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 valence 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 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 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 valence. In addition to those already indicated, the following have constant valence:
- 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 valence. 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 all 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.
VALENCE(Latin 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 valency has been one of the basic, fundamental concepts in chemistry. All students of chemistry must come across this concept. At first it seemed to them quite simple and unambiguous: hydrogen is monovalent, oxygen is divalent, etc. One of the manuals for applicants says this: “Valency 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 those printed in textbooks, must be applied very carefully.
In modern publications one can find different, often inconsistent, definitions. For example, this: “Valence is the ability of atoms to form a certain number of covalent bonds.” This definition is clear and unambiguous, but it is applicable only to compounds with covalent bonds. The valence of an atom is determined by the total number of electrons involved in the formation of a chemical bond; and the number of electron pairs with which a given atom is connected 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. After all, not all compounds have chemical bonds 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. Should hydrogen bonds be considered chemical bonds (for example, in water molecules)?
The question arises of what the valence of a nitrogen atom can be equal to in accordance with its different definitions. If valence is determined by the total number of electrons involved in the formation of chemical bonds with other atoms, then the maximum valence of a nitrogen atom should be considered equal to five, since the nitrogen atom can use all five of its external electrons - two s-electrons and three p-electrons - when forming chemical bonds. electrons. If valence is determined by the number of electron pairs with which a given atom is connected to others, then in this case the maximum valency of a nitrogen atom is four. In this case, three p-electrons form three covalent bonds with other atoms, and another 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 valence is determined only by the number of unpaired electrons in an atom, then the valence of nitrogen cannot be more than three, since the N atom cannot have more than three unpaired electrons (excitation of the 2s electron can only occur at the level with n = 3, which is energetically extremely unfavorable). Thus, 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 the resulting N+ cation will have four unpaired electrons, and the valence of this cation will be four. This happens, for example, in a molecule of nitric acid. Thus, different definitions of valence lead to different results even for simple molecules.
Which of these definitions is “correct” and is it even possible to give an unambiguous definition for valency? To answer these questions, it is useful to take an excursion into the past and consider how the concept of “valence” 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 mid-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) to form a chemical bond. One of the creators of the theory of chemical structure, Friedrich August Kekule, wrote: “Valence is a fundamental property of the atom, a property as constant and unchangeable as the atomic weight itself.” Kekule considered the valency 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 valency of oxygen and sulfur was 2, and the halogens were 1. In 1868, the German chemist K. G. Wichelhaus proposed using the term “atomicity” instead “valence” (in Latin valentia - strength). However, for a long time it was almost not used, at least in Russia (instead they spoke, for example, about “units of affinity”, “number of equivalents”, “number of shares”, etc.). It is significant that in Encyclopedic Dictionary of Brockhaus and Efron(almost all the articles on chemistry in this encyclopedia were reviewed, edited, and often written by D.I. Mendeleev) there is no article on “valence” at all. It is not found in Mendeleev’s classic work either. Basics 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 clearly demonstrate the difficulties that accompanied the concept of “valence” from the very beginning, it is appropriate to quote a concept that was popular at the beginning of the 20th century. in many countries, due to the great pedagogical talent of the author, the textbook of 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 so many unclear and imprecise definitions as the concept of valency " And further in the section Some oddities in views on valency the author writes:
“When the concept of valence was first constructed, it was believed - completely erroneously - that each element has one valency. Therefore, when considering pairs of compounds such 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 they distorted the formulas so as to fit them to this assumption. Thus, the formula of copper monochloride was written (and is often written to this day) like this: Cu 2 Cl 2. In this case, the formulas of two copper chloride compounds in a graphical representation take 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). Likewise... doubling the formula FeCl 2 gave Cl 2 >Fe–Fe 2, which allowed us to consider... iron as trivalent.” And then Smith makes a very important and relevant conclusion at all times: “It is quite contrary to the scientific method to invent or distort facts in order to support an idea which, not being based on experience, is the result of mere conjecture. However, the history of science shows that such errors are often observed.”
A review of the ideas of the beginning of the century about valency was given in 1912 by the Russian chemist L.A. Chugaev, who received worldwide recognition for his work on the chemistry of complex compounds. Chugaev clearly showed the difficulties associated with the definition and application of the concept of valence:
“Valence is a term used in chemistry in the same sense as “atomicity” to denote the maximum number of hydrogen atoms (or other monoatomic atoms or monoatomic radicals) with which an atom of a given element can be in direct connection (or which it is capable of replacing ). The word valency is also often used in the sense of a unit of valency, or a unit of affinity. Thus, they say that oxygen has two, nitrogen three, etc. The words valence 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 valence remained in use... The complication of the concept of valency began with the recognition that valency is a variable quantity... and in the sense of the matter it is always expressed as an integer.”
Chemists knew that many metals have variable valency, and they should talk, for example, about divalent, trivalent and hexavalent chromium. Chugaev said that even in the case of carbon, it was necessary to recognize the possibility that its valency may be different from 4, and CO is not the only exception: “Divalent carbon is very likely contained in carbilamines CH 3 -N=C, in fulminate acid and its salts C=NOH, C=NOMe, etc. We know that triatomic carbon also exists...” Discussing the theory of the German chemist I. Thiele about “partial” or partial valences, Chugaev spoke of it as “one of the first attempts expand the classical concept of valency and extend it to cases to which it, as such, is inapplicable. If Thiele came to the need... to allow for the “fragmentation” of units of valence, then there is a whole series of facts that force us, in another sense, to derive the concept of valence from the narrow framework in which it was originally contained. 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, whole values of their valence. Such values, generally speaking, are very few (elements exhibiting more than three different valences are rare)... Experience shows, however, that when all the above-mentioned valence units should be considered saturated, the ability of the molecules formed in this case for further addition does not yet reach limit. Thus, metal salts add water, ammonia, amines..., forming various hydrates, ammonia... etc. complex compounds that... we now classify as complex. The existence of such compounds that do not fit into the framework of the simplest idea 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 valence, there are also other, secondary ones. The latter are usually indicated by a dotted line.”
Indeed, what valency, for example, should be assigned to the cobalt atom in its chloride, which added six molecules of ammonia to form the compound CoCl 3 6NH 3 (or, what is the same, Co(NH 3) 6 Cl 3)? In it, a cobalt atom is combined simultaneously with nine chlorine and nitrogen atoms! 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) valency and secondary (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 them.
Next, Chugaev touches on R. Abegg’s theory of electrovalence, which can be positive (in higher oxygen compounds) or negative (in compounds with hydrogen). Moreover, the sum of the highest valences of elements for oxygen and hydrogen for groups IV to VII is equal to 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 by very peculiar formulas, very little reminiscent of ordinary valency values. These are, for example, the following compounds: NaCd 5, NaZn 12, FeZn 7, etc.”
Another famous Russian chemist I.A. Kablukov pointed out some difficulties in determining valence in his textbook Basic principles of inorganic chemistry, published in 1929. As for the coordination number, let us quote (in Russian translation) a textbook published in Berlin in 1933 by one of the founders of the modern theory of solutions, the Danish chemist Niels Bjerrum:
“Ordinary valence 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 valence numbers. In complex silver ions... most of them are directly bound to the central metal atom two atom or two groups of atoms, for example, Ag(NH 3) 2 +, Ag(CN) 2 –, Ag(S 2 O 3) 2 –... To describe this bond, the concept 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 +++ have in most cases 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 clear examples of the “special properties” of complex compounds in his textbook:
“Consider the following “molecular” platinum compounds: 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 break down into ions; the electrical conductivity of its solutions is extremely low; silver nitrate does not produce AgCl precipitate with it. Werner accepted that the chlorine atoms are bonded 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, secondary 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 disintegrate 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 solutions of salts 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 is a compound with the following structure: 2– + 2Cl–. Here we have a complex ion; the chlorine atoms in it are not precipitated by silver nitrate, and this complex forms an inner sphere of atoms around the nucleus - the Pt atom 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 large brackets. If we assume that Pt has four main valences, then only two are used in this complex, while the other two are held by the two outer chlorine atoms. In the first compound, all four valences 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 produces five ions; it is obvious that its structure is as follows: 4– + 4Cl – ... In the complex ion, all ammonia molecules are bonded to Pt by secondary valences; corresponding to the four main 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 to this compound the following structure 2– + 2K + . In the complex ion, the four main valences of Pt are used, but since the main valences of 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 striking differences in the properties of outwardly similar platinum complexes give an idea of the difficulties that chemists encountered when trying to unambiguously determine valence.
After the creation of electronic ideas about the structure of atoms and molecules, the concept of “electrovalence” began to be widely used. Since atoms can both give and accept electrons, electrovalency could be either positive or negative (nowadays, instead of electrovalency, the concept of oxidation state is used). How consistent were the new electronic ideas about valence with the previous ones? N. Bjerrum, in the already cited textbook, writes about this: “There is some dependence between the usual valence numbers and the new numbers introduced - electrovalency and coordination number -, 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 is in most cases 4, and its electrovalence is either +4 or –4. Since both numbers usually coincide for a carbon atom, carbon compounds are unsuitable for studying the difference between these two concepts.”
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) bonding and covalence appeared. In accordance with this theory, the valence 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 valence of an element was considered equal to the number of electrons in the outer electron shell of the atom (it coincides with the number of the group of the periodic table to which the given element belongs). According to other ideas, based on quantum chemical laws (they were developed by the 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 is precisely the definition given in a number of chemical encyclopedias.
However, facts are known that do not fit into this simple scheme. Thus, 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, can we determine the valence of atoms in compounds such as pentaborane B 5 H 9 and other boranes with “bridge” bonds, in which a 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 bonded to 10 carbon atoms at once); iron pentacarbonyl Fe(CO) 5 (the 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 not at all exceptional. As chemistry developed, such “valency violators” and compounds with various “exotic valences” became more and more numerous.
To circumvent 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, lone 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 a variety of complex compounds.
One of the conclusions is that the development of 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 valency and covalency, coordination number and degree oxidation, etc. That is, the concept of “valency” has “split” into a number of independent concepts, each of which operates in a certain area.” Apparently, the traditional concept of valence 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 entirely usual: the term “valence,” although used in modern chemistry, has very limited application, attempts to give it an unambiguous definition “for all occasions” are not very productive and are hardly necessary. 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 “valency” has mainly historical significance, while now chemists mainly use the more widespread, although somewhat artificial, concept of “degree” oxidation."
Ilya Leenson
In this article we will look at the methods and understand how to determine valence elements of the periodic table.
In chemistry, it is accepted that the valence of chemical elements can be determined by the group (column) in the periodic table. In reality, the valence of an element does not always correspond to the group number, but in most cases a certain valency using this method will give the correct result; often elements, depending on various factors, have more than one valency.
The unit of valence is taken to be the valency of a hydrogen atom equal to 1, that is, hydrogen is monovalent. Therefore, the valency of an element indicates how many hydrogen atoms one atom of the element in question is connected to. For example, HCl, where chlorine is monovalent; H2O, where oxygen is divalent; NH3, where nitrogen is trivalent.
How to determine valency using the periodic table.
The periodic table contains chemical elements that are placed in it according to certain principles and laws. Each element stands in place, which is determined by its characteristics and properties, and each element has its own number. Horizontal lines are called periods, which increase from the first line down. If a period consists of two rows (as indicated by numbering on the side), then such a period is called large. If it has only one row, it is called small.
In addition, there are groups in the table, of which there are eight in total. Elements are placed in vertical columns. Here their placement is uneven - on one side there are more elements (main group), on the other - fewer (side group).
Valence is the ability of an atom to form a certain number of chemical bonds with atoms of other elements. using the periodic table will help you understand knowledge of the types of valency.
For elements of secondary subgroups (and these include only metals), the valency must be remembered, especially since in most cases it is equal to I, II, less often III. You will also have to memorize the valencies of chemical elements that have more than two meanings. Or keep a table of element valences at hand at all times.
Algorithm for determining valency using the formulas of chemical elements.
1. Write down the formula of a chemical compound.
2. Designate the known valence of elements.
3. Find the least common multiple of valence 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. Check by multiplying the valence and index of each element. Their products must be equal.
Example: Let's determine the valence of hydrogen sulfide elements.
1. Let's write the formula:
2. Let us denote the known valency:
3. Find the least common multiple:
4. Find the ratio of the least common multiple to the number of sulfur atoms:
5. Let's check:
Table of characteristic valence values of some atoms of chemical compounds.
|
Elements |
Valence |
Connection examples |
|
H 2 , HF, Li 2 O, NaCl, KBr |
||
|
O, Mg, Ca, Sr, Ba, Zn |
H 2 O, MgCl 2, CaH 2, SrBr 2, BaO, ZnCl 2 |
|
|
CO 2, CH4, SiO 2, SiCl 4 |
||
|
CrCl 2, CrCl 3, CrO 3 |
||
|
H 2 S, SO 2, SO3 |
||
|
NH 3 , NH 4 Cl, HNO 3 |
||
|
PH 3, P 2 O 5, H 3 PO 4 |
||
|
SnCl 2, SnCl 4, PbO, PbO 2 |
||
|
HCl, ClF 3, BrF 5, IF 7 |
From the lesson materials you will learn that the constancy of the composition of a substance is explained by the presence of certain valence possibilities in the atoms of chemical elements; get acquainted with the concept of “valence of atoms of chemical elements”; learn to determine the valence of an element using the formula of a substance if the valence of another element is known.
Topic: Initial chemical ideas
Lesson: Valency of chemical elements
The composition of most substances is constant. For example, a water molecule always contains 2 hydrogen atoms and 1 oxygen atom - H 2 O. The question arises: why do substances have a constant composition?
Let's analyze the composition of the proposed substances: H 2 O, NaH, NH 3, CH 4, HCl. They all consist of atoms of two chemical elements, one of which is hydrogen. There can be 1,2,3,4 hydrogen atoms per atom of a chemical element. But in no substance will there be per hydrogen atom have to several atoms of another chemical element. Thus, a hydrogen atom can attach to itself a minimum number of atoms of another element, or rather, only one.
The property of atoms of a chemical element to attach to themselves a certain number of atoms of other elements is called valence.
Some chemical elements have constant valence values (for example, hydrogen(I) and oxygen(II)), others can exhibit several valence values (for example, iron(II,III), sulfur(II,IV,VI), carbon(II, IV)), they are called elements with variable valence. The valence values of some chemical elements are given in the textbook.
Knowing the valences of chemical elements, it is possible to explain why a substance has such a chemical formula. For example, the formula of water is H 2 O. Let us designate the valence capabilities of a chemical element using dashes. Hydrogen has a valence of I, and oxygen has a valence of II: H- and -O-. Each atom can fully utilize its valence capabilities if there are two hydrogen atoms per oxygen atom. The sequence of connections of atoms in a water molecule can be represented as the formula: H-O-H.
A formula that shows the sequence of atoms in a molecule is called graphic(or structural).
Rice. 1. Graphic formula of water
Knowing the formula of a substance consisting of atoms of two chemical elements and the valency of one of them, you can determine the valency of the other element.
Example 1. Let's determine the valency of carbon in the substance CH4. Knowing that the valence of hydrogen is always equal to I, and carbon has attached 4 hydrogen atoms to itself, we can say that the valence of carbon is equal to IV. The valence of atoms is indicated by a Roman numeral above the element sign: .
Example 2. Let's determine the valency of phosphorus in the compound P 2 O 5. To do this you need to do the following:
1. above the sign of oxygen, write down the value of its valence – II (oxygen has a constant valence value);
2. multiplying the valence of oxygen by the number of oxygen atoms in the molecule, find the total number of valence units – 2·5=10;
3. divide the resulting total number of valence units by the number of phosphorus atoms in the molecule – 10:2=5.
Thus, the valency of phosphorus in this compound is equal to V – .
1. Emelyanova E.O., Iodko A.G. Organization of cognitive activity of students in chemistry lessons in grades 8-9. Basic notes with practical tasks, tests: Part I. - M.: School Press, 2002. (p. 33)
2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 36-38)
3. Chemistry: 8th grade: textbook. for general education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§16)
4. Chemistry: inorg. chemistry: textbook. for 8th grade. general education institutions / G.E. Rudzitis, F.G. Feldman. – M.: Education, OJSC “Moscow Textbooks”, 2009. (§§11,12)
5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. – M.: Avanta+, 2003.
Additional web resources
1. Unified collection of digital educational resources ().
2. Electronic version of the journal “Chemistry and Life” ().
Homework
1. p.84 No. 2 from the textbook “Chemistry: 8th grade” (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005).
2. With. 37-38 No. 2,4,5,6 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.
Looking at the formulas of various compounds, it is easy to notice that number of atoms of the same element in the molecules of different substances is not identical. For example, HCl, NH 4 Cl, H 2 S, H 3 PO 4, etc. The number of hydrogen atoms in these compounds varies from 1 to 4. This is characteristic not only of hydrogen.
How can you guess which index to put next to the designation of a chemical element? How are the formulas of a substance made? This is easy to do when you know the valence of the elements that make up the molecule of a given substance.
– This is the property of an atom of a given element to attach, retain, or replace a certain number of atoms of another element in chemical reactions. The unit of valency is the valence of a hydrogen atom. Therefore, sometimes the definition of valence is formulated as follows: valence – This is the property of an atom of a given element to attach or replace a certain number of hydrogen atoms.
If one hydrogen atom is attached to one atom of a given element, then the element is monovalent, if two – divalent and etc. Hydrogen compounds are not known for all elements, but almost all elements form compounds with oxygen O. Oxygen is considered to be constantly divalent.
Constant valence:
I –
H, Na, Li, K, Rb, Cs
II –
O, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd
III –
B, Al, Ga, In
But what to do if the element does not combine with hydrogen? Then the valency of the required element is determined by the valency of the known element. Most often it is found using the valency of oxygen, because in compounds its valency is always 2. For example, it is not difficult to find the valence of elements in the following compounds: Na 2 O (valence of Na – 1, O – 2), Al 2 O 3 (valence of Al – 3, O – 2).
The chemical formula of a given substance can only be compiled by knowing the valency of the elements. For example, it is easy to create formulas for compounds such as CaO, BaO, CO, because the number of atoms in the molecules is the same, since the valences of the elements are equal.
What if the valences are different? When do we act in such a case? It is necessary to remember the following rule: in the formula of any chemical compound, the product of the valence of one element by the number of its atoms in the molecule is equal to the product of the valence by the number of atoms of another element. For example, if it is known that the valence of Mn in a compound is 7, and O – 2, then the formula of the compound will look like this: Mn 2 O 7.
How did we get the formula?
Let's consider an algorithm for compiling formulas by valence for compounds consisting of two chemical elements.
There is a rule that the number of valencies of one chemical element is equal to the number of valencies of another. Let us consider the example of the formation of a molecule consisting of manganese and oxygen.
We will compose in accordance with the algorithm:
1. We write down the symbols of chemical elements next to each other:
2.
We put the numbers of their valency over the chemical elements (the valence of a chemical element can be found in the table of the periodic system of Mendelev, for manganese –
7, at oxygen –
2.
3. Find the least common multiple (the smallest number that is divisible by 7 and 2 without a remainder). This number is 14. We divide it by the valences of the elements 14: 7 = 2, 14: 2 = 7, 2 and 7 will be the indices for phosphorus and oxygen, respectively. We substitute indices.
Knowing the valence of one chemical element, following the rule: valence of one element × the number of its atoms in the molecule = valence of another element × the number of atoms of this (other) element, you can determine the valence of another.
Mn 2 O 7 (7 2 = 2 7).
The concept of valence was introduced into chemistry before the structure of the atom became known. It has now been established that this property of an element is related to the number of external electrons. For many elements, the maximum valence follows from the position of these elements in the periodic table.
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