In order to learn how to compose chemical formulas, it is necessary to find out the patterns according to which the atoms of chemical elements are connected to each other in certain ratios. To do this, we compare the qualitative and quantitative composition of compounds whose formulas are HCl, H 2 O, NH 3, CH 4 (Fig. 12.1)

In terms of their qualitative composition, these substances are similar: each of the molecules contains hydrogen atoms. However, their quantitative composition is not the same. The atoms of chlorine, oxygen, nitrogen, carbon are connected to one, two, three and four hydrogen atoms, respectively.

This pattern was noticed at the beginning of the 11th century. J. Dalton. Over time, I. Ya. Berzelius discovered that the largest number of atoms connected to an atom of a chemical element does not exceed a certain value. In 1858, E. Frankland called the ability of atoms to bind or replace a certain number of other atoms as "connecting force" Term "valence"(from lat. valentia-"strength") was proposed in 1868 by the German chemist K. G. Wichelhaus.

Valence — common property atoms. It characterizes the ability of atoms chemically (by valence forces) to interact with each other.

The valency of many chemical elements was determined on the basis of experimental data on the quantitative and qualitative composition of substances. per unit of valence the valency of the hydrogen atom would be accepted. If an atom of a chemical element is connected to two monovalent atoms, then its valency is two. If it is connected to three monovalent atoms, then it is trivalent, etc.

The highest value of the valency of chemical elements is VIII .

Valency is indicated by Roman numerals. Let us denote the valency in the formulas of the considered compounds:

Also, scientists have found that many elements in different compounds exhibit different valency values. That is, there are chemical elements with constant and variable valency.

Is it possible to determine valence by the position of a chemical element in the periodic system? The maximum value of the element's valence coincides with the number of the group of the periodic system in which it is located. Nevertheless, there are exceptions - nitrogen, oxygen, fluorine, copper and some other elements. Remember: the group number is indicated by a Roman numeral above the corresponding vertical column of the periodic system.

Table. Chemical elements with constant valency

Element	Valence	Element	Valence
Hydrogen (H)		Calcium (Ca)
Sodium (Na)		Barium (Ba)
		Oxygen(O)
Beryllium(Be)		Aluminum (Al)
Magnesium (Mg)

Table. Chemical elements with variable valency

Element	Valence	Element	Valence
		Iron (Fe)
		Manganese (Mg)
			II, III, VI material from the site
Silver (AG)		Phosphorus (P)
Gold (Au)		Arsenic (As)
		Carbon (C)
Lead (Pb)		Silicon (Si)

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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, for those who are just starting to study chemistry, the question arises: How to determine the valence of an element? This is easy to do if you know a few rules.

Valencies constant and variable

Consider the compounds HF, H2S, and CaH2. In each of these examples, one hydrogen atom attaches to itself only one atom of another chemical element, which means that its valency is one. The valence value is written above the symbol of the chemical element in Roman numerals.

In the above example, the fluorine atom is bonded to only one univalent H atom, which means that its valency is also 1. The sulfur atom in H2S already attaches two H atoms to itself, so it is bivalent in this compound. Calcium is also bound to two hydrogen atoms in its CaH2 hydride, which means that its valency is two.

Oxygen in the vast majority of its compounds is divalent, that is, it forms two chemical bonds with other atoms.

In the first case, the sulfur atom attaches two oxygen atoms to itself, that is, it forms 4 chemical bonds in total (one oxygen forms two bonds, which means sulfur - two times 2), that is, its valency is 4.

In the SO3 compound, sulfur already attaches three O atoms, therefore its valency is 6 (it forms two bonds with each oxygen atom three times). The calcium atom attaches only one oxygen atom, forming two bonds with it, which means that its valence is the same as that of O, that is, it is equal to 2.

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

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

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

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

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

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

Determination of valence in a binary compound

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

Case one:

Case two:

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

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

If, instead of potassium, iron, or another element with variable valence, is present in the formula, we will need to know what is the valence of the acid residue. For example, you need to calculate the valencies of the atoms of all elements in combination with the formula FeSO4.

It should be noted that the term "valence" is more often used in organic chemistry. When formulating inorganic compounds, the concept of "oxidation state" is more often used.

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. Consequently, 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 valency.

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 valence: 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 divalent, 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.

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Valence- one of the main terms used in the theory of chemical structure. This concept defines the ability of an atom to form chemical bonds and quantitatively represents the number of bonds in which it participates.

Instruction

Valence(from Latin valentia - “strength”) - an indicator of the ability of an atom to attach other atoms to itself, forming chemical bonds with them inside the molecule. The total number of bonds in which an atom can participate is equal to the number of its unpaired electrons. Such bonds are called covalent.

Unpaired electrons are free electrons in the outer shell of an atom that pair with the outer electrons of another atom. Moreover, each such pair is called an electron pair, and such electrons are called valence. Based on this, valencies can sound like this: this is the number of electron pairs along which a given atom is connected to other atoms.

The maximum valence index of chemical elements of one group of the periodic system, as a rule, is equal to the serial number of the group. Different atoms of the same element can have different valencies. The polarity of the resulting is not taken into account, so the valence has no sign. It cannot be zero or negative.

The quantity of any chemical element is considered to be the number of univalent hydrogen atoms or divalent oxygen atoms. However, when determining valence, other elements can be used, the valency of which is precisely known.

Sometimes the concept of valence is identified with the concept of "oxidation state", but this is not true, although in some cases these indicators coincide. Oxidation state is a formal term that means the possible charge that an atom would receive if its electrons in electrons were transferred to more electronegative atoms. In this case, the oxidation state is expressed in units of charge and may have a sign, in contrast to valency. This term has become widespread in the inorganic, since in inorganic compounds one judges valency. Valence is also used in organic chemistry, since most organic compounds has a molecular structure.

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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

• Periodic 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.

", "a drug ". Usage within the framework of the modern definition is recorded in 1884 (German. Valenz). In 1789, William Higgins published a paper in which he suggested the existence of bonds between the smallest particles of matter.

However, an accurate and later fully confirmed understanding of the valence phenomenon was proposed in 1852 by the chemist Edward Frankland in a work in which he collected and rethought all the theories and assumptions that existed at that time on this subject. . Observing the ability to saturate different metals and comparing the composition of organic derivatives of metals with the composition of inorganic compounds, Frankland introduced the concept of " connecting force”, thus laying the foundation for the doctrine of valence. Although Frankland established some particular laws, his ideas were not developed.

Friedrich August Kekule played a decisive role in the creation of the theory of valency. In 1857, he showed that carbon is a four-basic (four-atom) element, and its simplest compound is methane CH 4 . Confident in the truth of his ideas about the valency of atoms, Kekule introduced them into his textbook of organic chemistry: basicity, according to the author, is a fundamental property of the atom, a property as constant and unchanging as atomic weight. In 1858, views that almost coincided with the ideas of Kekule were expressed in the article “ On the new chemical theory» Archibald Scott Cooper .

Three years later, in September 1861, A. M. Butlerov made the most important additions to the theory of valency. He made a clear distinction between a free atom and an atom that has entered into conjunction with another when its affinity is " connects and goes to new form ". Butlerov introduced the idea of ​​the completeness of the use of the forces of affinity and of " affinity tension”, that is, the energy non-equivalence of bonds, which is due to the mutual influence of atoms in a molecule. As a result of this mutual influence, atoms, depending on their structural environment, acquire different "chemical significance". Butlerov's theory made it possible to explain many experimental facts concerning the isomerism of organic compounds and their reactivity.

A huge advantage of the theory of valence was the possibility of a visual representation of the molecule. In the 1860s the first molecular models appeared. Already in 1864, A. Brown suggested using structural formulas in the form of circles with symbols of elements placed in them, connected by lines denoting the chemical bond between atoms; the number of lines corresponded to the valency of the atom. In 1865, A. von Hoffmann demonstrated the first ball-and-stick models in which croquet balls played the role of atoms. In 1866, drawings of stereochemical models appeared in Kekule's textbook, in which the carbon atom had a tetrahedral configuration.

Modern ideas about valence

Since the advent of the theory of chemical bonding, the concept of "valence" has undergone a significant evolution. At present, it does not have a strict scientific interpretation, therefore it is almost completely ousted from the scientific vocabulary and is used mainly for methodological purposes.

Basically, the valence of chemical elements is understood as the ability of its free atoms to form a certain number of covalent bonds. In compounds with covalent bonds, the valence of atoms is determined by the number of formed two-electron two-center bonds. It is this approach that was adopted in the theory of localized valence bonds, proposed in 1927 by W. Heitler and F. London in 1927. It is obvious that if an atom has n unpaired electrons and m lone electron pairs, then this atom can form n+m covalent bonds with other atoms. When assessing the maximum valence, one should proceed from the electronic configuration of a hypothetical, so-called. "excited" (valence) state. For example, the maximum valence of an atom of beryllium, boron and nitrogen is 4 (for example, in Be (OH) 4 2-, BF 4 - and NH 4 +), phosphorus - 5 (PCl 5), sulfur - 6 (H 2 SO 4) , chlorine - 7 (Cl 2 O 7).

In a number of cases, such characteristics of a molecular system as the degree of oxidation of an element, the effective charge on an atom, the coordination number of an atom, etc. are identified with valency. These characteristics can be close and even coincide quantitatively, but in no way identical to each other. For example, in the isoelectronic molecules of nitrogen N 2, carbon monoxide CO and cyanide ion CN - a triple bond is realized (that is, the valence of each atom is 3), however, the oxidation state of the elements is, respectively, 0, +2, −2, +2 and −3. In the ethane molecule (see figure), carbon is tetravalent, as in most organic compounds, while the oxidation state is formally -3.

This is especially true for molecules with delocalized chemical bonds, for example, in nitric acid, the oxidation state of nitrogen is +5, while nitrogen cannot have a valency higher than 4. The rule known from many school textbooks is “Maximum valence element is numerically equal to the group number in the Periodic Table" - refers solely to the oxidation state. The terms "permanent valence" and "variable valence" also predominantly refer to the oxidation state.

see also

Notes

Links

• Ugay Ya. A. Valence, chemical bond and oxidation state - the most important concepts of chemistry // Soros Educational Journal. - 1997. - No. 3. - S. 53-57.
• / Levchenkov S.I. Brief essay history of chemistry

Literature

• L. Pauling The nature of the chemical bond. M., L.: State. NTI chem. Literature, 1947.
• Cartmell, Fowles. Valency and structure of molecules. M.: Chemistry, 1979. 360 p.]
• Coulson Ch. Valence. M.: Mir, 1965.
• Marrel J., Kettle S., Tedder J. Valency theory. Per. from English. M.: Mir. 1968.
• Development of the doctrine of valency. Ed. Kuznetsova V.I. M.: Chemistry, 1977. 248s.
• Valence of atoms in molecules / Korolkov D. V. Fundamentals inorganic chemistry. - M.: Enlightenment, 1982. - S. 126.

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Synonyms:

See what "Valency" is in other dictionaries:

VALENCE, a measure of the "connectivity" of a chemical element, equal to the number of individual CHEMICAL BONDS that one ATOM can form. The valency of an atom is determined by the number of ELECTRONS at the highest (valence) level (external ... ... Scientific and technical encyclopedic dictionary

VALENCE- (from Latin valere to have a meaning), or atomicity, the number of hydrogen atoms or equivalent atoms or radicals, a given atom or radical can attach to a swarm. V. is one of the bases for the distribution of elements in the periodic system of D. I. ... ... Big Medical Encyclopedia

Valence- * valence * valence the term comes from lat. valid. 1. In chemistry, this is the ability of atoms of chemical elements to form a certain number of chemical bonds with atoms of other elements. In the light of the structure of the atom, V. is the ability of atoms ... ... Genetics. encyclopedic Dictionary

- (from lat. valentia force) in physics, a number showing how many hydrogen atoms a given atom can combine with or replace them. In psychology, valency is an English term for motivating ability. Philosophical ... ... Philosophical Encyclopedia

Atomic Dictionary of Russian synonyms. valency noun, number of synonyms: 1 atomicity (1) ASIS synonym dictionary. V.N. Trishin ... Synonym dictionary

VALENCE- (from lat. valentia - strong, durable, influential). The ability of a word to grammatically combine with other words in a sentence (for example, in verbs, valence determines the ability to combine with a subject, direct or indirect object) ... New dictionary methodological terms and concepts (theory and practice of teaching languages)

- (from the Latin valentia force), the ability of an atom of a chemical element to attach or replace a certain number of other atoms or atomic groups to form a chemical bond ... Modern Encyclopedia

- (from Latin valentia strength) the ability of an atom of a chemical element (or atomic group) to form a certain number of chemical bonds with other atoms (or atomic groups). Instead of valency, they often use more narrow concepts, for example… … Big Encyclopedic Dictionary

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 valency must be remembered:

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) valence of fluorine is I (and not VII), oxygen - II (and not VI), nitrogen - IV (the ability of nitrogen to show valency 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 valence 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 valency (II), in the molecule of this oxide there are five oxygen atoms and two chlorine atoms. Let the valence 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 valency 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 valence elements. Taking into account additional information, the solution is built according to the scheme of examples 1 and 2. Answer: Cl(I).

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

In examples 1 - 3, we determined the valency 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 options 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 smallest possible values ​​of 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 "Valence".