The highest valency of the elements. Valence

The level of knowledge about the structure of atoms and molecules in the 19th century did not allow explaining the reason why atoms form a certain number of bonds with other particles. But the ideas of scientists were ahead of their time, and valency is still being studied as one of the basic principles of chemistry.

From the history of the concept of "valency of chemical elements"

The outstanding English chemist of the 19th century, Edward Frankland, introduced the term "bond" into scientific use to describe the process of interaction of atoms with each other. The scientist noticed that some chemical elements form compounds with the same number of other atoms. For example, nitrogen attaches three hydrogen atoms to the ammonia molecule.

In May 1852, Frankland hypothesized that there was a specific number of chemical bonds that an atom could form with other tiny particles of matter. Frankland used the phrase "connecting force" to describe what would later be called valence. A British chemist determined how many chemical bonds form the atoms of individual elements known in the middle of the 19th century. Frankland's work was an important contribution to modern structural chemistry.

Development of views

German chemist F.A. Kekule proved in 1857 that carbon is a tetrabasic one. In its simplest compound - methane - there are bonds with 4 hydrogen atoms. The scientist used the term "basicity" to denote the property of elements to attach a strictly defined amount of other particles. In Russia, data on were systematized by A. M. Butlerov (1861). The theory of chemical bonding received further development thanks to the doctrine of the periodic change in the properties of elements. Its author is another outstanding D. I. Mendeleev. He proved that the valency of chemical elements in compounds and other properties are due to the position that they occupy in the periodic system.

Graphical representation of valency and chemical bonding

The possibility of a visual representation of molecules is one of the undoubted advantages of the theory of valency. The first models appeared in the 1860s, and since 1864, circles with a chemical sign inside have been used. Between the symbols of atoms, a dash is indicated and the number of these lines is equal to the value of the valency. In the same years, the first ball-and-stick models were made (see photo on the left). In 1866, Kekule proposed a stereochemical drawing of a carbon atom in the form of a tetrahedron, which he included in his textbook Organic Chemistry.

The valence of chemical elements and the formation of bonds were studied by G. Lewis, who published his works in 1923 after the name of the negatively charged smallest particles that make up the shells of atoms. In his book, Lewis used the dots around the four sides of the chemical element symbol to represent valence electrons.

Valency for hydrogen and oxygen

Before the creation of the periodic system, the valence of chemical elements in compounds was usually compared with those atoms for which it is known. Hydrogen and oxygen were chosen as standards. Another chemical element attracted or replaced a certain number of H and O atoms.

In this way, properties were determined in compounds with monovalent hydrogen (the valency of the second element is indicated by a Roman numeral):

• HCl - chlorine (I):
• H 2 O - oxygen (II);
• NH 3 - nitrogen (III);
• CH 4 - carbon (IV).

In oxides K 2 O, CO, N 2 O 3, SiO 2, SO 3, the oxygen valency of metals and non-metals was determined by doubling the number of added O atoms. The following values ​​were obtained: K (I), C (II), N (III) , Si (IV), S (VI).

How to determine the valency of chemical elements

There are regularities in the formation of a chemical bond involving shared electron pairs:

• The typical valency of hydrogen is I.
• The usual valency of oxygen is II.
• For non-metal elements, the lowest valence can be determined by formula 8 - the number of the group in which they are located in the periodic system. The highest, if possible, is determined by the group number.
• For elements of side subgroups, the maximum possible valence is the same as their group number in the periodic table.

The determination of the valence of chemical elements according to the formula of the compound is carried out using the following algorithm:

1. Write the known value for one of the elements above the chemical sign. For example, in Mn 2 O 7 the oxygen valency is II.
2. Calculate the total value, for which it is necessary to multiply the valence by the number of atoms of the same chemical element in the molecule: 2 * 7 \u003d 14.
3. Determine the valency of the second element for which it is unknown. Divide the value obtained in step 2 by the number of Mn atoms in the molecule.
4. 14: 2 = 7. in its highest oxide - VII.

Constant and variable valency

Valence values ​​for hydrogen and oxygen are different. For example, sulfur in the compound H 2 S is divalent, and in the formula SO 3 it is hexavalent. Carbon forms CO monoxide and CO 2 dioxide with oxygen. In the first compound, the valency of C is II, and in the second, IV. The same value in methane CH 4 .

Most elements exhibit not constant, but variable valency, for example, phosphorus, nitrogen, sulfur. The search for the main causes of this phenomenon led to the emergence of chemical bond theories, ideas about the valence shell of electrons, and molecular orbitals. The existence of different values ​​of the same property was explained from the standpoint of the structure of atoms and molecules.

Modern ideas about valency

All atoms consist of a positive nucleus surrounded by negatively charged electrons. The outer shell that they form is unfinished. The completed structure is the most stable, containing 8 electrons (an octet). The emergence of a chemical bond due to common electron pairs leads to an energetically favorable state of atoms.

The rule for compound formation is to complete the shell by accepting electrons or donating unpaired ones, whichever process is easier. If an atom provides for the formation of a chemical bond negative particles that do not have a pair, then it forms as many bonds as it has unpaired electrons. According to modern concepts, the valence of atoms of chemical elements is the ability to form a certain number of covalent bonds. For example, in a hydrogen sulfide molecule H 2 S, sulfur acquires valency II (-), since each atom takes part in the formation of two electron pairs. The "-" sign indicates the attraction of an electron pair to a more electronegative element. For a less electronegative value, “+” is added to the valence value.

With the donor-acceptor mechanism, electron pairs of one element and free valence orbitals of another element take part in the process.

The dependence of valence on the structure of the atom

Consider, using the example of carbon and oxygen, how the valence of chemical elements depends on the structure of the substance. The periodic table gives an idea of ​​the main characteristics of the carbon atom:

• chemical sign - C;
• element number - 6;
• core charge - +6;
• protons in the nucleus - 6;
• electrons - 6, including 4 external ones, of which 2 form a pair, 2 are unpaired.

If the carbon atom in CO monoxide forms two bonds, then only 6 negative particles come to its use. To acquire an octet, it is necessary that the pairs form 4 external negative particles. Carbon has valency IV (+) in dioxide and IV (-) in methane.

The serial number of oxygen is 8, the valence shell consists of six electrons, 2 of which do not form a pair and take part in chemical bonding and interaction with other atoms. The typical valency of oxygen is II (-).

Valency and oxidation state

In many cases it is more convenient to use the concept of "oxidation state". This is the name given to the charge an atom would acquire if all the bonding electrons were transferred to an element that has a higher electronegativity (EO) value. The oxidation number in a simple substance is zero. A “-” sign is added to the oxidation state of a more EO element, a “+” sign is added to a less electronegative one. For example, for metals of the main subgroups, oxidation states and ion charges are typical, equal to the group number with a “+” sign. In most cases, the valency and oxidation state of atoms in the same compound are numerically the same. Only when interacting with more electronegative atoms, the oxidation state is positive, with elements in which the EO is lower, it is negative. The concept of "valency" is often applied only to substances of a molecular structure.

When considering the chemical elements, one can notice that the number of atoms in the same element in different substances varies. How to write down the formula correctly and not make a mistake in the index of a chemical element? This is easy to do if you have an idea of ​​\u200b\u200bwhat valence is.

What is valence for?

The valence of chemical elements is the ability of the atoms of an element to form chemical bonds, that is, to attach other atoms to themselves. A quantitative measure of valency is the number of bonds that a given atom forms with other atoms or atomic groups.

Currently, valence is the number of covalent bonds (including those that have arisen by the donor-acceptor mechanism) by which a given atom is connected to others. This does not take into account the polarity of the bonds, which means that the valence has no sign and cannot be equal to zero.

A covalent chemical bond is a bond carried out through the formation of common (bonding) electron pairs. If there is one common electron pair between two atoms, then such a bond is called single, if two - double, if three - triple.

How to find valence?

The first question that worries 8th grade students who have begun to study chemistry is how to determine the valency of chemical elements? The valency of a chemical element can be viewed in a special table of the valency of chemical elements

Rice. 1. Table of valency of chemical elements

The valency of hydrogen is taken as unity, since a hydrogen atom can form one bond with other atoms. The valency of other elements is expressed by a number that shows how many hydrogen atoms an atom of a given element can attach to itself. For example, the valence of chlorine in a molecule of hydrogen chloride is equal to one. Therefore, the formula for hydrogen chloride will look like this: HCl. Since both chlorine and hydrogen have a valency of one, no index is used. Both chlorine and hydrogen are monovalent, since one hydrogen atom corresponds to one chlorine atom.

Consider another example: the valence of carbon in methane is four, the valence of hydrogen is always one. Therefore, the index 4 should be placed next to hydrogen. Thus, the formula for methane looks like this: CH 4.

Many elements form compounds with oxygen. Oxygen is always divalent. Therefore, in the formula of water H 2 O, where monovalent hydrogen and divalent oxygen always occur, the index 2 is placed next to hydrogen. This means that the water molecule consists of two hydrogen atoms and one oxygen atom.

Rice. 2. Graphic formula of water

Not all chemical elements have a constant valency, for some it may vary depending on the compounds where this element is used. Elements with constant valency include hydrogen and oxygen, elements with variable valence include, for example, iron, sulfur, carbon.

How to determine valence by the formula?

If you don’t have a valency table before your eyes, but there is a formula for a chemical compound, then it is possible to determine the valence by the formula. Take for example the formula manganese oxide - Mn 2 O 7

Rice. 3. Manganese oxide

As you know, oxygen is divalent. To find out what valency manganese has, it is necessary to multiply the oxygen valence by the number of gas atoms in this compound:

The resulting number is divided by the number of manganese atoms in the compound. It turns out:

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

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

Alkali metals, hydrogen, and halogens are considered monovalent;

But trivalent boron and aluminum.

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

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

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

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

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

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

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

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

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

The following rules apply:

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

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

Alas, there are exceptions to both rules.

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

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

It is useful to remember the following rules:

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

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

5) Aluminum always has a valence of 3.

6) Hydrogen always has a valence of 1.

7) Oxygen almost always has a valence of 2.

8) Carbon almost always has a valence of 4.

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

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

According to this definition, the valency of nitrogen in HNO3 is 4, not 5. Nitrogen cannot be pentavalent, because in this case, 10 electrons would circle around the nitrogen atom. And this cannot be, because the maximum of electrons is 8.

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

There are chemical elements with both constant and variable valency, which varies depending on which element it (the given element) is in conjunction with or enters.

Valencies of some chemical elements:

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

So, valency can be determined by periodic table:

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





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

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

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

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

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



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

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

An element can have one or more valences.

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

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

Some elements may have different valencies in different 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.

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

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

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

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

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

In this article, we will look at ways and understand how to determine valency elements of the periodic table.

In chemistry, it is accepted that the valence of chemical elements can be recognized by the group (column) in the periodic table. In reality, the valency of an element does not always correspond to the group number, but in most cases a certain valence using this method will give the correct result; often elements, depending on various factors, have more than one valency.

The unit of valence is the valency of the 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 bivalent; NH3, where nitrogen is trivalent.

How to determine valence according to the periodic table.

The periodic table contains chemical elements that are placed in it according to certain principles and laws. Each element stands in its place, which is determined by its characteristics and properties, and each element has its own number. The horizontal lines are called periods, which increase from the first line down. If the period consists of two rows (which is indicated on the side by numbering), then such a period is called a large one. If it has only one row, then it is called small.

In addition, there are groups in the table, of which there are only eight. Items are arranged in columns vertically. Here their placement is uneven - on the one hand there are more elements (main group), on the other - less (side group).

Valency is the ability of an atom to form a certain number of chemical bonds with atoms of other elements. according to the periodic table will help to understand the knowledge of the types of valency.

For elements of secondary subgroups (and only metals belong to them), the valence 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 values. Or constantly keep at hand the valency table of elements.

Algorithm for determining valence by the formulas of chemical elements.

1. Write down the formula of a chemical compound.

2. Designate the known valency of the elements.

3. Find the least common multiple of valency and index.

4. Find the ratio of the least common multiple to the number of atoms of the second element. This is the required valence.

5. Make a check by multiplying the valency and index of each element. Their works must be equal.

Example: determine the valency of the elements of hydrogen sulfide.

1. Let's write the formula:

2. 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 valency values ​​of some atoms of chemical compounds.

Considering the formulas of various compounds, it is easy to see that number of atoms the same element in the molecules of different substances is not the same. 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 typical not only for hydrogen.

How to guess which index to put next to the designation of a chemical element? How are the formulas of a substance formed? 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, hold or replace a certain number of atoms of another element in chemical reactions. The unit of valency is the valency of the 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 univalent 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 bivalent.

Permanent 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 valence of oxygen, because in compounds its valency is always 2. For example, it will not be difficult to find the valence of elements in the following compounds: Na 2 O (valence Na – 1,O – 2), Al 2 O 3 (Al – 3,O – 2).

The chemical formula of a given substance can be compiled only by knowing the valency of the elements. For example, it is easy to formulate 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 valencies 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 valency of Mn in a compound is 7, and O – 2, then the compound formula will look like this Mn 2 O 7.

How did we get the formula?

Consider the algorithm for compiling formulas by valency for those consisting of two chemical elements.

There is a rule that the number of valences in one chemical element is equal to the number of valences in another. 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 next the symbols of chemical elements:

2. We put over the chemical elements the numbers of their valence (the valency of a chemical element can be found in the periodic table of Mendelev, for manganese – 7, have oxygen – 2.

3. Find the least common multiple (the smallest number that is divisible without a remainder by 7 and by 2). This number is 14. We divide it by the valencies of the elements 14: 7 \u003d 2, 14: 2 \u003d 7, 2 and 7 will be indices, respectively, for phosphorus and oxygen. We substitute indexes.

Knowing the valence of one chemical element, following the rule: the valency of one element × the number of its atoms in a molecule = the valency of another element × the number of atoms of this (another) element, one can determine the valence of another.

Mn 2 O 7 (7 2 = 2 7).

The concept of valency was introduced into chemistry before the structure of the atom was known. It has now been established that this property of an element is related to the number of outer electrons. For many elements, the maximum valence results from the position of those elements in the periodic table.

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