences of chemical elements, it is possible to explain why a substance has such a chemical formula. For example, the formula of water is H 2 O. Let us designate the valence capabilities of a chemical element using dashes. Hydrogen has a valence of I, and oxygen has a valence of II: H- and -O-. Each atom can fully utilize its valence capabilities if there are two hydrogen atoms per oxygen atom. The sequence of connections of atoms in a water molecule can be represented as the formula: H-O-H.

A formula that shows the sequence of atoms in a molecule is called graphic(or structural).

Rice. 1. Graphic formula of water

Knowing the formula of a substance consisting of atoms of two chemical elements and the valency of one of them, you can determine the valency of the other element.

Example 1. Let's determine the valency of carbon in the substance CH4. Knowing that the valence of hydrogen is always equal to I, and carbon has attached 4 hydrogen atoms to itself, we can say that the valence of carbon is equal to IV. The valence of atoms is indicated by a Roman numeral above the element sign: .

Example 2. Let's determine the valency of phosphorus in the compound P 2 O 5. To do this you need to do the following:

1. above the sign of oxygen, write down the value of its valence – II (oxygen has a constant valence value);

2. multiplying the valence of oxygen by the number of oxygen atoms in the molecule, find total number valence units – 2·5=10;

3. divide the resulting total number of valency units by the number of phosphorus atoms in the molecule – 10:2=5.

Thus, the valence of phosphorus in this compound is equal to V – .

1. Emelyanova E.O., Iodko A.G. Organization cognitive activity students in chemistry lessons in grades 8-9. Basic notes with practical tasks, tests: Part I. - M.: School Press, 2002. (p. 33)

2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 36-38)

3. Chemistry: 8th grade: textbook. for general education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§16)

4. Chemistry: inorg. chemistry: textbook. for 8th grade. general education institutions / G.E. Rudzitis, F.G. Feldman. – M.: Education, OJSC “Moscow Textbooks”, 2009. (§§11,12)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. – M.: Avanta+, 2003.

Additional web resources

1. Unified collection of digital educational resources ().

2. Electronic version of the journal “Chemistry and Life” ().

Homework

1. p.84 No. 2 from the textbook “Chemistry: 8th grade” (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005).

2. With. 37-38 No. 2,4,5,6 from Workbook in chemistry: 8th grade: to the textbook P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

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 valence 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 example given, the fluorine atom is bonded to only one monovalent H atom, which means its valency is also 1. The sulfur atom in H2S already attaches two H atoms to itself, so it is divalent in this compound. Calcium in its hydride CaH2 is also bound to two hydrogen atoms, which means its valence 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 indicated, 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 valency. Moreover, its maximum value in most cases coincides with the number of the group in which the element is located in the Periodic Table (rule 1).

There are many exceptions to this rule. Thus, element 1 of group copper exhibits valences 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 valency in a binary compound

Let's consider how to determine the valency of an element in a binary (of two elements) compound. There are two options here: in a compound, the valency 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 all chemical substances consist of diatomic molecules. How to determine the valency 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.

Looking at the formulas of various compounds, it is easy to notice that number of atoms of the same element in the molecules of different substances is not identical. 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 of hydrogen.

How can you guess which index to put next to the designation of a chemical element? How are the formulas of a substance made? This is easy to do when you know the valency of the elements that make up the molecule of a given substance.

– This is the property of an atom of a given element to attach, retain, or replace a certain number of atoms of another element in chemical reactions. The unit of valency is the valence of a hydrogen atom. Therefore, sometimes the definition of valence is formulated as follows: valence – This is the property of an atom of a given element to attach or replace a certain number of hydrogen atoms.

If one hydrogen atom is attached to one atom of a given element, then the element is monovalent, if two – divalent and etc. Hydrogen compounds are not known for all elements, but almost all elements form compounds with oxygen O. Oxygen is considered to be constantly divalent.

Constant 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 combine with hydrogen? Then the valence of the required element is determined by the valence of the known element. Most often it is found using the valency of oxygen, because in compounds its valency is always 2. For example, it is not difficult to find the valence of elements in the following compounds: Na 2 O (valence of Na – 1, O – 2), Al 2 O 3 (valence of Al – 3, O – 2).

The chemical formula of a given substance can only be compiled by knowing the valency of the elements. For example, it is easy to create formulas for compounds such as CaO, BaO, CO, because the number of atoms in the molecules is the same, since the valences of the elements are equal.

What if the valences are different? When do we act in such a case? Need to remember next 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 valency by the number of atoms of another element. For example, if it is known that the valence of Mn in a compound is 7, and O – 2, then the formula of the compound will look like this: Mn 2 O 7.

How did we get the formula?

Let's consider an algorithm for compiling formulas by valence for compounds consisting of two chemical elements.

There is a rule that the number of valencies of one chemical element is equal to the number of valencies of another. Let us consider the example of the formation of a molecule consisting of manganese and oxygen.
We will compose in accordance with the algorithm:

1. We write down the symbols of chemical elements next to each other:

MnO

2. We put the numbers of their valency over the chemical elements (the valence of a chemical element can be found in the table of the periodic system of Mendelev, for manganese – 7, at oxygen – 2.

3. Find the least common multiple (the smallest number that is divisible by 7 and 2 without a remainder). This number is 14. We divide it by the valences of the elements 14: 7 = 2, 14: 2 = 7, 2 and 7 will be the indices for phosphorus and oxygen, respectively. We substitute indices.

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

Mn 2 O 7 (7 2 = 2 7).

2x = 14,

x = 7.

The concept of valence was introduced into chemistry before the structure of the atom became known. It has now been established that this property of an element is related to the number of external electrons. For many elements, the maximum valence follows from the position of these elements in the periodic table.

People often hear the word “valence” without fully understanding what it is. So what is valency? Valency is one of the terms used in chemical structure. Valence essentially 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 valence 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 the outer electron of another atom. Each pair of such electrons is called an “electron”, and each of the electrons is called a valence. So the definition of the word “valence” is the number of electron pairs with which one atom is connected to another atom.

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

The maximum valence of chemical elements from one group of the periodic system of Mendeleev is equal to the serial number of this group. Atoms of the same element can have different valencies in different chemical compounds. Polarity covalent bonds, which are formed, are not taken into account. This is why valence has no sign. Also, valence cannot be a negative value and equal zero.

Sometimes the concept of “valence” is equated with the concept of “oxidation state,” but this is not true, although sometimes these indicators do coincide. Oxidation number 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 may have some 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. Conversely, in organic chemistry, valence is used because most organic compounds have a molecular structure.

Now you know what the valency of chemical elements is!