Valency is the ability of atoms to attach to themselves a certain number of other atoms.

One atom of another monovalent element is combined 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 of a given element can combine with. The valency of an element is the number of bonds that an atom forms:

Na – monovalent (one bond)

H – monovalent (one bond)

O – divalent (two bonds for each atom)

S – hexavalent (forms six bonds with neighboring atoms)

Rules for determining valency
elements in connections

1. Valence 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. Higher valence equal to group number .

4. Lowest valence 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 located in “A” subgroups, the valence is equal to the group number.

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

For example: sulfur has the highest valency VI and the lowest (8 – 6) equal to II; phosphorus exhibits valences V and III.

7. Valence can be constant or variable.

The valency of elements must be known in order to compose chemical formulas of compounds.

Algorithm for composing the formula of a phosphorus oxide compound

Sequencing	Formulating phosphorus oxide
1. Write the symbols of the elements	R O
2. Determine the valencies of elements	V II P O
3. Find the least common multiple of the numerical values ​​of valences	5 2 = 10
4. Find the relationships between atoms of elements by dividing the found smallest multiple by the corresponding valencies of the elements	10: 5 = 2, 10: 2 = 5; P:O=2:5
5. Write indices for element symbols	R 2 O 5
6. Formula of the compound (oxide)	R 2 O 5

Remember!

Features of compiling chemical formulas of compounds.

1) The lowest valence is shown by the element that is located to the right and above in D.I. Mendeleev’s table, and the highest valence is shown by the element located to the left and below.

For example, in combination with oxygen, sulfur exhibits the highest valency VI, and oxygen the lowest valency II. Thus, the formula for sulfur oxide will be SO 3.

In the compound of silicon with carbon, the first exhibits the highest valency IV, and the second - the lowest IV. So the formula – SiC. This is silicon carbide, the basis of refractory and abrasive materials.

2) The metal atom comes first in the formula.

2) In the formulas of compounds, the non-metal atom exhibiting the lowest valency always comes in second place, and the name of such a compound ends in “id”.

For example,SaO – calcium oxide, NaCl - sodium chloride, PbS – lead sulfide.

Now you can write the formulas for any compounds of metals and non-metals.

In chemistry lessons you have already become acquainted with the concept of valency. chemical elements. We have collected all in one place useful information about this question. Use it when you prepare for the State Exam and the Unified State Exam.

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 chemical bonds with other atoms.

From Latin the word “valency” is translated as “strength, ability.” A very correct name, right?

The concept of “valence” is one of the basic ones in chemistry. It was introduced even before scientists knew the structure of the atom (back in 1853). Therefore, as we studied the structure of the atom, it underwent some changes.

Thus, from the point of view of electronic theory, valence is directly related to the number of outer electrons of an element’s atom. This means that “valency” refers to the number of electron pairs that an atom has with 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 were chemists of the 19th century able to describe valence even when they believed that there were no particles smaller than an atom? This is not to say that it was so simple - they relied on chemical analysis.

Through 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 the exact mass of each element in a sample of pure (without impurities) substance was.

True, 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 is 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 such as acidic residues, but not of all elements (except for the known valency of hydrogen) individually.

As you have already noticed, valence 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 valence of the acid residue. So, for example, in H 2 SO 3 the valency of SO 3 is I, in HСlO 3 the valency of СlO 3 is I.

In a similar way, if the valence 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 them. Knowing the valence of elements helps to correctly place the indices. In the same way, by looking at the molecular formula and indices, you can tell the valences of the constituent elements.

You do tasks like this 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, you can easily determine the valence of another element.

To do this, you just need to remember that in a substance of a molecular nature, the number of valences of both elements is equal. Therefore, use the least common multiple (corresponding to the number of free valencies required for the compound) to determine the valence of an element that is unknown to you.

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 valency II participate in the formation of a chemical bond. Their least common multiple is 6.

• Example: you have the 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 valence of Mn is VII.

In a similar way, you can do the opposite: write down the correct chemical formula of a substance, knowing the valences of its elements.

• Example: to correctly write the formula of phosphorus oxide, we take into account the valency of oxygen (II) and phosphorus (V). This means that the least common multiple of P and O is 10. Therefore, the formula has next view: P 2 O 5 .

Knowing well the properties of elements that they exhibit in various compounds, it is possible to 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 so that covalent bonds in substances have become more visual and understandable for you, write their structural formulas. The lines between the elements represent the bonds (valency) that arise between their atoms:

Valency characteristics

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

Valency can be:

• constant (metals of the main subgroups);
• variable (non-metals and metals of secondary groups):
• higher valence;
• lowest valence.

The following remains constant in various chemical compounds:

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

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

Valence and electron theory

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

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

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 numbers of the groups 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 Lithium Beryllium Carbon Nitrogen / Nitrogen Oxygen Fluorine Neon / Neon Sodium/Sodium Magnesium / Magnesium Aluminum Silicon Phosphorus / Phosphorus Sulfur/Sulfur Chlorine Argon / Argon Potassium/Potassium Calcium Scandium / Scandium Titanium Vanadium Chrome / Chromium Manganese / Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic/Arsenic Selenium Bromine Krypton / Krypton Rubidium / Rubidium Strontium / Strontium Yttrium / Yttrium Zirconium / Zirconium Niobium / Niobium Molybdenum Technetium / Technetium Ruthenium / Ruthenium Rhodium Palladium Silver Cadmium Indium Tin/Tin Antimony / Antimony Tellurium / Tellurium Iodine / Iodine Xenon / Xenon Cesium Barium / Barium Lanthanum / Lanthanum Cerium Praseodymium / Praseodymium Neodymium / Neodymium Promethium / Promethium Samarium / Samarium Europium Gadolinium / Gadolinium Terbium / Terbium Dysprosium / Dysprosium Holmium Erbium Thulium Ytterbium / Ytterbium Lutetium / Lutetium Hafnium / Hafnium Tantalum / Tantalum Tungsten/Tungsten Rhenium / Rhenium Osmium / Osmium Iridium / Iridium Platinum Gold Mercury Thalium / Thallium Lead/Lead Bismuth Polonium Astatine Radon / Radon Francium Radium Actinium Thorium Proactinium / Protactinium Uranium / 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

Those valences that the elements possessing them rarely exhibit are given in parentheses.

Valency and oxidation state

Thus, speaking about the degree of oxidation, it is meant that an atom in a substance of ionic (which is important) nature has a certain conventional 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 valence and oxidation state can be the same (H 2 O, CH 4, etc.) or different (H 2 O 2, HNO 3 ).

Conclusion

By deepening your knowledge of the structure of atoms, you will learn more deeply and in more detail about valency. This description of chemical elements is not exhaustive. But it has great practical significance. What have you yourself been convinced of more than once while solving problems and conducting chemical experiments on lessons.

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

We hope you find this material useful in preparing your homework and self-preparing for tests and exams.

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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 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 valency. In addition to those already indicated, the following have constant valency:

• 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 valence 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 valence 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.

The first stumbling block for students of chemistry. A big mistake is the approach when the student does not try to understand valence, expecting that knowledge about this will then apply itself. But this approach is incorrect, since without understanding this we run into the dead end of our inability to compose even the simplest formula.

What is the “valence” of elements?

Valence is a word taken by scientists from the Latin language, which translated means strength and opportunity. Of course, the name is not accidental and can greatly help us in understanding the essence of the term. After all, valency characterizes an atom from the point of view of its ability to form bonds with other atoms. In other words, valency can be considered as the ability of an atom to form bonds through which molecules appear.

Designate element valency always in Roman numerals only. You can see its value for different atoms in a special table.

What are the characteristics of the valency of elements?

All substances that have valency are characterized by the fact that it is either constant (in all connections) or variable. Constant valency- characteristics of a very small group of substances (hydrogen, fluorine, sodium, potassium, oxygen, etc. There are many more atoms in the world that have variable valence. In different reactions, interacting with different atoms, they become differently valent. For example, nitrogen in the compound NH3 has a valency of III, since it is associated with three atoms, but in nature it has a valence of from one to four. Once again, different valences are a more common phenomenon.

The influence of the valence of elements in chemical reactions.

Even after scientists learned that an atom is not the smallest particle in the world, they were already operating with this concept. They understood that there is an internal factor that influences the course of chemical reactions of various substances. Due to the fact that scientists saw the structure of the molecule differently, the concept of “ element valency"experienced several metamorphoses.

The valence of a substance is determined by the number of outer electrons of an atom. The number of electrons an atom has, the maximum number of connections it can make. Thus, “valency” refers to the number of electron pairs of atoms.

Although the electronic theory appeared much later, after the “splitting” of the atom into smaller particles, scientists were still quite successful in determining valence in most cases. They succeeded in this thanks to chemical analysis of substances.

It was hard work: first of all, it was necessary to determine the mass of the element in its pure form. Next, using chemical analysis, scientists determined what the composition of the compound was, and only then could they calculate how many atoms a molecule of the substance contained.

This method is still used, but is not universal. This makes it convenient to identify an element in a simple compound of substances. For example, with monovalent hydrogen, or divalent oxygen.

But even when working with acids, the method is not particularly successful. No, we can partially use it, for example, when determining the valency of compounds of acidic residues.

It looks like this: using the knowledge that the valence of oxygen is always equal to two, we can easily calculate the valence of the entire acid residue. For example, in H 2 SO 3 the valence of SO 3 is I, in HСlO 3 the valence of ClO 3 is I.

Valence of elements in formulas.

As we said above, the concept “ valence of elements"related to the electronic structure of the atom. But this is not the only type of connection that exists in nature. Chemists are also familiar with ionic, crystalline and other forms of structure of matter. For such structures, valence is no longer so relevant, but when working with formulas of molecular reactions, we must definitely take it into account.

In order to make a formula, we must arrange all the indices that balance the number of atoms that enter into the reaction. Only knowing the valency of substances can we correctly place the indices. Conversely, knowing the molecular formula and having indices, you can find out the valency of the elements that make up the substance.

To carry out such calculations, it is important to remember that the valencies of both elements that enter into the reaction will be equal, which means that for the search it is necessary to find the least common multiple.

For example, let's take iron oxide. Our chemical bond involves iron and oxygen. In this reaction, iron has a valence of III, and oxygen has a valence of II. By easy calculations we determine that the least common multiple is 6. This means that the formula looks like Fe 2 O 3.

Unusual ways to determine the valency of elements.

There are more non-standard ones, but interesting ways determining the valency of a substance. If you know the properties of an element well, then you can even determine the valency visually. For example, copper. Its oxides will be red and black, and its hydroxides will be yellow and blue.

Visibility.

In order to element valency was more clear, they recommend writing structural formulas. When creating them, we write symbols for atoms, and then draw lines based on valency. There, each line indicates the connections of each of the elements and it turns out very clearly.

Different chemical elements differ in their ability to form chemical bonds, that is, to combine with other atoms. Therefore, in complex substances they can only be present in certain proportions. Let's figure out how to determine valency using the periodic table.

There is such a definition of valence: this is the ability of an atom to form a certain number of chemical bonds. Unlike , this quantity is always only positive and is denoted by Roman numerals.

This characteristic for hydrogen is used as a unit, which is taken equal to I. This property shows how many monovalent atoms a given element can combine with. For oxygen, this value is always equal to II.

It is necessary to know this characteristic in order to correctly write chemical formulas of substances and equations. Knowing this value will help establish the relationship between the number of atoms of different types in a molecule.

This concept originated in chemistry in the 19th century. Frankland started a theory explaining the combination of atoms in various proportions, but his ideas about the “binding force” were not very widespread. A vital role in the development of the theory belonged to Kekula. He called the property of forming a certain number of bonds basicity. Kekulé believed that this was a fundamental and unchanging property of every type of atom. Butlerov made important additions to the theory. With the development of this theory, it became possible to visually depict molecules. This was very helpful in studying the structure of various substances.

How can the periodic table help?

You can find valency by looking at the group number in the short-period version. For most elements for which this characteristic is constant (takes only one value), it coincides with the group number.

Such properties have main subgroups. Why? The group number corresponds to the number of electrons in the outer shell. These electrons are called valence electrons. They are responsible for the ability to connect with other atoms.

The group consists of elements with a similar electronic shell structure, and the nuclear charge increases from top to bottom. In short-term form, each group is divided into main and secondary subgroups. Representatives of the main subgroups are s and p elements, representatives of the side subgroups have electrons in d and f orbitals.

How to determine the valence of chemical elements if it changes? It can coincide with the group number or be equal to the group number minus eight, and also take other values.

Important! The higher and to the right the element, the less its ability to form relationships. The more it is shifted down and to the left, the larger it is.

The way valence changes in the periodic table for a particular type of atom depends on the structure of its electron shell. Sulfur, for example, can be di-, tetra- and hexavalent.

In the ground (unexcited) state of sulfur, two unpaired electrons are located in the 3p sublevel. In this state, it can combine with two hydrogen atoms and form hydrogen sulfide. If sulfur goes into a more excited state, then one electron will move to the free 3d sublevel, and there will be 4 unpaired electrons.

Sulfur will become tetravalent. If you give it even more energy, then another electron will move from the 3s sublevel to 3d. Sulfur will go into an even more excited state and become hexavalent.

Constant and variable

Sometimes the ability to form chemical bonds may change. It depends on which compound the element is included in. For example, sulfur in H2S is divalent, in SO2 it is tetravalent, and in SO3 it is hexavalent. The largest of these values ​​is called the highest, and the smallest is called the lowest. The highest and lowest valencies according to the periodic table can be established as follows: the highest coincides with the group number, and the lowest is equal to 8 minus the group number.

How to determine the valence of chemical elements and whether it changes? We need to establish whether we are dealing with a metal or a non-metal. If it is a metal, you need to establish whether it belongs to the main or secondary subgroup.

• Metals of the main subgroups have a constant ability to form chemical bonds.
• For metals of secondary subgroups - variable.
• For non-metals it is also variable. In most cases, it takes two meanings - higher and lower, but sometimes there can be a greater number of options. Examples are sulfur, chlorine, bromine, iodine, chromium and others.

In compounds, the lowest valence is shown by the element that is higher and to the right in the periodic table, respectively, the highest is the one that is to the left and lower.

Often the ability to form chemical bonds takes on more than two meanings. Then you won’t be able to recognize them from the table, but you will need to learn them. Examples of such substances:

• carbon;
• sulfur;
• chlorine;
• bromine.

How to determine the valence of an element in the formula of a compound? If it is known for other components of the substance, this is not difficult. For example, you need to calculate this property for chlorine in NaCl. Sodium - element main subgroup first group, so it is monovalent. Consequently, chlorine in this substance can also create only one bond and is also monovalent.

Important! However, it is not always possible to find out this property for all atoms in a complex substance. Let's take HClO4 as an example. Knowing the properties of hydrogen, we can only establish that ClO4 is a monovalent residue.

How else can you find out this value?

The ability to form a certain number of connections does not always coincide with the group number, and in some cases it will simply have to be learned. Here the table of valency of chemical elements will come to the rescue, which shows the values ​​of this value. The 8th grade chemistry textbook provides values ​​for the ability to combine with other atoms of the most common types of atoms.

H, F, Li, Na, K	1
O, Mg, Ca, Ba, Sr, Zn	2
B, Al	3
C, Si	4
Cu	1, 2
Fe	2, 3
Cr	2, 3, 6
S	2, 4, 6
N	3, 4
P	3, 5
Sn, Pb	2, 4
Cl, Br, I	1, 3, 5, 7

Application

It is worth saying that chemists currently hardly use the concept of valency according to the periodic table. Instead, the concept of oxidation state is used for the ability of a substance to form a certain number of relationships, for substances with structure - covalence, and for substances with an ionic structure - ion charge.

However, the concept under consideration is used for methodological purposes. With its help it is easy to explain why atoms different types combine in the ratios that we observe, and why these ratios are different for different compounds.

On this moment the approach according to which the combination of elements into new substances was always explained using valency according to the periodic table, regardless of the type of bond in the compound, is outdated. Now we know that for ionic, covalent, and metallic bonds there are different mechanisms for combining atoms into molecules.

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Let's sum it up

Using the periodic table, it is not possible to determine the ability to form chemical bonds for all elements. For those that exhibit one valency according to the periodic table, in most cases it is equal to the group number. If there are two options for this value, then it can be equal to the group number or eight minus the group number. There are also special tables by which you can find out this characteristic.