Chemical elements and their valency. Determining the valency of chemical elements

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 valency 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 valency:

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 valence of the required element is determined by the valence 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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One of the important topics in the study of school is the course regarding valence. This will be discussed in the article.

Valence - what is it?

Valence in chemistry means the property of the atoms of a chemical element to bind atoms of another element to themselves. Translated from Latin - strength. It is expressed in numbers. For example, the valence of hydrogen will always be equal to one. If we take the formula of water - H2O, it can be represented as H - O - H. One oxygen atom was able to bind two hydrogen atoms to itself. This means that the number of bonds that oxygen creates is two. And the valence of this element will be equal to two.

In turn, hydrogen will be divalent. Its atom can be connected to only one atom of a chemical element. In this case with oxygen. More precisely, atoms, depending on the valency of the element, form pairs of electrons. How many such pairs are formed - this will be the valence. The numeric value is called the index. Oxygen has an index of 2.

How to determine the valence of chemical elements using Dmitry Mendeleev’s table

Looking at the periodic table of elements, you will notice vertical rows. They are called groups of elements. Valence also depends on the group. Elements of the first group have the first valence. Second - second. Third - third. And so on.

There are also elements with a constant valence index. For example, hydrogen, halogen group, silver and so on. They definitely need to be learned.

How to determine the valence of chemical elements using formulas?

Sometimes it is difficult to determine valence from the periodic table. Then you need to look at the specific chemical formula. Let's take FeO oxide. Here, iron, like oxygen, will have a valency index of two. But in Fe2O3 oxide it’s different. Iron will be ferric.

You must always remember the different ways to determine valence and not forget them. Know its constant numerical values. Which elements have them? And, of course, use the table of chemical elements. And also study individual chemical formulas. It is better to present them in schematic form: H – O – H, for example. Then the connections are visible. And the number of dashes (dashes) will be the numerical value of the valence.

The concept of “valence” has been formed in chemistry since the beginning of the 19th century. The English scientist E. Frankland noticed that all elements can form only a certain number of bonds with atoms of other elements. He called it "connective force." Later, the German scientist F.A. Kekule studied methane and came to the conclusion that one carbon atom can attach only four hydrogen atoms under normal conditions.

He called it basicity. The basicity of carbon is four. That is, carbon can form four bonds with other elements.

The concept was further developed in the works of D.I. Mendeleev. Dmitry Ivanovich developed the doctrine of periodic changes in the properties of simple substances. He defined connecting force as the ability of an element to attach a certain number of atoms of another element.

Determination from the periodic table

The periodic table makes it easy to determine the basicity of elements. For this you need be able to read the periodic table. The table has eight groups vertically, and periods are arranged horizontally. If the period consists of two rows, then it is called large, and if it consists of one, it is called small. Elements are distributed unevenly vertically in columns and groups. Valency is always indicated by Roman numerals.

To determine valence, you need to know what it is. For metals of the main subgroups it is always constant, but for non-metals and metals of secondary subgroups it can be variable.

The constant is equal to the group number. A variable can be higher or lower. The highest variable is equal to the group number, and the low one is calculated by the formula: eight minus the group number . When determining, you need to remember:

for hydrogen it is equal to I;
for oxygen - II.

If a compound has a hydrogen or oxygen atom, then determining its valency is not difficult, especially if we have a hydride or oxide.

Formula and algorithm

The lowest valency is for those elements that are located to the right and higher in the table. And, conversely, if the element is lower and to the left, then it will be higher. To define it, you need to follow the universal algorithm:

Example: let's take the ammonia compound - NH3. We know that the hydrogen atom has a constant valence and is equal to I. We multiply I by 3 (the number of atoms) - the smallest multiple is 3. Nitrogen in this formula has an index of one. Hence the conclusion: we divide 3 by 1 and we find that for nitrogen it is equal to IIII.

The value for hydrogen and oxygen is always easy to determine. It is more difficult when it needs to be determined without them. For example , compound SiCl4. How to determine the valence of elements in this case? Chlorine is in group 7. This means its valence is either 7 or 1 (eight minus the group number). Silicon is in the fourth group, which means its potential for forming bonds is four. It becomes logical that chlorine exhibits the lowest valency in this situation and it is equal to I.

Modern chemistry textbooks always contain a table of the valence of chemical elements. This makes the task much easier for students. The topic is studied in the eighth grade - in the course of inorganic chemistry.

Modern representations

Modern ideas about valency based on the structure of atoms. An atom consists of a nucleus and electrons rotating in orbitals.

The nucleus itself is made up of protons and neutrons, which determine atomic weight. For a substance to be stable, its energy levels must be filled and have eight electrons.

When interacting, elements strive for stability and either give up their unpaired electrons or accept them. The interaction occurs according to the principle of “which is easier” - giving or accepting electrons. This also determines how valence changes in the periodic table. The number of unpaired electrons in the outer energy orbital is equal to the group number.

As an example

Alkali metal sodium is in the first group of Mendeleev's periodic table. This means that it has one unpaired electron in its outer energy level. Chlorine is in the seventh group. This means that chlorine has seven unpaired electrons. Chlorine needs exactly one electron to complete its energy level. Sodium gives up its electron to it and becomes stable in the compound. Chlorine receives an additional electron and also becomes stable. As a result, a bond and a strong connection appears - NaCl - the famous table salt. The valency of chlorine and sodium in this case will be equal to 1.

In order to determine the valence of a particular substance, you need to look at Mendeleev’s periodic table of chemical elements; the designations in Roman numerals will be the valences of certain substances in this table. For example, BUT, hydrogen (H) will always be monovalent, and oxygen (O) will always be divalent. Here is a cheat sheet below that I think will help you)

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

Alkali metals, hydrogen, and halogens are considered monovalent;

But boron and aluminum are trivalent.

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

The lowest valence is determined by subtracting the group number from 8. Non-metals are endowed with a lower valence to a greater extent.

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

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

Valency can be determined using the periodic table. The highest valence of an element is always equal to the number of the group in which it is found.

Nonmetals most often have the lowest variable valency. To find out the lowest valency, the group number is subtracted from 8 - the result will be the desired value. For example, sulfur is in group 6 and its highest valency is VI, the lowest valency will be II (86 = 2).

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

As is known, valence can be constant (when a chemical element always forms the same number of bonds with other atoms) and variable (when, depending on a particular substance, the valency of the same element changes).

The periodic system of chemical elements by D.I. Mendeleev will help us determine valence.

The following rules apply:

1) Maximum The valence of a chemical element is equal to the group number. For example, chlorine is in the 7th group, which means it has a maximum valency of 7. Sulfur: it is in the 6th group, which means it has a maximum valence of 6.

2) Minimum valence for non-metals equals 8 minus the group number. For example, the minimum valence of the same chlorine is 8 7, that is, 1.

Alas, there are exceptions to both rules.

For example, copper is in group 1, but the maximum valence of copper is not 1, but 2.

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

It is useful to remember the following rules:

3) All alkaline metals (metals of group I, the main subgroup) always have valence 1. For example, the valency of sodium is always 1 because it is an alkali metal.

4) All alkaline earth metals (metals of group II, the main subgroup) always have valence 2. For example, the valence of magnesium is always 2 because it is an alkaline earth metal.

5) Aluminum always has a valency of 3.

6) Hydrogen always has a valence of 1.

7) Oxygen almost always has a valence of 2.

8) Carbon almost always has a valence of 4.

It should be remembered that definitions of valency may differ in different sources.

More or less accurately, valency can be defined as the number of shared electron pairs through which a given atom is connected to others.

According to this definition, the valency of nitrogen in HNO3 is 4, not 5. Nitrogen cannot be pentavalent, because in this case there would be 10 electrons circling the nitrogen atom. But this cannot happen, because the maximum number of electrons is 8.

The valence of any chemical element is its property, or rather the property of its atoms (atoms of this element) to hold a certain number of atoms, but of another chemical element.

There are chemical elements with both constant and variable valence, which changes depending on which element it (this element) is in combination with or enters into.

Valencies of some chemical elements:

Let's now move on to how the valency of an element is determined from the table.

So, valence can be determined by periodic table:

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

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

The valence of any element can be determined from the periodic table itself, by the group number.

At least this can be done in the case of metals, because their valence is equal to the group number.

The story with non-metals is a little different: their highest valence (in compounds with oxygen) is also equal to the group number, but the lowest valence (in compounds with hydrogen and metals) must be determined using the following formula: 8 - group number.

The more you work with chemical elements, the better you remember their valency. To get started, this cheat sheet will suffice:

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

Valency is the ability of atoms of some chemical elements to attach to themselves atoms of other elements. To successfully write formulas and correctly solve problems, you need to know well how to determine valency. First you need to learn all the elements with constant valency. Here they are: 1. Hydrogen, halogens, alkali metals (always monovalent); 2. Oxygen and alkaline earth metals (divalent); 3. B and Al (trivalent). To determine valency using the periodic table, you need to find out which group the chemical element is in and determine whether it is in the main group or a secondary one.

An element can have one or more valencies.

The maximum valency of an element is equal to the number of valence electrons. We can determine valency by knowing the location of an element on the periodic table. The maximum valence number is equal to the number of the group in which the required element is located.

Valence is indicated by a Roman numeral and is typically written in the upper right corner of the element symbol.

Some elements may have different valencies in different compounds.

For example, sulfur has the following valencies:

II in H2S compound
IV in SO2 compound
VI in SO3 compound

The rules for determining valence are not as easy to use, so they need to be remembered.

Determining valency using the periodic table is simple. As a rule, it corresponds to the number of the group in which the element is located. But there are elements that can have different valencies in different compounds. In this case we are talking about constant and variable valency. The variable can be maximum, equal to the group number, or it can be minimum or intermediate.

But it is much more interesting to determine the valency in compounds. There are a number of rules for this. First of all, it is easy to determine the valence of elements if one element in a compound has a constant valence, for example, oxygen or hydrogen. On the left is a reducing agent, that is, an element with a positive valence, on the right is an oxidizing agent, that is, an element with a negative valence. The index of an element with a constant valence is multiplied by this valence and divided by the index of an element with an unknown valence.

Example: silicon oxides. The valence of oxygen is -2. Let's find the valency of silicon.

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

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

", "a drug ". Use within the modern definition was 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 valency 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 various 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 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 valence. 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 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 " 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 valency 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 valency

Since the emergence of the theory of chemical bonding, the concept of “valence” 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 valency 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 valency 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 valence” also primarily refer to the oxidation state.

Notes

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 valence. Ed. Kuznetsova V. I. M.: Khimiya, 1977. 248 p.
Valence of atoms in molecules / Korolkov D.V. Fundamentals of inorganic chemistry. - M.: Education, 1982. - P. 126.

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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 valency 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 atom, V. 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... Synonym dictionary

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, valence determines the ability to combine with the subject, direct or indirect object) ... New dictionary of 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