Table of elements with constant valency. Valence. Definition of valency

Valence is the ability of an atom of a given element to form a certain number of chemical bonds.

Figuratively speaking, valency 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: valence 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 valence you need to remember:

The remaining elements may exhibit different valency.

The highest valency of an element in most cases coincides with the number of the group in which the element is located.

For example, manganese is in group VII (side subgroup), the highest valency of Mn is seven. Silicon is located in group IV (the main subgroup), its highest valency is four.

It should be remembered, however, that the highest valency is not always the only possible one. For example, the highest valency of chlorine is seven (check it out!), but compounds are known in which this element exhibits valences VI, V, IV, III, II, I.

It is important to remember a few exceptions: the maximum (and only) valency of fluorine is I (and not VII), oxygen - II (and not VI), nitrogen - IV (the ability of nitrogen to show valence V is a popular myth that is found even in some school textbooks).

Valency and oxidation state are not identical concepts.

These concepts are close enough, but they should not be confused! The oxidation state has a sign (+ or -), valence - no; the oxidation state of an element in a substance can be zero, the valence is zero only if we are dealing with an isolated atom; the numerical value of the oxidation state may NOT coincide with the valency. For example, the valence of nitrogen in N 2 is III, and the oxidation state = 0. The valency of carbon in formic acid is IV, and the oxidation state is +2.

If the valency of one of the elements in a binary compound is known, the valency of the other can be found.

This is done very simply. Remember the formal rule: the product of the number of atoms of the first element in a molecule and its valency must be equal to the same product for the second element.

In compound A x B y: valency (A) x = valence (B) y

Example 1. Find the valencies of all elements in the NH 3 compound.

Solution. We know the valency of hydrogen - it is constant and equal to I. We multiply the valency of H by the number of hydrogen atoms in the ammonia molecule: 1 3 \u003d 3. Therefore, for nitrogen, the product of 1 (number of N atoms) by X (nitrogen valency) should also be equal to 3. Obviously, X = 3. Answer: N(III), H(I).

Example 2. Find the valencies of all elements in the Cl 2 O 5 molecule.

Solution. Oxygen has a constant valence (II), in the molecule of this oxide there are five oxygen atoms and two chlorine atoms. Let the valency of chlorine \u003d X. We make an equation: 5 2 \u003d 2 X. Obviously, X \u003d 5. Answer: Cl (V), O (II).

Example 3. Find the valence of chlorine in the SCl 2 molecule, if it is known that the valency of sulfur is II.

Solution. If the authors of the problem had not told us the valency of sulfur, it would have been impossible to solve it. Both S and Cl are variable valency elements. Taking into account additional information, the solution is built according to the scheme of examples 1 and 2. Answer: Cl(I).

Knowing the valency of two elements, you can draw up a formula for a binary compound.

In examples 1 - 3, we determined the valence using the formula, now let's try to do the reverse procedure.

Example 4. Write the formula for the compound of calcium and hydrogen.

Solution. The valencies of calcium and hydrogen are known - II and I, respectively. Let the formula of the desired compound be Ca x H y. We again compose the well-known equation: 2 x \u003d 1 y. As one of the solutions to this equation, we can take x = 1, y = 2. Answer: CaH 2 .

"And why exactly CaH 2? - you ask. - After all, the variants Ca 2 H 4 and Ca 4 H 8 and even Ca 10 H 20 do not contradict our rule!"

The answer is simple: take the minimum possible values x and y. In the given example, these minimum (natural!) values are exactly equal to 1 and 2.

"So, compounds like N 2 O 4 or C 6 H 6 are impossible? - you ask. - Should these formulas be replaced with NO 2 and CH?"

No, they are possible. Moreover, N 2 O 4 and NO 2 are completely different substances. But the CH formula does not correspond to any real stable substance at all (unlike C 6 H 6).

Despite all that has been said, in most cases you can be guided by the rule: take smallest values indexes.

Example 5. Write the formula for the compound of sulfur with fluorine, if it is known that the valency of sulfur is six.

Solution. Let the compound formula be S x F y . The valency of sulfur is given (VI), the valency of fluorine is constant (I). Again we make the equation: 6 x \u003d 1 y. It is easy to understand that the smallest possible values of the variables are 1 and 6. Answer: SF 6 .

Here, in fact, are all the main points.

Now check yourself! I propose to go a little test on the topic "Valency".

One chemical element to attach or replace a certain number of atoms of another.

The valency of the hydrogen atom is taken as a unit of valency, 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 bivalent; NH3 where nitrogen is trivalent.

Table of elements with constant valency.

Substance formulas can be compiled according to the valences of their constituent elements. And vice versa, knowing the valencies of the elements, you can compose from them chemical formula.

Algorithm for compiling formulas of substances by valency.

1. Write down the symbols of the elements.

2. Determine the valencies of the elements included in the formula.

3. Find the least common multiple of the numerical values of the valency.

4. Find the relationship between the atoms of the elements by dividing the found least common multiple by the corresponding valencies of the elements.

5. Write down the indices of the elements in the chemical formula.

Example: Write the chemical formula for phosphorus oxide.

1. Let's write the symbols:

2. Define the valencies:

4. Find the relationship between atoms:

5. Let's write the indices:

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:

H 2 S

2. Denote the known valency:

H 2 S

3. Find the least common multiple:

H 2 S

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

H 2 S

5. Let's check.

In order to determine the valency 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, HO, hydrogen (H) will always be monovalent a, and oxygen (O) will always be divalent. Below is a cheat sheet that I hope will help you)

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

Alkali metals, hydrogen, and halogens are considered monovalent;

But trivalent boron and aluminum.

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

The lower valency is found out by subtracting the group number from 8. Non-metals are endowed with lower valency to a greater extent.

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

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

Valence can be determined from the periodic table. The highest valence of an element is always equal to the number of the group in which it is located.

Non-metals most often have a lower variable valence. To find out lower 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 valence is VI, the lowest valence will be II (86 = 2).

According to the school definition, valency is the ability of a chemical element to form one or another number of chemical bonds with other atoms.

As you know, valence is 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 of D. I. Mendeleev will help us determine the valence.

The following rules apply:

1) Maximum the valency of a chemical element is equal to the group number. For example, chlorine is in the 7th group, which means that its maximum valency is 7. Sulfur: it is in the 6th group, which means that it does not have 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 the 1st group, however, the maximum valence of copper is not 1, but 2.

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

It is useful to remember the following rules:

3) All alkaline metals (metals of group I, 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, 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 valence 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 the definitions of valency may differ.

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

According to this definition, the valency of nitrogen in HNO3 is 4, not 5. Nitrogen cannot be pentavalent, because in this case, 10 electrons would circle around the nitrogen atom. And this cannot be, because the maximum 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 valency, which varies depending on which element it (the given element) is in conjunction with or enters.

Valencies of some chemical elements:

Now let's move on to how the valence of an element is determined from the table.

So, valency can be determined by periodic table:

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

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

The valency 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.

With non-metals, a slightly different story: their highest valency (in compounds with oxygen) is also equal to the group number, but the lower valency (in compounds with hydrogen and metals) must be determined by the following formula: 8 - group number.

The more you work with chemical elements, the better you remember their valency. And for starters, this cheat sheet is enough:

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

Valetity is the ability of atoms of some chemical elements to attach atoms of other elements to themselves. For successful writing formulas, right decision tasks, you need to know well how to determine valency. 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 (bivalent); 3. B and Al (trivalent). To determine the valence according to the periodic table, you need to find out in which group the chemical element is located and determine whether it is in the main group or side.

An element can have one or more valences.

The maximum valence of elements is equal to the number of valence electrons. We can determine valence by knowing the location of the element in the 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 usually written in the upper right corner of the element symbol.

Some elements may have different valencies in different connections.

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.

It is easy to determine the valence according to the periodic table. As a rule, it corresponds to the number of the group in which the element is located. But there are elements that in different compounds can have different valencies. In this case, we are talking about constant and variable valency. 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 the elements if one element in the compound has a constant valency, for example, it is oxygen or hydrogen. On the left is a reducing agent, that is, an element with a positive valency, on the right is an oxidizing agent, that is, an element with a negative valence. The index of an element with constant valency is multiplied by that valence and divided by the index of an element with unknown valence.

Example: silicon oxides. The valency of oxygen is -2. Find the valency of silicon.

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

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

Instruction

For example, two substances– HCl and H2O. It is well known to everyone and water. The first substance contains one hydrogen atom (H) and one chlorine atom (Cl). This suggests that in this compound they form one, that is, they hold one atom near them. Hence, valence and one and the other is equal to 1. It is just as easy to determine valence elements that make up the water molecule. It contains two hydrogens and one oxygen atom. Therefore, the oxygen atom formed two bonds to attach two hydrogens, and they, in turn, formed one bond each. Means, valence oxygen is 2, and hydrogen is 1.

But sometimes you have to face substances mi more complex in terms of the properties of their constituent atoms. There are two types of elements: with a constant (, hydrogen, etc.) and non-permanent valence Yu. For atoms of the second type, this number depends on the compound in which they are included. An example is (S). It can have valences of 2, 4, 6, and sometimes even 8. Determining the ability of elements such as sulfur to hold other atoms around is a little more difficult. To do this, you need to know other components substances.

Remember the rule: the product of the number of atoms by valence of one element in the compound must match the same product for another element. This can be verified by again referring to the water molecule (H2O):
2 (amount of hydrogen) * 1 (its valence) = 2
1 (amount of oxygen) * 2 (its valence) = 2
2 = 2 means everything is defined correctly.

Now test this algorithm on a more complex substance, for example, N2O5 - oxide. It was previously stated that oxygen has a constant valence 2, so you can compose:
2 (valence oxygen) * 5 (its amount) \u003d X (unknown valence nitrogen) * 2 (its amount)
By simple arithmetic calculations, it can be determined that valence nitrogen in this compound is 5.

Valence- this is the ability of chemical elements to hold a certain number of atoms of other elements. At the same time, this is the number of bonds formed by a given atom with other atoms. Determining valency is quite simple.

Instruction

Please note that the valence of atoms of some elements is constant, while others are variable, that is, it tends to change. For example, hydrogen in all compounds is monovalent, since it forms only one. Oxygen is able to form two bonds, while being divalent. But y can be II, IV or VI. It all depends on the element with which it connects. Thus, sulfur is an element with variable valence.

Note that in molecules of hydrogen compounds, it is very easy to calculate the valency. Hydrogen is always monovalent, and this indicator for the element associated with it will be equal to the number of hydrogen atoms in this molecule. For example, in CaH2, calcium will be divalent.

Remember the main rule for determining valency: the product of the valence index of an atom of an element and the number of its atoms in any molecule, the product of the valence index of an atom of the second element and the number of its atoms in a given molecule.

Look at the letter formula denoting this equality: V1 x K1 \u003d V2 x K2, where V is the valence of the atoms of the elements, and K is the number of atoms in the molecule. With its help, it is easy to determine the valence index of any element if the rest of the data is known.

Consider the example of the sulfur oxide molecule SO2. Oxygen in all compounds is bivalent, therefore, substituting the values in the proportion: Voxygen x Oxygen \u003d Vsulfur x Kser, we get: 2 x 2 \u003d Vsulfur x 2. From here, Vsulfur \u003d 4/2 \u003d 2. Thus, the valency of sulfur in this molecule is 2.

Related videos

This is the ability of an atom to interact with other atoms, forming chemical bonds with them. Many scientists, first of all, the German Kekule and our compatriot Butlerov, made a great contribution to the creation of the theory of valency. Electrons, which take part in the formation of a chemical bond, are called valence.

You will need

Mendeleev table.

Instruction

Remember the atom. He is our solar system: a massive nucleus (“star”) is located in the center, and electrons (“”) revolve around it. The size of the nucleus, although almost the entire mass of the atom is concentrated in it, is negligible compared to the distance to the electron orbits. Which of the electrons of an atom will most easily enter into interactions with the electrons of other atoms? It is not difficult to understand that those that are farthest from the nucleus are on the outer electron shell.

VALENCE(lat. valentia - strength) the ability of an atom to attach or replace a certain number of other atoms or groups of atoms.

For many decades, the concept of valence has been one of the basic, fundamental concepts in chemistry. All students of chemistry must have come across this concept. At first, it seemed to them quite simple and unambiguous: hydrogen is monovalent, oxygen is bivalent, and so on. In one of the manuals for applicants, it says so: “Valence is the number of chemical bonds formed by an atom in a compound.” But what then, in accordance with this definition, is the valence of carbon in iron carbide Fe 3 C, in iron carbonyl Fe 2 (CO) 9, in the long-known salts K 3 Fe (CN) 6 and K 4 Fe (CN) 6? And even in sodium chloride, each atom in the NaCl crystal is bonded to six other atoms! So many definitions, even printed in textbooks, must be applied very carefully.

In modern publications, one can find different, often inconsistent definitions. For example, this: "Valency is the ability of atoms to form a certain number of covalent bonds." This definition is clear, unambiguous, but it is applicable only for compounds with covalent bonds. Determine the valence of an atom and total number electrons involved in the formation of a chemical bond; and the number of electron pairs by which a given atom is bonded to other atoms; and the number of its unpaired electrons participating in the formation of common electron pairs. Another frequently encountered definition of valence as the number of chemical bonds by which a given atom is connected to other atoms also causes difficulties, since it is not always possible to clearly define what a chemical bond is. Indeed, not in all compounds chemical bonds are formed by pairs of electrons. The simplest example is ionic crystals, such as sodium chloride; in it, each sodium atom forms a bond (ionic) with six chlorine atoms, and vice versa. Is it necessary to consider hydrogen bonds as chemical bonds (for example, in water molecules)?

The question arises of what the valence of the nitrogen atom can be equal to in accordance with its various definitions. If valence is determined by the total number of electrons involved in the formation of chemical bonds with other atoms, then the maximum valency of the nitrogen atom should be considered equal to five, since the nitrogen atom can use all its five outer electrons in the formation of chemical bonds - two s-electrons and three p- electrons. If valence is determined by the number of electron pairs with which a given atom is bonded to others, then in this case the maximum valence of a nitrogen atom is four. In this case, three p-electrons form three covalent bonds with other atoms, and one more bond is formed due to two 2s-electrons of nitrogen. An example is the reaction of ammonia with acids to form an ammonium cation. Finally, if the valence is determined only by the number of unpaired electrons in the atom, then the nitrogen valency cannot be more than three, since the N atom cannot have more than three unpaired electrons (excitation of the 2s electron can occur only to the level with n = 3, which is energetically extremely unfavorable). So, in halides, nitrogen forms only three covalent bonds, and there are no such compounds as NF 5 , NCl 5 or NBr 5 (unlike the completely stable PF 3 , PCl 3 and PBr 3). But if a nitrogen atom transfers one of its 2s electrons to another atom, then four unpaired electrons will remain in the resulting N + cation, and the valence of this cation will be four. This happens, for example, in the molecule of nitric acid. Thus, different definitions of valency lead to different results even in the case of simple molecules.

Which of these definitions is “correct” and is it possible to give an unambiguous definition for valence. To answer these questions, it is useful to make an excursion into the past and consider how the concept of "valence" has changed with the development of chemistry.

The idea of the valence of elements (which, however, did not receive recognition at that time) was first expressed in the middle of the 19th century. English chemist E. Frankland: he spoke about a certain "saturation capacity" of metals and oxygen. Subsequently, valency began to be understood as the ability of an atom to attach or replace a certain number of other atoms (or groups of atoms) with the formation of a chemical bond. One of the creators of the theory of chemical structure, Friedrich August Kekule, wrote: “Valency is a fundamental property of the atom, a property as constant and unchanging as the atomic weight itself.” Kekule considered the valence of an element to be a constant value. By the end of the 1850s, most chemists believed that the valence (then called "atomicity") of carbon was 4, the valencies of oxygen and sulfur were 2, and the valencies of halogens were 1. In 1868, the German chemist K.G. "valence" (in Latin valentia - strength). However, for a long time it was almost never used, at least in Russia (instead of it, they spoke, for example, about “units of affinity”, “number of equivalents”, “number of shares”, etc.). It is significant that in encyclopedic dictionary Brockhaus and Efron(almost all the articles on chemistry in this encyclopedia were looked through, corrected, and often written by D.I. Mendeleev) there is no article “valence” at all. It is not in the classic work of Mendeleev either. Fundamentals of Chemistry(he only occasionally mentions the concept of "atomicity", without dwelling on it in detail and without giving it an unambiguous definition).

In order to visually show the difficulties that accompanied the concept of "valence" from the very beginning, it is appropriate to quote the popular at the beginning of the 20th century. many countries, due to the great pedagogical talent of the author, a textbook by the American chemist Alexander Smith, published by him in 1917 (in Russian translation - in 1911, 1916 and 1931): “Not a single concept in chemistry has received such a number of unclear and inaccurate definitions as the concept of valence ". And further in the section Some oddities in the views on valence the author writes:

“When the concept of valency was first constructed, then it was believed - quite erroneously - that each element has one valency. Therefore, considering such pairs of compounds as CuCl and CuCl 2, or ... FeCl 2 and FeCl 3, we proceeded from the assumption that copper Always is divalent, and iron is trivalent, and on this basis the formulas were distorted in such a way as to fit them to this assumption. Thus, the formula for copper chloride was written (and often written even now) as follows: Cu 2 Cl 2. In this case, the formulas of two copper chloride compounds in graphic image get the form: Cl–Cu–Cu–Cl and Cl–Cu–Cl. In both cases, each copper atom holds (on paper) two units and is therefore divalent (on paper). Similarly... doubling the formula FeCl 2 gave Cl 2 >Fe–Fe 2, which allowed us to consider... ferric iron.” And then Smith makes a very important and timeless conclusion: “It is quite disgusting scientific method- to invent or distort facts in order to support a belief that, while not based on experience, is the result of mere conjecture. However, the history of science shows that such errors are often observed.

In 1912, the Russian chemist L.A. Chugaev, who received world recognition for his work on the chemistry of complex compounds, gave a review of the ideas of the beginning of the century about valency. Chugaev clearly showed the difficulties associated with the definition and application of the concept of valency:

“Valency is a term used in chemistry in the same sense as “atomicity” to refer to the maximum number of hydrogen atoms (or other monatomic atoms or monatomic radicals) with which an atom of a given element can be in direct connection (or which it can replace ). The word valence is often also used in the sense of a unit of valency, or a unit of affinity. So, they say that oxygen has two, nitrogen three valences, etc. The words valency and "atomicity" were previously used without any distinction, but as the very concepts expressed by them lost their original simplicity and became more complicated, for a number of cases only the word valency remained in use ... The complication of the concept of valency began with the recognition that valency is a variable quantity ... moreover, in the sense of the matter, it is always expressed as an integer.

Chemists knew that many metals have variable valence, and they should have talked, for example, about divalent, trivalent and hexavalent chromium. Chugaev said that even in the case of carbon, he had to admit the possibility that its valence could be different from 4, and CO is not the only exception: “Divalent carbon is very likely found in carbylamines CH 3 -N = C, in its salts C=NOH, C=NOMe, etc. We know that there is also a triatomic carbon...” extend the classical concept of valence and extend it to cases to which it, as such, is inapplicable. If Thiele came to the need ... to allow the "fragmentation" of units of valence, then there are a number of facts that make it necessary to deduce the concept of valence in another sense from the narrow framework in which it was originally enclosed. We have seen that the study of the simplest (mostly binary ...) compounds formed by chemical elements, for each of these latter, forces us to assume certain, always small and, of course, integer values of their valency. Such values, generally speaking, are very few (elements exhibiting more than three different valences are rare) ... Experience shows, however, that when all the above units of valence should be considered saturated, the ability of the molecules formed in this case to further addition does not at all reach limit. So, metal salts add water, ammonia, amines .., forming various hydrates, ammoniates ... etc. complex compounds, which ... we now classify as complex. The existence of such compounds, which do not fit into the framework of the simplest concept of valence, naturally required its expansion and the introduction of additional hypotheses. One of these hypotheses, proposed by A. Werner, is that along with the main, or basic, units of valency, there are other, secondary ones. The latter are usually indicated by a dotted line.

Indeed, what valence, for example, should be attributed to the cobalt atom in its chloride, which added six molecules of ammonia to form the compound CoCl 3 6NH 3 (or, which is the same, Co (NH 3) 6 Cl 3)? In it, a cobalt atom is connected simultaneously with nine atoms of chlorine and nitrogen! D.I. Mendeleev wrote on this occasion about the little-studied "forces of residual affinity." And the Swiss chemist A. Werner, who created the theory of complex compounds, introduced the concepts of main (primary) valence and side (secondary) valence (in modern chemistry, these concepts correspond to the oxidation state and coordination number). Both valences can be variable, and in some cases it is very difficult or even impossible to distinguish between them.

Further, Chugaev touches on R. Abegg's theory of electrovalency, which can be positive (in higher oxygen compounds) or negative (in compounds with hydrogen). In this case, the sum of the higher valences of elements in oxygen and hydrogen for groups from IV to VII is 8. The presentation in many chemistry textbooks is still based on this theory. In conclusion, Chugaev mentions chemical compounds, for which the concept of valence is practically inapplicable - intermetallic compounds, the composition of which "is often expressed in very peculiar formulas, very little resembling the usual values of valency. Such, for example, are the following compounds: NaCd 5 , NaZn 12 , FeZn 7 and others.

Some difficulties in determining valence were pointed out by another famous Russian chemist I.A. Kablukov in his textbook Basic beginnings inorganic chemistry , published in 1929. As for the coordination number, we will quote (in Russian translation) a textbook published in Berlin in 1933 by one of the creators modern theory solutions of the Danish chemist Nils Bjerrum:

“The usual valency numbers give no idea of the characteristic properties exhibited by many atoms in numerous complex compounds. To explain the ability of atoms or ions to form complex compounds, a new special series of numbers was introduced for atoms and ions, different from the usual valency numbers. In complex silver ions ... directly connected to the central metal atom for the most part two an atom or two groups of atoms, for example, Ag (NH 3) 2 +, Ag (CN) 2 -, Ag (S 2 O 3) 2 - ... To describe this connection, the concept was introduced coordination number and assign a coordination number of 2 to Ag + ions. As can be seen from the examples given, the groups associated with central atom, can be neutral molecules (NH 3) and ions (CN -, S 2 O 3 -). The divalent copper ion Cu ++ and the trivalent gold ion Au +++ in most cases have a coordination number of 4. The coordination number of an atom, of course, does not yet indicate what kind of bond exists between the central atom and other atoms or groups of atoms associated with it; but it turned out to be an excellent tool for the systematics of complex compounds.

A. Smith gives very illustrative examples of the “special properties” of complex compounds in his textbook:

“Consider the following “molecular” compounds of platinum: PtCl 4 2NH 3 , PtCl 4 4NH 3 , PtCl 4 6NH 3 and PtCl 4 2KCl. A closer study of these compounds reveals a number of remarkable features. The first compound in solution practically does not decompose into ions; the electrical conductivity of its solutions is extremely low; silver nitrate does not precipitate AgCl with it. Werner assumed that the chlorine atoms are bound to the platinum atom by ordinary valences; Werner called them the main ones, and the ammonia molecules are connected to the platinum atom by additional, side valences. This compound, according to Werner, has the following structure:

Large brackets indicate the integrity of a group of atoms, a complex that does not decompose when the compound is dissolved.

The second compound has different properties from the first; this is an electrolyte, the electrical conductivity of its solutions is of the same order as the electrical conductivity of salt solutions that decompose into three ions (K 2 SO 4, BaCl 2, MgCl 2); silver nitrate precipitates two out of four atoms. According to Werner, this compound has the following structure: 2– + 2Cl –. Here we have a complex ion, chlorine atoms in it are not precipitated by silver nitrate, and this complex forms around the nucleus - the Pt atom - the inner sphere of atoms in the compound, the chlorine atoms split off in the form of ions form the outer sphere of the atoms, which is why we write them outside of large brackets. If we assume that Pt has four main valences, then only two are used in this complex, while the other two hold two external chlorine atoms. In the first compound, all four valencies of platinum are used in the complex itself, as a result of which this compound is not an electrolyte.

In the third compound, all four chlorine atoms are precipitated by silver nitrate; the high electrical conductivity of this salt shows that it gives five ions; it is obvious that its structure is as follows: 4– + 4Cl – ... In the complex ion, all ammonia molecules are associated with Pt by side valences; corresponding to the four principal valences of platinum, there are four chlorine atoms in the outer sphere.

In the fourth compound, silver nitrate does not precipitate chlorine at all, the electrical conductivity of its solutions indicates decomposition into three ions, and exchange reactions reveal potassium ions. We attribute the following structure to this compound: 2– + 2K + . In the complex ion, the four main valences of Pt are used, but since the main valences of the two chlorine atoms are not used, two positive monovalent ions (2K +, 2NH 4 +, etc.) can be retained in the outer sphere.

The given examples of the striking difference in the properties of outwardly similar platinum complexes give an idea of the difficulties that chemists encountered when trying to unambiguously determine valency.

After the creation of electronic ideas about the structure of atoms and molecules, the concept of "electrovalency" began to be widely used. Since atoms can both donate and accept electrons, the electrovalence could be either positive or negative (now the concept of oxidation state is used instead of electrovalence). To what extent did the new electronic ideas about valence agree with the old ones? N. Bjerrum in the already cited textbook writes about this: “Between ordinary numbers valencies and introduced new numbers - electrovalence and coordination number - there is some dependence, but they are by no means identical. The old concept of valency has split into two new concepts. On this occasion, Bjerrum made an important note: “The coordination number of carbon in most cases is 4, and its electrovalence is either +4 or -4. Since both numbers usually coincide for the carbon atom, carbon compounds are not suitable for studying the difference between these two concepts on them.

Within the framework of the electronic theory of chemical bonding, developed in the works of the American physical chemist G. Lewis and the German physicist W. Kossel, such concepts as donor-acceptor (coordination) bond and covalence appeared. In accordance with this theory, the valency of an atom was determined by the number of its electrons participating in the formation of common electron pairs with other atoms. In this case, the maximum valency of the element was considered equal to the number electrons in the outer electron shell of an atom (it coincides with the group number of the periodic table to which the element belongs). According to other concepts based on quantum chemical laws (they were developed by German physicists W. Heitler and F. London), not all external electrons should be counted, but only unpaired ones (in the ground or excited state of the atom); this definition is given in a number of chemical encyclopedias.

However, facts are known that do not fit into this a simple circuit. So, in a number of compounds (for example, in ozone), a pair of electrons can hold not two, but three nuclei; in other molecules, the chemical bond may be carried out by a single electron. It is impossible to describe such connections without using the apparatus of quantum chemistry. How, for example, to determine the valency of atoms in such compounds as pentaborane B 5 H 9 and other boranes with "bridge" bonds, in which the hydrogen atom is bonded to two boron atoms at once; ferrocene Fe (C 5 H 5) 2 (an iron atom with an oxidation state of +2 is immediately connected to 10 carbon atoms); iron pentacarbonyl Fe(CO) 5 (an iron atom in the zero oxidation state is bonded to five carbon atoms); sodium pentacarbonyl chromate Na 2 Cr (CO) 5 (oxidation state of chromium-2)? Such "non-classical" cases are by no means exceptional. Similar "valency breakers", compounds with various "exotic valences", as chemistry developed, became more and more.

To get around some difficulties, a definition was given according to which, when determining the valence of an atom, it is necessary to take into account the total number of unpaired electrons, unshared electron pairs and vacant orbitals involved in the formation of chemical bonds. Vacant orbitals are directly involved in the formation of donor-acceptor bonds in various complex compounds.

One of the conclusions is that the development of the theory and the acquisition of new experimental data led to the fact that attempts to achieve a clear understanding of the nature of valence divided this concept into a number of new concepts, such as main and secondary valency, ionic valence and covalence, coordination number and degree oxidation, etc. That is, the concept of "valency" "split" into a number of independent concepts, each of which operates in a certain area. Apparently, the traditional concept of valency has a clear and unambiguous meaning only for compounds in which all chemical bonds are two-center (i.e., connecting only two atoms) and each bond is carried out by a pair of electrons located between two neighboring atoms, in other words, for covalent compounds such as HCl, CO 2 , C 5 H 12, etc.

The second conclusion is not quite common: the term "valence", although it is used in modern chemistry, has a very limited use, attempts to give it an unambiguous definition "for all occasions" are not very productive and are hardly needed. It is not for nothing that the authors of many textbooks, especially those published abroad, do without this concept at all or limit themselves to pointing out that the concept of "valence" has mainly historical significance, while now chemists mainly use the more common, albeit somewhat artificial, concept of "degree oxidation."

Ilya Leenson