Valency of elements in compounds. Valence

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How to determine the valency of chemical elements? This question is faced by everyone who is just starting to get acquainted with chemistry. First, let's find out what it is. Valence can be thought of as the property of the atoms of one element to hold a certain number of atoms of another element.

Elements with constant and variable valency

For example, the formula H-O-H shows that each H atom is connected to only one atom (in this case, oxygen). It follows that its valency is 1. The O atom in the water molecule is bonded to two univalent H atoms, which means it is divalent. Valence values are written in Roman numerals above the symbols of the elements:

The valencies of hydrogen and oxygen are constant. However, there are exceptions for oxygen. For example, in the hydroxonium ion H3O+, oxygen is trivalent. There are other elements with constant valency.

Li, Na, K, F are monovalent;
Be, Mg, Ca, Sr, Ba, Cd, Zn - have a valency equal to II;
Al, B are trivalent.

Now let's determine the valence of sulfur in the compounds H2S, SO2 and SO3.

In the first case, one sulfur atom is bonded to two univalent H atoms, which means that its valency is two. In the second example, there are two oxygen atoms per sulfur atom, which is known to be divalent. We get the valency of sulfur equal to IV. In the third case, one S atom attaches three O atoms, which means that the valence of sulfur is equal to VI (the valence of atoms of one element multiplied by their number).

As you can see, sulfur can be two-, four- and hexavalent:

Such elements are said to have variable valency.

Rules for determining valencies

The maximum valence for the atoms of a given element coincides with the number of the group in which it is located in the Periodic system. For example, for Ca it is 2, for sulfur it is 6, for chlorine it is 7. There are also many exceptions to this rule:
- an element of group 6, O, has valency II (in H3O + - III);
- monovalent F (instead of 7);
- bi- and trivalent iron, usually an element of group VIII;
-N can only hold 4 atoms near itself, not 5, as the group number implies;
- one- and two-valent copper, located in group I.
The minimum valence value for elements in which it is variable is determined by the formula: group number in PS - 8. So, the lowest valence of sulfur 8 - 6 = 2, fluorine and other halogens - (8 - 7) = 1, nitrogen and phosphorus - (8 - 5)= 3 and so on.
In a compound, the sum of the valency units of the atoms of one element must correspond to the total valency of the other.
In a water molecule H-O-H, the valency of H is equal to I, there are 2 such atoms, which means that there are 2 valency units in hydrogen (1 × 2 = 2). The same value has the valency of oxygen.
In a compound consisting of atoms of two types, the element located in second place has the lowest valence.
The valence of the acid residue coincides with the number of H atoms in the acid formula, the valency of the OH group is I.
In a compound formed by the atoms of three elements, the atom that is in the middle of the formula is called the central one. The O atoms are directly connected to it, and the rest of the atoms form bonds with oxygen.

We use these rules to complete tasks.

From the materials of the lesson, 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 "valency of atoms of chemical elements"; learn to determine the valence of an element by the formula of a substance, if the valency 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. All of them 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 no substance will per hydrogen atom have to a few atoms of another chemical element. Thus, a hydrogen atom can attach to itself the minimum number of atoms of another element, or rather, only one.

The property of the atoms of a chemical element to attach to itself 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 may show several valence values (for example, iron (II, III), sulfur (II, IV, VI), carbon (II, IV)), they are called elements with variable valency. The valency values of some chemical elements are given in the textbook.

Knowing the valencies of chemical elements, it is possible to explain why a substance has just such a chemical formula. For example, the formula for water is H 2 O. Let's denote the valence capabilities of a chemical element with dashes. Hydrogen has valency I, and oxygen has II: H- and -O-. Each atom can fully use its valence capabilities if there are two hydrogen atoms per oxygen atom. The sequence of connecting atoms in a water molecule can be represented as a formula: H-O-H.

The formula that shows the sequence of connection 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 valence of one of them, it is possible to determine the valence of another element.

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

Example 2 Let us determine the valence of phosphorus in the compound P 2 O 5. To do this, you must perform the following steps:

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

2. multiplying the oxygen valency by the number of oxygen atoms in the molecule, find the total number of 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 of cognitive activity of students at chemistry lessons in grades 8-9. Supporting 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. Grade 8” / 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 institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§16)

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

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

Additional web resources

1. A single 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 №№ 2,4,5,6 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / 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 "valence". Most often, this term refers to the ability of atoms of one element to attach a certain number of atoms of other elements. Often, for those who are just starting to study chemistry, the question arises: How to determine the valence of an element? This is easy to do if you know a few rules.

Valencies constant and variable

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 that its valency is one. The valence 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 univalent H atom, which means that 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 is also bound to two hydrogen atoms in its CaH2 hydride, which means that 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 valency is 6 (it forms two bonds with each oxygen atom three times). The calcium atom attaches only one oxygen atom, forming two bonds with it, which means that its valence is the same as that of O, that is, it is equal to 2.

Note that the H atom is univalent in any compound. Always (except for the hydronium ion H3O (+)) is 2 oxygen valency. Calcium forms two chemical bonds with both hydrogen and oxygen. These are elements with constant valency. In addition to those already indicated, the following have a constant valency:

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

The sulfur atom, in contrast to the cases considered, in combination with hydrogen has a valence equal to II, and with oxygen it can be both four- and six-valent. 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 system (rule 1).

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

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

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

Determination of valence in a binary compound

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

Case one:

Case two:

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

Not all chemicals are made up of diatomic molecules. How to determine the valency of an element in a three-element particle? Let's consider this question on the example of the formulas of two compounds K2Cr2O7.

If, instead of potassium, iron, or another element with variable valency, is present in the formula, we will need to know what is the valency of the acid residue. For example, you need to calculate the valencies 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 formulating inorganic compounds, the concept of "oxidation state" is more often used.

Considering the formulas of various compounds, it is easy to see that number of atoms the same element in the molecules of different substances is not the same. 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 typical not only for hydrogen.

How to guess which index to put next to the designation of a chemical element? How are the formulas of a substance formed? 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, hold or replace a certain number of atoms of another element in chemical reactions. The unit of valency is the valency of the 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 univalent 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.

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 combine with hydrogen? Then the valency of the required element is determined by the valency of the known element. Most often, it is found using the valence of oxygen, because in 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 (valence Na – 1,O – 2), Al 2 O 3 (Al – 3,O – 2).

The chemical formula of a given substance can be compiled only by knowing the valency of the elements. For example, it is easy to formulate 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 valencies are different? When do we act in such a 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 by the number of atoms of another element. For example, if it is known that the valency of Mn in a compound is 7, and O – 2, then the compound formula will look like this Mn 2 O 7.

How did we get the formula?

Consider the algorithm for compiling formulas by valency for those consisting of two chemical elements.

There is a rule that the number of valences in one chemical element is equal to the number of valences in another. 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 next the symbols of chemical elements:

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

3. Find the least common multiple (the smallest number that is divisible without a remainder by 7 and by 2). This number is 14. We divide it by the valencies of the elements 14: 7 \u003d 2, 14: 2 \u003d 7, 2 and 7 will be indices, respectively, for phosphorus and oxygen. We substitute indexes.

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

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

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

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