Valency of all elements in chemical compounds table. Constant and variable valency

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

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One atom of another monovalent element combines with one atom of a monovalent element(HCl) . Two monovalent atoms combine with an atom of a divalent 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 of a given element can combine with. 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 valence
elements in connections

1. Shaft hydrogen take 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, carbon in the CO 2 compound (carbon dioxide) has a valence of IV.

3. Supreme shaft is equal to group number .

4. lower valency equals 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 "A" subgroups, the shaft is equal to the group number.

6. In non-metals, two valences are mainly manifested: higher and lower.

Figuratively speaking, a shaft is the number of "hands" with which an atom clings to other atoms. Naturally, atoms have no "hands"; their role is played by the so-called. valence electrons.

It can be said 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 (there are relatively few of them) and elements with variable valency (of which the majority).

Elements with constant valency must be remembered.

In this article, we will look at ways and understand how to determine valency elements of the periodic table.

In chemistry it is accepted that the valence chemical elements can be recognized by the group (column) in the periodic table. In reality, the valency of an element does not always correspond to the group number, but in most cases a certain valence using this method will give the correct result; often elements, depending on various factors, have more than one valence.

The unit of valence is the valency of the hydrogen atom, equal to 1, that is, hydrogen is monovalent. Therefore, the valency of an element indicates how many hydrogen atoms one atom of the element in question is connected to. For example, HCl, where chlorine is monovalent; H2O, where oxygen is divalent; NH3, where nitrogen is trivalent.

How to determine valence according to the periodic table.

The periodic table contains chemical elements that are placed in it according to certain principles and laws. Each element stands in its place, which is determined by its characteristics and properties, and each element has its own number. horizontal lines are called periods, which increase from the first row down. If the period consists of two rows (which is indicated on the side by numbering), then such a period is called a large one. If it has only one row, then it is called small.

In addition, there are groups in the table, of which there are only eight. Items are arranged in columns vertically. Here their placement is uneven - on the one hand there are more elements (main group), on the other - less (side group).

Valency is the ability of an atom to form a certain number of chemical bonds with atoms of other elements. according to the periodic table will help to understand the knowledge of the types of valency.

For elements of secondary subgroups (and only metals belong to them), the valence must be remembered, especially since in most cases it is equal to I, II, less often III. You will also have to memorize the valencies of chemical elements that have more than two values. Or constantly keep at hand the valency table of elements.

Algorithm for determining valence by the formulas of chemical elements.

1. Write down the formula of a chemical compound.

2. Designate the known valency of the elements.

3. Find the least common multiple of valency and index.

4. Find the ratio of the least common multiple to the number of atoms of the second element. This is the desired valency.

5. Make a check by multiplying the valency and index of each element. Their works must be equal.

Example: determine the valency of the elements of hydrogen sulfide.

1. Let's write the formula:

2. Denote the known valency:

3. Find the least common multiple:

4. Find the ratio of the least common multiple to the number of sulfur atoms:

5. Let's check:

Table of characteristic valency values ​​of some atoms of chemical compounds.

Elements	Valence	Connection examples
		H 2 , HF, Li 2 O, NaCl, KBr
O, Mg, Ca, Sr, Ba, Zn		H 2 O, MgCl 2, CaH 2, SrBr 2, BaO, ZnCl 2
		CO 2 , CH4, SiO 2 , SiCl 4
		CrCl 2 , CrCl 3 , CrO 3
		H2S, SO2, SO3
		NH 3 , NH 4 Cl, HNO 3
		PH 3 , P 2 O 5 , H 3 PO 4
		SnCl 2 , SnCl 4 , PbO, PbO 2
		HCl, ClF 3 , BrF 5 , IF 7

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.

– is the property of an atom of a given element to attach, retain, or replace in chemical reactions a certain number of atoms of another element. 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 valence (the valency 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 ( smallest number, which is evenly divisible by 7 and 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 valency 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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In chemistry lessons, you have already got acquainted with the concept of the valency of chemical elements. We have collected all in one place useful information about this question. Use it when preparing for the GIA and the Unified State Examination.

Valency and chemical analysis

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

From Latin, the word "valence" is translated as "strength, ability." Very true name, right?

The concept of "valence" is one of the main ones in chemistry. It was introduced even before the structure of the atom became known to scientists (back in 1853). Therefore, as the structure of the atom was studied, it underwent some changes.

So, from the point of view of electronic theory, valency is directly related to the number of external electrons of an atom of an element. This means that by "valency" is meant the number of electron pairs by which an atom is bonded to other atoms.

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

You may ask, how could chemists of the 19th century be able to describe valency even when they believed that there were no particles smaller than an atom? It cannot be said that it was so simple - they relied on chemical analysis.

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

Admittedly, this method is not without flaws. Because the valence of an element can be determined in this way only in its simple combination with always monovalent hydrogen (hydride) or always divalent oxygen (oxide). For example, the valency of nitrogen in NH 3 - III, since one hydrogen atom is bonded to three nitrogen atoms. And the valency of carbon in methane (CH 4), according to the same principle, is IV.

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

As you have already noticed, valency is indicated by Roman numerals.

Valency and acids

Since the valence of hydrogen remains unchanged and is well known to you, you can easily determine the valency of the acid residue. So, for example, in H 2 SO 3 the valency of SO 3 is I, in HClO 3 the valency of ClO 3 is I.

In a similar way, if the valency of the acid residue is known, it is easy to write down the correct formula of the acid: NO 2 (I) - HNO 2, S 4 O 6 (II) - H 2 S 4 O 6.

Valency and formulas

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

Indices in the molecular formulas of substances reflect the number of atoms of the elements that make up their composition. Knowing the valency of the elements helps to correctly arrange the indices. In the same way, by looking at the molecular formula and indices, you can name the valences of the constituent elements.

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

To do this, you just need to remember that in a substance of molecular nature, the number of valencies of both elements are equal. Therefore, use the least common multiple (corresponding to the number of free valences required for the connection) to determine the valence of the element that you do not know.

To make it clear, let's take the formula of iron oxide Fe 2 O 3. Here, two iron atoms with valence III and 3 oxygen atoms with valence II participate in the formation of a chemical bond. Their least common multiple is 6.

• Example: you have formulas Mn 2 O 7 . You know the valence of oxygen, it is easy to calculate that the least common multiple is 14, hence the valency of Mn is VII.

Similarly, you can do the opposite: write down the correct chemical formula of a substance, knowing the valencies of its constituent elements.

• Example: in order to correctly write down the formula of phosphorus oxide, we take into account the valency of oxygen (II) and phosphorus (V). Hence, the least common multiple for P and O is 10. Therefore, the formula has the following form: P 2 O 5.

Knowing well the properties of the elements that they exhibit in various compounds, one can determine their valence even by appearance such connections.

For example: copper oxides are red (Cu 2 O) and black (CuO) in color. Copper hydroxides are colored yellow (CuOH) and blue (Cu(OH) 2).

And to make covalent bonds in substances more clear and understandable for you, write their structural formulas. The dashes between the elements depict the bonds (valencies) that arise between their atoms:

Valency characteristics

Today, the determination of the valency of elements is based on knowledge about the structure of the outer electron shells of their atoms.

Valence can be:

• constant (metals of the main subgroups);
• variable (non-metals and metals of side groups):
• highest valence;
• lower valency.

The constant in various chemical compounds remains:

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

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

Valence and electronic theory

Within the framework of the electronic theory, the valence of an atom is determined on the basis of the number of unpaired electrons that participate in the formation of electron pairs with the electrons of other atoms.

Only electrons located on the outer shell of the atom participate in the formation of chemical bonds. Therefore, the maximum valence of a chemical element is the number of electrons in the outer electron shell of its atom.

The concept of valence is closely related to the Periodic Law, discovered by D. I. Mendeleev. If you look closely at the periodic table, you can easily notice: the position of an element in the periodic table and its valency are inextricably linked. The highest valency of elements that belong to the same group corresponds to the ordinal number of the group in the periodic system.

You will find out the lowest valency when you subtract the group number of the element that interests you from the number of groups in the periodic table (there are eight of them).

For example, the valency of many metals coincides with the group numbers in the table of periodic elements to which they belong.

Table of valency of chemical elements

Serial number chem. element (atomic number)	Name	chemical symbol	Valence
1	Hydrogen Helium / Helium Lithium / Lithium Beryllium / Beryllium Carbon / Carbon Nitrogen / Nitrogen Oxygen / Oxygen Fluorine / Fluorine Neon / Neon Sodium Magnesium / Magnesium Aluminum Silicon / Silicon Phosphorus / Phosphorus Sulfur Chlorine / Chlorine Argon / Argon Potassium / Potassium Calcium / Calcium Scandium / Scandium Titanium / Titanium Vanadium / Vanadium Chromium / Chromium Manganese / Manganese Iron / Iron Cobalt / Cobalt Nickel / Nickel Copper Zinc / Zinc Gallium / Gallium Germanium / Germanium Arsenic / Arsenic Selenium / Selenium Bromine / Bromine Krypton / Krypton Rubidium / Rubidium Strontium / Strontium Yttrium / Yttrium Zirconium / Zirconium Niobium / Niobium Molybdenum / Molybdenum Technetium / Technetium Ruthenium / Ruthenium Rhodium Palladium / Palladium Silver / Silver Cadmium / Cadmium Indium / Indium Tin / Tin Antimony / Antimony Tellurium / Tellurium Iodine / Iodine Xenon / Xenon Cesium / Cesium Barium / Barium Lanthanum / Lanthanum Cerium / Cerium Praseodymium / Praseodymium Neodymium / Neodymium Promethium / Promethium Samaria / Samarium Europium / Europium Gadolinium / Gadolinium Terbium / Terbium Dysprosium / Dysprosium Holmium / Holmium Erbium / Erbium Thulium / Thulium Ytterbium / Ytterbium Lutetium / Lutetium Hafnium / Hafnium Tantalum / Tantalum Tungsten / Tungsten Rhenium / Rhenium Osmium / Osmium Iridium / Iridium Platinum / Platinum Gold / Gold Mercury / Mercury Waist / Thallium Lead / Lead Bismuth / Bismuth Polonium / Polonium Astatine / Astatine Radon / Radon Francium / Francium Radium / Radium Actinium / Actinium Thorium / Thorium Proactinium / Protactinium Uranus / Uranium	H	I (I), II, III, IV, V I, (II), III, (IV), V, VII II, (III), IV, VI, VII II, III, (IV), VI (I), II, (III), (IV) I, (III), (IV), V (II), (III), IV (II), III, (IV), V (II), III, (IV), (V), VI (II), III, IV, (VI), (VII), VIII (II), (III), IV, (VI) I, (III), (IV), V, VII (II), (III), (IV), (V), VI (I), II, (III), IV, (V), VI, VII (II), III, IV, VI, VIII (I), (II), III, IV, VI (I), II, (III), IV, VI (II), III, (IV), (V) No data No data (II), III, IV, (V), VI

In brackets are given those valences that the elements possessing them rarely show.

Valency and oxidation state

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

It is interesting that for an atom of the same element, depending on the elements with which it forms a chemical compound, the valency and oxidation state can be the same (H 2 O, CH 4, etc.) and differ (H 2 O 2, HNO 3 ).

Conclusion

Deepening your knowledge of the structure of atoms, you will learn more deeply and in more detail about valency. This characterization of chemical elements is not exhaustive. But she has a big applied value. What have you yourself seen more than once, solving problems and conducting chemical experiments on lessons.

This article is designed to help you organize your knowledge of valency. And also to recall how it can be determined and where valence is used.

We hope that this material will be useful for you in preparing homework and self-preparation for tests and exams.

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Often people hear the word "valency" without fully understanding what it is. So what is valence? 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 the outer electron of 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 not needed, use simple formulas, where the valency 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. 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, valence is used in organic chemistry, because most organic compounds have a molecular structure.

Now you know what the valency of chemical elements is!