How to determine valency chemical elements? This question is faced by everyone who is just starting to get acquainted with chemistry. First, let's find out what it is. Valency can be considered as the property of atoms of one element to hold a certain number of atoms of another element.
Elements with constant and variable valency
For example, from formulas H-O-H it is clear that each H atom is connected to only one atom (in in this case with oxygen). It follows that its valency is 1. The O atom in a water molecule is bonded to two monovalent H atoms, which means it is divalent. Valence values are written in Roman numerals above the symbols of the elements:
The valencies of hydrogen and oxygen are constant. However, there are exceptions for oxygen. For example, in the hydronium ion H3O+, oxygen is trivalent. There are other elements with constant valence.
- Li, Na, K, F – monovalent;
- Be, Mg, Ca, Sr, Ba, Cd, Zn – have a valence of II;
- Al, B are trivalent.
Now let's determine the valency of sulfur in the compounds H2S, SO2 and SO3.
In the first case, one sulfur atom is bonded to two monovalent H atoms, which means its valence is two. In the second example, for one sulfur atom there are two oxygen atoms, which, as is known, is divalent. We obtain a valence of sulfur equal to IV. In the third case, one S atom attaches three O atoms, which means that the valence of sulfur is equal to VI (the valency of the atoms of one element multiplied by their number).
As you can see, sulfur can be di-, tetra- and hexavalent:
Such elements are said to have variable valency.
Rules for determining valencies
- The maximum valency for the atoms of a given element coincides with the number of the group in which it is located in the Periodic Table. For example, for Ca it is 2, for sulfur – 6, for chlorine – 7. There are also many exceptions to this rule:
-element of group 6, O, has valency II (in H3O+ – III);
- monovalent F (instead of 7);
-usually di- and trivalent iron, an element of group VIII;
-N can only hold 4 atoms near itself, and not 5, as follows from the group number;
- mono- and divalent copper, located in group I. - The minimum valence value for elements for which it is variable is determined by the formula: Group No. in PS - 8. Thus, the lowest valency of sulfur 8 - 6 = 2, fluorine and other halogens - (8 - 7) = 1, nitrogen and phosphorus - (8 – 5)= 3 and so on.
- In a compound, the sum of the valence units of the atoms of one element must correspond to the total valency of the other.
- In a molecule water N-O-N the valence of H is equal to I, there are 2 such atoms, which means that hydrogen has 2 valence units in total (1×2=2). The valency of oxygen has the same meaning.
- In a compound consisting of two types of atoms, the element located in second place has the lowest valence.
- The valence of the acid residue coincides with the number of H atoms in the acid formula, the valence of the OH group is equal to I.
- In a compound formed by atoms of three elements, the atom that is in the middle of the formula is called the central one. The O atoms are directly bonded to it, and the remaining atoms form bonds with oxygen.
We use these rules to complete tasks.
Valency 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 do not have any “hands”; their role is played by the so-called. valence electrons.
You can say it 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 (of which there are relatively few) and elements with variable valence (of which the majority are).
Elements with constant valence must be remembered:
The remaining elements may exhibit different valencies.
The highest valence 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 valence of Mn is seven. Silicon is located in group IV (main subgroup), its highest valency is four.
It should be remembered, however, that the highest valence is not always the only possible one. For example, the highest valence of chlorine is seven (make sure of this!), but compounds in which this element exhibits valences VI, V, IV, III, II, I are known.
It's 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 exhibit valence V is a popular myth that is found even in some school textbooks).
Valence and oxidation state are not identical concepts.
These concepts are quite close, but they should not be confused! The oxidation state has a sign (+ or -), the valence does not; the oxidation state of an element in a substance can be zero, the valency 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 valency 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 = +2.
If the valence of one of the elements in a binary compound is known, the valency of the other can be found.
This is done quite simply. Remember the formal rule: the product of the number of atoms of the first element in a molecule and its valence must be equal to the same product for the second element.
In the compound A x B y: valence (A) x = valence (B) y
Example 1. Find the valencies of all elements in the compound NH 3.
Solution. We know the valency of hydrogen - it is constant and equal to I. We multiply the valency H by the number of hydrogen atoms in the ammonia molecule: 1 3 = 3. Therefore, for nitrogen, the product of 1 (the number of atoms N) by X (the valence of nitrogen) should also be equal to 3. Obviously, X = 3. Answer: N(III), H(I).
Example 2. Find the valences of all elements in the Cl 2 O 5 molecule.
Solution. Oxygen has a constant valency (II); the molecule of this oxide contains five oxygen atoms and two chlorine atoms. Let the valence of chlorine = X. Let's create the equation: 5 2 = 2 X. Obviously, X = 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 valence of sulfur, it would have been impossible to solve it. Both S and Cl are elements with variable valency. Taking into account additional information, the solution is constructed according to the scheme of examples 1 and 2. Answer: Cl(I).
Knowing the valencies of two elements, you can create a formula for a binary compound.
In examples 1 - 3, we determined valency using the formula; now let's try to do the reverse procedure.
Example 4. Write a 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 = 1 y. As one of the solutions to this equation, we can take x = 1, y = 2. Answer: CaH 2.
“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 example given, these minimum (natural!) values are exactly 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 formula CH 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 follow the rule: take smallest values indexes.
Example 5. Write a formula for the compound of sulfur and fluorine if it is known that the valency of sulfur is six.
Solution. Let the formula of the compound be S x F y . The valence of sulfur is given (VI), the valency of fluorine is constant (I). We formulate the equation again: 6 x = 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 suggest you go through a short test on the topic "Valence".
Lesson objectives.
Didactic:
- Based on the students’ knowledge, repeat the concepts “ chemical formula”;
- promote the formation in students of the concept of “valence” and the ability to determine the valence of atoms of elements using the formulas of substances;
- to focus schoolchildren’s attention on the possibility of integrating chemistry and mathematics courses.
Educational:
- continue to develop the skills to formulate definitions;
- explain the meaning of the concepts studied and explain the sequence of actions when determining valency using the formula of a substance;
- promote enrichment vocabulary, development of emotions, creativity;
- develop the ability to highlight the main, essential, compare, generalize, develop diction and speech.
Educational:
- foster a sense of camaraderie and the ability to work collectively;
- increase the level of aesthetic education of students;
- guide students towards healthy image life.
Planned learning outcomes:
- Students should be able to formulate the definition of “valency”, know the valence of hydrogen and oxygen atoms in compounds, and use it to determine the valence of atoms of other elements in binary compounds,
- Be able to explain the meaning of the concept of “valency” and the sequence of actions when determining the valence of atoms of elements using the formulas of substances.
Concepts introduced for the first time in class: valence, constant and variable valency.
Organizational forms: conversation, individual assignments, independent work.
Learning Tools: algorithm for determining valence.
Demonstration equipment: ball-and-stick models of molecules of hydrogen chloride, water, ammonia, methane.
Equipment for students: on each table “Algorithm for determining valence.”
Leading task: individual task – prepare a report on the topic “Evolution of the concept of “valency”.
Lesson progress
I. Orientative-motivational stage.
1. Frontal conversation with students on the completed topic “Chemical formula”.
Exercise: What does it say here? (Teacher demonstration of formulas printed on separate sheets of paper).
2. Individual work based on the cards of three students on the topic “Relative molecular mass.” (Carry out the solution on the board). Teacher check.
Card No. 1. Calculate the relative molecular weight of these substances: NaCl, K 2 O.
Reference data:
- Ar (Na) = 23
- Ar (Cl) = 35.5
- Ar (K) = 39
- Ar (O) = 16
Card No. 2. Calculate the relative molecular weight of these substances: CuO, SO 2.
Reference data:
- Ar (Cu) = 64
- Ar (O) = 16
- Ar (S) =3 2
Card number 3. Calculate the relative molecular weight of these substances: CH 4, NO.
Reference data:
- Ar (C) = 12
- Ar (H) = 1
- Ar (N) = 14
- Ar (O) = 16
3. Independent work students in notebooks.
The task is of an information and computational nature (the condition is written in the handout).
The effectiveness of toothpastes in preventing caries can be compared by the content of active fluoride in them, which can interact with tooth enamel. Toothpaste“Crest” (made in the USA) contains, as indicated on the packaging, SnF 2, and toothpaste “FM extra DENT” (made in Bulgaria) contains NaF. Calculate which of these two pastes is more potent for caries prevention.
Examination: one student reads the solution orally.
II. Operational and executive stage.
1. Teacher's explanation. Statement of the problem.
The concept of valence.
– Until now, we have used ready-made formulas given in the textbook. Chemical formulas can be derived based on data on the composition of substances. But most often, when drawing up chemical formulas, the patterns that the elements obey when connecting with each other are taken into account.
Exercise: compare the qualitative and quantitative composition in molecules: HCl, H 2 O, NH 3, CH 4.
Conversation with students:
– What do the molecules have in common?
Suggested answer: Presence of hydrogen atoms.
– How do they differ from each other?
Suggested answer:
- HCl – one chlorine atom holds one hydrogen atom,
- H 2 O – one oxygen atom holds two hydrogen atoms,
- NH 3 – one nitrogen atom holds three hydrogen atoms,
- CH 4 – one carbon atom holds four hydrogen atoms.
Demonstration of ball-and-stick models.
Problem: Why do different atoms hold different numbers of hydrogen atoms?
(We listen to the students' answers.)
Conclusion: Atoms have different abilities to hold a certain number of other atoms in compounds. This is called valence. The word “valence” comes from Lat. valentia – strength.
Notebook entry:
Valence is the property of atoms to hold a certain number of other atoms in a compound.
Valence is indicated by Roman numerals.
Notes on the board and in notebooks:
| I II H2O |
I III H3N |
I IV H4C |
The valency of the hydrogen atom is taken to be one, and that of oxygen is II.
2. Evolution of the concept of “valence” (student message).
- IN early XIX century, J. Dalton formulated the law of multiple relations, from which it followed that each atom of one element can combine with one, two, three, etc. atoms of another element (as, for example, in the compounds of atoms with hydrogen that we considered).
In the middle of the 19th century, when the exact relative weights of atoms were determined (I.Ya. Berzelius and others), it became clear that the largest number of atoms with which a given atom can combine does not exceed a certain value, depending on its nature. This ability to bind or replace a certain number of other atoms was called “valence” by E. Frankland in 1853.
Since at that time there were no compounds known for hydrogen where it was bonded to more than one atom of any other element, the hydrogen atom was chosen as the standard, having a valence of 1.
At the end of the 50s. XIX century A.S. Cooper and A. Kekule postulated the principle of constant tetravalency of carbon in organic compounds. The concept of valency formed an important part of A.M.’s theory of chemical structure. Butlerov in 1861
Periodic law D.I. Mendeleev in 1869 revealed the dependence of the valence of an element on its position in the periodic table.
Contributions to the evolution of the concept of “valency” over the years were made by V. Kossel, A. Werner, and G. Lewis.
Since the 30s. In the 20th century, ideas about the nature and character of valence constantly expanded and deepened. Significant progress was made in 1927, when W. Heitler and F. London performed the first quantitative quantum chemical calculation of the hydrogen molecule H 2 .
3. Determination of the valence of atoms of elements in compounds.
Rule for determining valency: the number of valence units of all atoms of one element is equal to the number of valence units of all atoms of another element.
Valency determination algorithm.
Valency determination algorithm |
Example |
|
| 1. Write down the formula of the substance. | H2S, Cu2O | |
| 2. Designate the known valence of the element | I H2S, |
|
| 3. Find the number of valence units of atoms of a known element by multiplying the valency of the element by the number of its atoms | 2 I H2S |
2 |
| 4. Divide the number of valency units of the atoms by the number of atoms of the other element. The resulting answer is the desired valency | 2 I II H2S |
2 |
| 5. Do a check, that is, count the number of valence units of each element | I II H2S (2=2) |
I II Cu2O (2=2) |
4. Exercise: determine the valence of elements in substances ( simulator: students come to the board in a line). The task is in the handout.
SiH 4, CrO 3, H 2 S, CO 2, CO, SO 3, SO 2, Fe 2 O 3, FeO, HCl, HBr, Cl 2 O 5, Cl 2 O 7, PH 3, K 2 O, Al 2 O 3, P 2 O 5, NO 2, N 2 O 5, Cr 2 O 3, SiO 2, B 2 O 3, SiH 4, Mn 2 O 7, MnO, CuO, N 2 O 3.
III. Evaluative-reflective stage.
Primary test of knowledge acquisition.
IN within three minutes you need to complete one of three tasks of your choice. Choose only the task that you can handle. The task is in the handout.
- Reproductive level (“3”). Determine the valence of atoms of chemical elements using the formulas of compounds: NH 3, Au 2 O 3, SiH 4, CuO.
- Application layer (“4”). From the given series, write down only those formulas in which the metal atoms are divalent: MnO, Fe 2 O 3, CrO 3, CuO, K 2 O, CaH 2.
- Creative level (“5”). Find a pattern in the sequence of formulas: N 2 O, NO, N 2 O 3 and put the valencies above each element.
Random check. Student consultant ready-made template checks 4 student notebooks.
Work on mistakes. Answers are on the back of the board.
IV. Summing up the lesson.
Conversation with students:
- What problem did we pose at the beginning of the lesson?
- What conclusion have we reached?
- Define “valency”.
- What is the valency of a hydrogen atom? Oxygen?
- How to determine the valency of an atom in a compound?
Assessing the work of students as a whole and individual students.
Homework: § 4, pp. 23–25, ex. on page 25.
- Thank you for the lesson. Goodbye.
", "drug". Use within modern definition 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 phenomenon of valence was proposed in 1852 by the chemist Edward Frankland in a work in which he collected and reinterpreted all the theories and assumptions that existed at that time in this regard. . 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", thereby laying the foundation for the doctrine of valency. 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 valence. In 1857, he showed that carbon is a tetrabasic (four-atomic) element, and its simplest compound is methane CH 4. Confident of the truth of his ideas about the valence of atoms, Kekule introduced them into his textbook of organic chemistry: basicity, according to the author, is a fundamental property of an atom, a property as constant and unchangeable as atomic weight. In 1858, views almost coinciding with the ideas of Kekule were expressed in the article “ About 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 valence. He made a clear distinction between a free atom and an atom that has entered into combination with another when its affinity " connects and goes to new uniform " Butlerov introduced the concept of the complete use of the forces of affinity and the “ affinity tension", that is, the energetic nonequivalence of bonds, which is due to the mutual influence of atoms in the 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 valence theory was the possibility of a visual representation of the molecule. In the 1860s. the first molecular models appeared. Already in 1864 A. Brown proposed using structural formulas in the form of circles with symbols of elements placed in them, connected by lines indicating 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 the role of atoms was played by croquet balls. In 1866, drawings of stereochemical models in which the carbon atom had a tetrahedral configuration appeared in Kekule's textbook.
Modern ideas about valence
Since the emergence of the theory of chemical bonding, the concept of “valency” has undergone significant evolution. Currently, it does not have a strict scientific interpretation, therefore it is almost completely crowded out of 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 two-electron two-center bonds formed. This is precisely the approach adopted in the theory of localized valence bonds, proposed in 1927 by W. Heitler and F. London in 1927. Obviously, 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 valency, one should proceed from the electronic configuration of the hypothetical, so-called. “excited” (valence) state. For example, the maximum valence of a beryllium, boron and nitrogen atom 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 some cases, such characteristics of a molecular system as the oxidation state of an element, the effective charge on an atom, the coordination number of an atom, etc. are identified with valence. These characteristics may be close and even coincide quantitatively, but are 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), but 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 equal to −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 concepts of “constant valency” and “variable valency” also primarily refer to the oxidation state.
See also
Notes
Links
- Ugay Ya. A. Valency, chemical bond and oxidation state are the most important concepts of chemistry // Soros educational journal. - 1997. - No. 3. - P. 53-57.
- / Levchenkov S. I. Brief essay history of chemistry
Literature
- L. Pawling The nature of the chemical bond. M., L.: State. NTI chem. literature, 1947.
- Cartmell, Foles. Valence and structure of molecules. M.: Chemistry, 1979. 360 pp.]
- Coulson Ch. Valence. M.: Mir, 1965.
- Murrell J., Kettle S., Tedder J. Valence theory. Per. from English M.: Mir. 1968.
- Development of the doctrine of valency. Ed. Kuznetsova V.I. M.: Chemistry, 1977. 248 p.
- Valence of atoms in molecules / Korolkov D.V. Fundamentals of inorganic chemistry. - M.: Education, 1982. - P. 126.
Wikimedia Foundation. 2010.
Synonyms:See what “Valency” is in other dictionaries:
VALENCE, a measure of the “connecting power” of a chemical element, equal to the number of individual CHEMICAL BONDS that one ATOM can form. The valence of an atom is determined by the number of ELECTRONS at the highest (valence) level (external... ... Scientific and technical encyclopedic dictionary
VALENCE- (from the Latin valere to mean), or atomicity, the number of hydrogen atoms or equivalent atoms or radicals, a given atom or radical can join the swarm. V. is one of the basis for the distribution of elements in the periodic table D.I.... ... Great Medical Encyclopedia
Valence- * valence * valence the term comes from lat. having power. 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 V. atom, this is the ability of atoms... ... Genetics. Encyclopedic Dictionary
- (from Latin valentia force) in physics, a number showing how many hydrogen atoms a given atom can combine with or replace them. In psychology, valence is a designation coming from England for motivating ability. Philosophical... ... Philosophical Encyclopedia
Atomicity Dictionary of Russian synonyms. valency noun, number of synonyms: 1 atomicity (1) ASIS Dictionary of Synonyms. V.N. Trishin... Dictionary of synonyms
VALENCE- (from Latin valentia - strong, durable, influential). The ability of a word to grammatically combine with other words in a sentence (for example, for verbs, valency determines the ability to combine with the subject, direct or indirect object) … New dictionary methodological terms and concepts (theory and practice of language teaching)
- (from 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 force) 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, narrower concepts are often used, for example... ... Big Encyclopedic Dictionary
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However, an accurate and later fully confirmed understanding of the phenomenon of valence was proposed in 1852 by the chemist Eduard Frankland in a work in which he collected and reinterpreted all the theories and assumptions that existed at that time in this regard. Observing the ability to saturate various metals and comparing the composition of organic derivatives of metals with the composition of inorganic compounds, Frankland introduced the concept of “ connecting force» ( connecting weight), thereby laying the foundation for the doctrine of valency. 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 tetrabasic (four-atomic) 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 an atom, a property as constant and unchangeable as atomic weight. In 1858, views almost coinciding with the ideas of Kekule were expressed in the article “ About 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 valence. He made a clear distinction between a free atom and an atom that has entered into combination with another when its affinity " binds and transforms into a new form" Butlerov introduced the concept of the complete use of the forces of affinity and the “ affinity tension", that is, the energetic nonequivalence of bonds, which is due to the mutual influence of atoms in the 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 valence theory was the possibility of a visual representation of the molecule. In the 1860s. the first molecular models appeared. Already in 1864, A. Brown proposed using structural formulas in the form of circles with symbols of elements placed in them, connected by lines indicating 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 in which the carbon atom had a tetrahedral configuration appeared in Kekule's textbook.
Initially, the valency of the hydrogen atom was taken as the unit of valence. The valency of another element can be expressed by the number of hydrogen atoms that adds to itself or replaces one atom of this other element. The valence determined in this way is called the valence in hydrogen compounds or the hydrogen valency: for example, in the compounds HCl, H 2 O, NH 3, CH 4, the hydrogen valence of chlorine is one, oxygen - two, nitrogen - three, carbon - four.
The valence of oxygen is usually equal to two. Therefore, knowing the composition or formula of an oxygen compound of a given element, one can determine its valency as twice the number of oxygen atoms that can attach one atom of a given element. The valency determined in this way is called the valency of the element in oxygen compounds or oxygen valence: thus, in the compounds K 2 O, CO, N 2 O 3, SiO 2, SO 3, the oxygen valency of potassium is one, carbon - two, nitrogen - three, silicon - four, sulfur - six.
For most elements, the valency values in hydrogen and oxygen compounds are different: for example, the valency of sulfur in hydrogen is two (H 2 S), and in oxygen six (SO 3). In addition, most elements exhibit different valencies in their different compounds [some elements may have neither hydrides nor oxides]. For example, carbon forms two oxides with oxygen: carbon monoxide CO and carbon dioxide CO 2 . In carbon monoxide, the valence of carbon is two, and in carbon dioxide it is four (some elements can also form peroxides). From the examples considered, it follows that, as a rule, it is impossible to characterize the valency of an element with any one number and/or method.
Modern ideas about valence
Since the emergence of the theory of chemical bonding, the concept of “valency” has undergone significant evolution. Currently, it does not have a strict scientific interpretation, therefore it is almost completely crowded out of scientific vocabulary and is used mainly for methodological purposes.
Basically, the valence of a chemical element is usually understood as the ability of its free atoms (in a narrower sense, a measure of its ability) to form a certain number of covalent bonds. In compounds with covalent bonds, the valence of atoms is determined by the number of two-electron two-center bonds formed. This is precisely the approach adopted in the theory of localized valence bonds, proposed in 1927 by W. Heitler and F. London. Obviously, 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 valency, one should proceed from the electronic configuration of the hypothetical, so-called. “excited” (valence) state. For example, the maximum valence of an atom of boron, carbon and nitrogen is 4 (for example, in −, CH 4 and +), phosphorus - 5 (PCl 5), sulfur - 6 (H 2 SO 4), chlorine - 7 (Cl 2 O 7 ).
The number of bonds that an atom can form is equal to the number of its unpaired electrons used to form common electron pairs (molecular two-electron clouds). A covalent bond can also be formed by a donor-acceptor mechanism. Moreover, in both cases the polarity of the formed bonds is not taken into account, and therefore the valence has no sign - it can be neither positive nor negative, in contrast to the oxidation state(N 2, NO 2, NH 3 and +).In addition to the valence of hydrogen and oxygen, the ability of atoms of a given element to combine with each other or with atoms of other elements in a number of cases can be expressed [often identified] in other ways: for example, the oxidation state of the element (the conditional charge of an atom under the assumption that the substance consists of ions), covalence (the number of chemical bonds formed by an atom of a given element, including with the element of the same name; see below), coordination number of an atom (the number of atoms immediately surrounding a given atom), etc. These characteristics can be are close and even coincide quantitatively, but are in no way identical to each other. For example, in the isoelectronic molecules of nitrogen N2, carbon monoxide CO and cyanide ion CN−, a triple bond is realized (that is, the valence of each atom is 3), but 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 −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 concepts of “constant valency” and “variable valency” also primarily refer to the oxidation state.
Covalency element (a measure of the valence capabilities of elements; saturation capacity) is determined total number unpaired electrons [valence electron pairs] both in the normal and excited states of the atom, or, in other words, the number of covalent bonds formed by the atom (carbon 2s 2 2p 2 II is covalent, and in the excited state C* 2s 1 2p 3 - IV -covalent; thus, in CO and CO 2 the valency is II or IV, and covalency - II And/or IV). Thus, the covalency of nitrogen in molecules N 2 , NH 3 , Al≡N and cyanamide Ca=N-C≡N is three, the covalency of oxygen in molecules H 2 O and CO 2 is two, the covalency of carbon in molecules CH 4 , CO 2 and crystal ( diamond) - four.
In the classical and/or post-quantum chemical concept, the number of optical (valence) electrons at a given excitation energy can be determined from the electronic absorption spectra of diatomic molecules. According to this method, the reciprocal value of the tangent of the slope of the correlation straight line/straight lines (with relevant values of molecular electronic terms, which are formed by relative sums of atomic ones) corresponds to the number of pairs of valence electrons, that is, valence in its classical sense.
There is a simple relationship between the valence [stoichiometric] of a given compound, the molar mass of its atoms and its equivalent mass, which follows directly from atomic theory and the definition of the concept of “equivalent mass”. CO - valence, since most inorganic substances have a non-molecular structure, while most organic substances have a molecular structure. These two concepts cannot be identified, even if they coincide numerically. The term “valence electrons” is also widely used, that is, the most weakly associated with the nucleus of an atom, most often the outer electrons.
Based on the valence of elements, true formulas of compounds can be compiled, and, conversely, based on true formulas, the valencies of elements in given compounds can be determined. In this case, it is necessary to adhere to the principle that the product of the valence of one element by the number of its atoms is equal to the product of the valence of the second element by the number of its atoms. So, to create the formula of nitric oxide (III), you should write above the symbol of the valency of the elements N I I I (\displaystyle (\stackrel (III)(\mbox(N)))) O I I (\displaystyle (\stackrel (II)(\mbox(O)))). Having determined the lowest common denominator and dividing it into the appropriate valencies, we obtain the atomic ratio of nitrogen to oxygen, namely 2: 3. Therefore, the formula of nitrogen oxide (III) corresponds to N + 3 2 O − 2 3 (\displaystyle (\stackrel (+3)(\mbox(N)))_(2)(\stackrel (-2)(\mbox(O)))_(3)). To determine valency, do the same in reverse.
