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Valence- the ability of elements to attach other elements to themselves.

talking plain language, is a number indicating how many elements a certain atom can attach to itself.

The key point in chemistry is the correct recording of the formulas of compounds.

There are several rules that make it easier for us correct compilation formulas.

The valency of all metals of the main subgroups is equal to the group number:

The figure shows an example of the main and secondary subgroups of group I.

2. The valence of oxygen is two

3. The valence of hydrogen is equal to one

4. Non-metals exhibit two types of valency:

Inferior (8th group number)
Higher (equal to group number)

A) In compounds with metals, non-metals exhibit a lower valency!

B) In binary compounds, the sum of the valency of one type of atom is equal to the sum of the valency of another type of atom!

The valence of aluminum is three (aluminum-metal Group III). The valency of oxygen is two. The sum of valence for two aluminum atoms is 6. The sum of valency for three oxygen atoms is also 6.

1) Determine the valencies of the elements in the compounds:

The valency of aluminum is III. In formula 1 atom => the total valency is also 3. Therefore, for all chlorine atoms, the valence will also be 3 (the rule of binary compounds). 3:3=1. The valency of chlorine is 1.

The valency of oxygen is 2. In the compound there are 3 oxygen atoms => the total valency is 6. For two atoms the total valency is 6 => for one iron atom - 3 (6: 2 = 3)

2) Write the formulas of the compound consisting of:

sodium and oxygen

The valency of oxygen is II.

Sodium metal of the first group of the main subgroup => its valency is I.

Different chemical elements differ in their ability to create chemical bonds, that is, to combine with other atoms. Therefore, in complex substances, they can be found only in certain proportions. Let's figure out how to determine the valence according to the periodic table.

There is such a definition of valence: this is the ability of an atom to form a certain number of chemical bonds. Unlike , this value is always only positive and is indicated by Roman numerals.

This characteristic for hydrogen is used as a unit, which is taken equal to I. This property shows how many monovalent atoms this element can combine with. For oxygen, this value is always equal to II.

Knowing this characteristic is necessary in order to correctly write down chemical formulas substances and equations. Knowing this value will help to establish the ratio between the number of atoms various types in a molecule.

This concept arose in chemistry in the 19th century. Frankland started the theory explaining the combination of atoms in various ratios, but his ideas about the "binding force" were not very common. The decisive role in the development of the theory belonged to Kekula. He called the property of forming a certain number of bonds basicity. Kekule believed that this is a fundamental and unchanging property of every kind of atom. Important additions to the theory were made by Butlerov. With the development of this theory, it became possible to visualize molecules. This helped a lot in studying the structure of various substances.

How can the periodic table help?

You can find valence by looking at the group number in the short period version. For most elements for which this characteristic is constant (takes only one value), it coincides with the group number.

Such properties have main subgroups. Why? The group number corresponds to the number of electrons in the outer shell. These electrons are called valence electrons. They are responsible for the ability to combine with other atoms.

The group consists of elements with a similar structure of the electron shell, and the charge of the nucleus increases from top to bottom. In the short period form, each group is divided into main and secondary subgroups. Representatives of the main subgroups are s and p elements, representatives of secondary subgroups have electrons in d and f orbitals.

How to determine valence chemical elements if it changes? It can be the same as the group number, or be equal to the group number minus eight, or take on other values.

Important! The higher and to the right the element, the less its ability to form relationships. The more it is shifted down and to the left, the larger it is.

How the valency changes in the periodic table for a particular type of atom depends on the structure of its electron shell. Sulfur, for example, can be di-, tetra- and hexavalent.

In the ground (unexcited) state, sulfur has two unpaired electrons at the 3p sublevel. In this state, it can combine with two hydrogen atoms and form hydrogen sulfide. If sulfur goes into a more excited state, then one electron will go to the free 3d sublevel, and there will be 4 unpaired electrons.

Sulfur will become tetravalent. If we give it even more energy, then one more electron will move from the 3s sublevel to 3d. Sulfur will go into an even more excited state and become hexavalent.

Constant and variable

Sometimes the ability to form chemical bonds can change. It depends on which connection the element is in. For example, sulfur in H2S is divalent, in SO2 it is tetravalent, and in SO3 it is hexavalent. The largest of these values is called the highest, and the smallest - the lowest. The highest and lowest valencies according to the periodic table can be set as follows: the highest coincides with the group number, and the lowest is equal to 8 minus the group number.

How to determine the valency of chemical elements and whether it changes? We need to establish whether we are dealing with metal or non-metal. If it is a metal, you need to establish whether it belongs to the main or secondary subgroup.

In metals of the main subgroups, the ability to form chemical relationships is constant.
For metals of secondary subgroups - a variable.
Non-metals also have a variable. In most cases, it takes two values - higher and lower, but sometimes it can be more options. Examples are sulfur, chlorine, bromine, iodine, chromium and others.

In compounds, the lower valency is shown by the element that is higher and to the right in the periodic table, respectively, the higher - the one that is to the left and lower.

Often the ability to form chemical bonds takes on more than two values. Then you won’t be able to recognize them from the table, but you will need to learn them. Examples of such substances:

carbon;
sulfur;
chlorine;
bromine.

How to determine the valency of an element in a compound formula? If it is known for other constituents of the substance, this is not difficult. For example, you want to calculate this property for chlorine in NaCl. Sodium is an element of the main subgroup of the first group, so it is monovalent. Therefore, chlorine in this substance can also create only one bond and is also monovalent.

Important! However, it is not always possible to find out this property for all atoms in a complex substance. Let's take HClO4 as an example. Knowing the properties of hydrogen, one can only establish that ClO4 is a univalent residue.

How else can you find this value?

The ability to form a certain number of bonds does not always coincide with the group number, and in some cases it will simply have to be memorized. Here on help will come table of valency of chemical elements, where the values of this quantity are given. In the chemistry textbook for grade 8, the values \u200b\u200bof the ability to combine with other atoms of the most common types of atoms are given.

H, F, Li, Na, K	1
O, Mg, Ca, Ba, Sr, Zn	2
B,Al	3
C, Si	4
Cu	1, 2
Fe	2, 3
Cr	2, 3, 6
S	2, 4, 6
N	3, 4
P	3, 5
Sn, Pb	2, 4
Cl, Br, I	1, 3, 5, 7

Application

It is worth saying that chemists at present almost do not use the concept of valence according to the periodic table. Instead, for the ability of a substance to form a certain number of relationships, the concept of the degree of oxidation is used, for substances with a structure - covalence, and for substances of an ionic structure - the charge of the ion.

However, the concept under consideration is used for methodological purposes. With it, it is easy to explain why atoms different types combine in the ratios that we observe, and why these ratios are different for different compounds.

On this moment the approach according to which the combination of elements into new substances was always explained using valence according to the periodic table, regardless of the type of bond in the compound, is outdated. Now we know that for ionic, covalent, metallic bonds there are different mechanisms association of atoms into molecules.

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Summing up

According to the periodic table, it is not possible to determine the ability to form chemical bonds for all elements. For those that show one valency according to the periodic table, in most cases it is equal to the group number. If there are two options for this value, then it can be equal to the group number or eight minus the group number. There are also special tables by which you can find out this characteristic.

Valency is the ability of an atom given element form a certain number of chemical bonds.

Figuratively speaking, valency is the number of "hands" with which an atom clings to other atoms. Naturally, atoms have no "hands"; their role is played by the so-called. valence electrons.

It can be said differently: valence is the ability of an atom of a given element to attach a certain number of other atoms.

The following principles must be clearly understood:

There are elements with constant valence (there are relatively few of them) and elements with variable valency (of which the majority).

Elements with constant valency must be remembered:

The remaining elements may exhibit different valency.

The highest valency of an element in most cases coincides with the number of the group in which the element is located.

For example, manganese is found in VII group(side subgroup), the highest valency of Mn is seven. Silicon is located in group IV (the main subgroup), its highest valency is four.

It should be remembered, however, that the highest valency is not always the only possible one. For example, the highest valency of chlorine is seven (check it out!), but compounds are known in which this element exhibits valences VI, V, IV, III, II, I.

It is important to remember a few exceptions: the maximum (and only) valency of fluorine is I (and not VII), oxygen - II (and not VI), nitrogen - IV (the ability of nitrogen to show valence V is a popular myth that is found even in some school textbooks).

Valency and oxidation state are not identical concepts.

These concepts are close enough, but they should not be confused! The oxidation state has a sign (+ or -), valence - no; the oxidation state of an element in a substance can be zero, the valence is zero only if we are dealing with an isolated atom; the numerical value of the oxidation state may NOT coincide with the valency. For example, the valence of nitrogen in N 2 is III, and the oxidation state = 0. The valency of carbon in formic acid is IV, and the oxidation state is +2.

If the valency of one of the elements in a binary compound is known, the valency of the other can be found.

This is done very simply. Remember the formal rule: the product of the number of atoms of the first element in a molecule and its valency must be equal to the same product for the second element.

In compound A x B y: valency (A) x = valence (B) y

Example 1. Find the valencies of all elements in the NH 3 compound.

Solution. We know the valency of hydrogen - it is constant and equal to I. We multiply the valency of H by the number of hydrogen atoms in the ammonia molecule: 1 3 \u003d 3. Therefore, for nitrogen, the product of 1 (number of N atoms) by X (nitrogen valency) should also be equal to 3. Obviously, X = 3. Answer: N(III), H(I).

Example 2. Find the valencies of all elements in the Cl 2 O 5 molecule.

Solution. Oxygen has a constant valence (II), in the molecule of this oxide there are five oxygen atoms and two chlorine atoms. Let the valency of chlorine \u003d X. We make an equation: 5 2 \u003d 2 X. Obviously, X \u003d 5. Answer: Cl (V), O (II).

Example 3. Find the 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 valency of sulfur, it would have been impossible to solve it. Both S and Cl are variable valence elements. Taking into account additional information, the solution is built according to the scheme of examples 1 and 2. Answer: Cl(I).

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

In examples 1 - 3, we determined the valence using the formula, now let's try to do the reverse procedure.

Example 4. Write the formula for the compound of calcium and hydrogen.

Solution. The valencies of calcium and hydrogen are known - II and I, respectively. Let the formula of the desired compound be Ca x H y. We again compose the well-known equation: 2 x \u003d 1 y. As one of the solutions to this equation, we can take x = 1, y = 2. Answer: CaH 2 .

"And why exactly CaH 2? - you ask. - After all, the 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 given example, these minimum (natural!) values are exactly equal to 1 and 2.

"So, compounds like N 2 O 4 or C 6 H 6 are impossible? - you ask. - Should these formulas be replaced with NO 2 and CH?"

No, they are possible. Moreover, N 2 O 4 and NO 2 are completely different substances. But the CH formula does not correspond to any real stable substance at all (unlike C 6 H 6).

Despite all that has been said, in most cases you can be guided by the rule: take smallest values indexes.

Example 5. Write the formula for the compound of sulfur with fluorine, if it is known that the valency of sulfur is six.

Solution. Let the compound formula be S x F y . The valency of sulfur is given (VI), the valency of fluorine is constant (I). Again we make the equation: 6 x \u003d 1 y. It is easy to understand that the smallest possible values of the variables are 1 and 6. Answer: SF 6 .

Here, in fact, are all the main points.

Now check yourself! I propose to go a little test on the topic "Valence".

concept valence comes from the Latin word "valentia" and was known as early as the middle of the 19th century. The first "extensive" mention of valence was in the works of J. Dalton, who argued that all substances consist of atoms interconnected in certain proportions. Then, Frankland introduced the very concept of valence, which found further development in the works of Kekule, who spoke about the relationship between valency and chemical bonding, A.M. Butlerov, who in his theory of structure organic compounds associated valence with the reactivity of one or another chemical compound and D.I. Mendeleev (in the Periodic system of chemical elements, the highest valency of an element is determined by the group number).

DEFINITION

Valence is the number of covalent bonds that an atom can form in combination with a covalent bond.

The valency of an element is determined by the number of unpaired electrons in an atom, since they take part in the formation of a chemical bond between atoms in compound molecules.

The ground state of an atom (the state with minimum energy) is characterized by the electronic configuration of the atom, which corresponds to the position of the element in the Periodic system. An excited state is a new energy state of an atom, with a new distribution of electrons within the valence level.

The electronic configurations of electrons in an atom can be represented not only in the form electronic formulas, but also with the help of electron-graphic formulas (energy, quantum cells). Each cell indicates an orbital, the arrow indicates an electron, the direction of the arrow (up or down) indicates the spin of the electron, a free cell indicates a free orbital that an electron can occupy when excited. If there are 2 electrons in a cell, such electrons are called paired, if electron 1 is unpaired. For example:

6 C 1s 2 2s 2 2p 2

Orbitals fill in the following way: first one electron with the same spins, and then the second electron with opposite spins. Since the 2p sublevel has three orbitals with same energy, then each of the two electrons occupied one orbital. One orbital remained free.

Determination of the valency of an element by electron-graphic formulas

The valence of an element can be determined by the electron-graphic formulas of the electronic configurations of electrons in an atom. Consider two atoms, nitrogen and phosphorus.

7 N 1s 2 2s 2 2p 3

Because the valency of an element is determined by the number of unpaired electrons, therefore, the valence of nitrogen is III. Since the nitrogen atom has no free orbitals, an excited state is impossible for this element. However, III is not the maximum nitrogen valence, the maximum nitrogen valency is V and is determined by the group number. Therefore, it should be remembered that with the help of electron-graphic formulas it is not always possible to determine the highest valence, as well as all the valences characteristic of this element.

15 P 1s 2 2s 2 2p 6 3s 2 3p 3

In the ground state, the phosphorus atom has 3 unpaired electrons, therefore, the valency of phosphorus is III. However, there are free d-orbitals in the phosphorus atom, therefore, electrons located on the 2s sublevel are able to depair and occupy vacant orbitals of the d-sublevel, i.e. go into an excited state.

Now the phosphorus atom has 5 unpaired electrons, therefore, phosphorus also has a valency equal to V.

Elements having multiple valency values

Elements of IVA - VIIA groups can have several valency values, and, as a rule, the valence changes in steps of 2 units. This phenomenon is due to the fact that electrons participate in the formation of a chemical bond in pairs.

Unlike the elements of the main subgroups, the elements of the B-subgroups, in most compounds, do not show a higher valency equal to the group number, for example, copper and gold. In general, transitional elements exhibit a great variety chemical properties, which is explained by a large set of valences.

Consider the electronic graphic formulas of the elements and establish, in connection with which the elements have different valences (Fig. 1).

Tasks: determine the valence possibilities of As and Cl atoms in the ground and excited states.