Mendeleev valency of the formula of a substance. Determine the valence of chemical elements

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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element;
> predict the possible values ​​of the valency of an element based on its placement in the periodic system;
> determine the valence values ​​of elements in binary compounds according to their formulas;
> draw up formulas for binary compounds based on the valency values ​​of the elements.

The valence value of an element, if necessary, is indicated in the chemical formula by a Roman numeral above its symbol: In mathematical calculations and text, Arabic numerals are used for this.

Determine the valency of the elements in the molecules of ammonia NH 3 and methane CH 4.

Information about the valence of elements in a substance can be represented in another way. First, the symbols of each atom in the molecule are written at a certain distance from each other. Then the monovalent atom is connected to another with one dash, two dashes are drawn from the divalent atom, etc.:

Such formulas are called graphic. They show the order in which atoms are connected in molecules.

The simple hydrogen molecule has the graphic formula H-H. Similar are the graphic formulas of the molecules of fluorine, chlorine, bromine, iodine. The graphic formula of the oxygen molecule is 0=0, and molecules nitrogen.

When compiling such formulas for molecules of complex substances, it should be borne in mind that the atoms of one element, as a rule, are not interconnected.

Draw the graphic formulas of ammonia and methane molecules.

From the graphic formula of the molecule, it is easy to determine the valency of each atom. The valency value is equal to the number of dashes that come from the atom.

For compounds of ionic and atomic structure, graphic formulas are not used.

The valence of an element and its placement in the periodic system.

Some elements have a constant valency.

This is interesting

At the beginning of the XIX century. in views on the composition of chemical compounds dominated
principle of "maximum simplicity". So, the formula for water was written HO, not H 2 O.

Hydrogen and Fluor are always monovalent, and Oxygen- divalent. Other elements with constant valence are in groups I-III of the periodic system, and the valence value of each element coincides with the group number. Thus, the group I element Lithium is monovalent, the group II element Magnesium is bivalent, and the group III element Boron is trivalent. The exceptions are the elements of group I Cuprum (valence values ​​- I and 2) and Aurum (I and 3).

Most of the elements have variable valency. Here are its values ​​for some of them:

Plumbum (Group IV) - 2.4;
Phosphorus (V group) - 3.5;
Chromium (VI group) - 2, 3, 6;
Sulfur (VI group) - 2, 4, 6;
Mangan (VII group) - 2, 4, 6, 7;
Chlorine (VII group) - I, 3, 5, 7.

An important rule follows from this information: the maximum value of the valence of an element coincides with the number of the group in which it is located1. Because in periodic system eight groups, then the valency values ​​of the elements can be from I to 8.

There is one more rule: the valence value of a non-metallic element in combination with Hydrogen or with a metallic element is 8 minus the number of the group in which the element is located. We will confirm it with examples of compounds of elements with Hydrogen. Group VII element Iodine in hydrogen iodine HI is monovalent (8-7=1), group VI element Oxygen in water H 2 O is divalent (8-6 = 2), group V element Nitrogen in ammonia
NH3 is trivalent (8 - 5 = 3).

Determination of the valence of elements in a binary compound by its formula.

Binary 2 is a compound formed by two elements.

1 There are several exceptions.
2 The term comes from the Latin word binarius - double; consisting of two parts.

This is interesting

The formulas of compounds formed by three or more elements are different.

It is necessary to find out the value of the valence of an element in a compound when the element has a variable valency. How do they do this exercise Let's show an example.

Let's find the value of the valency of Iodine in its combination with Oxygen, which has the formula I 2 O 5.

You know that oxygen is a divalent element. Let's write the value of its valency above the symbol of this element in the chemical formula of the compound:. There are 2 * 5 = 10 valence units per 5 oxygen atoms. They need to be "distributed" between two atoms of Iodine (10: 2 = 5). It follows from this that iodine in the compound is pentavalent.

The compound formula with the designation of the valency of the elements -

Determine the valence of elements in compounds with the formulas CO 2 and Cl 2 O 7.

Compilation of chemical formulas of compounds according to the valency of the elements.

Let's perform the task opposite to the previous one - we will make the chemical formula of the compound of Sulfur with Oxygen, in which Sulfur is hexavalent.

First, we write down the symbols of the elements that form the compound, and indicate the valence values ​​above them:. Then we find the smallest number that is divisible without a remainder by both valency values. This number is 6. We divide it by the valence value of each element and get the corresponding indices in the chemical formula of the compound: .

To check the chemical formula, the rule is used: the products of the valence values ​​of each element by the number of its atoms in the formula are the same. These products are for the chemical formula just derived: 6 -1 = 2-3.

Remember that in the formulas of compounds, including binary ones, the symbols of metallic elements are first written down, and then non-metallic ones. If the compound is formed only by non-metallic elements and among them is Oxygen or Fluor, then these elements are recorded last.

This is interesting

The order of writing the elements in the formula for the combination of Oxygen with Fluor is: OF 2.

Write the chemical formulas of Boron compounds with Fluor and Oxygen.

The reasons for connecting atoms to each other and the explanation of the valency values ​​of elements are related to the structure of atoms. This material will be covered in 8th grade.

conclusions

Valence is the ability of an atom to combine with a certain number of the same or other atoms.

There are elements with constant and variable valency. Hydrogen and Fluor are always monovalent, Oxygen is bivalent.

The valence values ​​of the elements are reflected in the graphic formulas of the molecules by the corresponding number of dashes near the atoms.

The products of the valence values ​​of each element by the number of its atoms in the formula of a binary compound are the same.

?
75. What is valency? What are the maximum and minimum values ​​of the valency of chemical elements.

76. Indicate the symbols of elements with constant valency: K, Ca, Cu, Cl, Zn, F, H.

77. Determine the valency of all elements in compounds that have the following formulas:

78. Determine the valence of elements in compounds with the following formulas:
a) BaH 2 , V 2 O 5 , MoS 3 , SiF 4 , Li 3 P; b) CuS, TiCI 4 , Ca 3 N 2 , P 2 O 3 , Mn 2 O 7 .

79. Write formulas for compounds formed by elements with constant valency: Na...H..., Ba...F..., Al...О..., AI...F....

80. Make formulas of compounds using the indicated valencies of some elements:

81. Write the formulas of compounds with Oxygen of the following elements: a) Lithium; b) magnesium; c) Osmium (shows valency 4 and 8).

82. Draw the graphic formulas of the molecules CI 2 O, PH 3, SO 3.

83. Determine the valency of the elements according to the graphic formulas of molecules:

At leisure

"Constructing" Molecules

Rice. 45. Model of the methane molecule CH 4

According to graphic formulas, it is possible to make models of molecules (Fig. 45). The most convenient material for this is plasticine. Ballatoms are made from it (plasticine of different colors is used for atoms of various elements). The balls are connected with matches; each match replaces one dash in the graphic formula of the molecule.

Make models of molecules H 2 , O 2 , H 2 O (has an angular shape), NH3 (has a pyramid shape), CO 2 (has a linear shape).

Valence is the ability of atoms to attach a certain number of other atoms to themselves.

One atom of another monovalent element combines with one atom of a monovalent element(HCl) . Two monovalent atoms combine with an atom of a divalent element(H2O) or one divalent atom(CaO) . This means that the valence of an element can be represented as a number that shows how many atoms of a monovalent element an atom of a given element can combine with. The valence of an element is the number of bonds that an atom forms:

Na – monovalent (one bond)

H – monovalent (one bond)

O - divalent (two bonds per atom)

S - hexavalent (forms six bonds with neighboring atoms)

Rules for determining valence
elements in connections

1. Valency hydrogen take for I(unit). Then, in accordance with the formula of water H 2 O, two hydrogen atoms are attached to one oxygen atom.

2. Oxygen in its compounds always exhibits valence II. Therefore, carbon in the CO 2 compound (carbon dioxide) has a valence of IV.

3. Highest valency is equal to group number .

4. lower valency equals the difference between the number 8 (the number of groups in the table) and the number of the group in which this element is located, i.e. 8 - N groups .

5. For metals in the "A" subgroups, the valence is equal to the group number.

6. In non-metals, two valencies are mainly manifested: higher and lower.

For example: sulfur has a higher valence VI and a lower one (8 - 6) equal to II; phosphorus exhibits valencies V and III.

7. Valency can be constant or variable.

The valency of elements must be known in order to compose the chemical formulas of compounds.

Algorithm for formulating a compound of phosphorus oxide

Features of compiling chemical formulas of compounds.

1) The element that is in the Mendeleev table to the right and above shows the lowest valence, and the element located to the left and below shows the highest valence.

For example, in combination with oxygen, sulfur exhibits a higher valency VI, and oxygen a lower II. So the formula for sulfur oxide would be SO 3.

In the combination of silicon with carbon, the former exhibits a higher valency IV, and the latter a lower IV. So the formula – SiC. It is silicon carbide, the basis of refractory and abrasive materials.

2) The metal atom is in the first place in the formula.

2) In the formulas of compounds, the non-metal atom, which exhibits the lowest valency, always comes in second place, and the name of such a compound ends in "id".

For example,CaO - calcium oxide, NaCl - sodium chloride, PbS - lead sulfide.

Now you yourself can write the formulas of any compounds of metals with non-metals.

Instruction

For example, two substances– HCl and H2O. It is well known to everyone and water. The first substance contains one hydrogen atom (H) and one chlorine atom (Cl). This suggests that in this compound they form one, that is, they hold one atom near them. Hence, valence and one and the other is equal to 1. It is just as easy to determine valence elements that make up the water molecule. It contains two hydrogens and one oxygen atom. Therefore, the oxygen atom formed two bonds to attach two hydrogens, and they, in turn, formed one bond each. Means, valence oxygen is 2, and hydrogen is 1.

But sometimes you have to face substances mi more complex in terms of the properties of their constituent atoms. There are two types of elements: with a constant (, hydrogen, etc.) and non-permanent valence Yu. For atoms of the second type, this number depends on the compound in which they are included. An example is (S). It can have valences of 2, 4, 6, and sometimes even 8. Determining the ability of elements such as sulfur to hold other atoms around is a little more difficult. To do this, you need to know other components substances.

Remember the rule: the product of the number of atoms by valence of one element in the compound must match the same product for another element. This can be verified by again referring to the water molecule (H2O):
2 (amount of hydrogen) * 1 (its valence) = 2
1 (amount of oxygen) * 2 (its valence) = 2
2 = 2 means everything is defined correctly.

Now test this algorithm on a more complex substance, for example, N2O5 - oxide. It was previously stated that oxygen has a constant valence 2, so you can compose:
2 (valence oxygen) * 5 (its amount) \u003d X (unknown valence nitrogen) * 2 (its amount)
By simple arithmetic calculations, it can be determined that valence nitrogen in this compound is 5.

Valence- this is the ability of chemical elements to hold a certain number of atoms of other elements. At the same time, this is the number of bonds formed by a given atom with other atoms. Determining valency is quite simple.

Instruction

Please note that the valence of atoms of some elements is constant, while others are variable, that is, it tends to change. For example, hydrogen in all compounds is monovalent, since it forms only one. Oxygen is able to form two bonds, while being divalent. But y can be II, IV or VI. It all depends on the element with which it connects. Thus, sulfur is an element with variable valency.

Note that in molecules of hydrogen compounds, it is very easy to calculate the valency. Hydrogen is always monovalent, and this indicator for the element associated with it will be equal to the number of hydrogen atoms in this molecule. For example, in CaH2, calcium will be divalent.

Remember the main rule for determining valency: the product of the valence index of an atom of an element and the number of its atoms in any molecule, the product of the valence index of an atom of the second element and the number of its atoms in a given molecule.

Look at the letter formula denoting this equality: V1 x K1 \u003d V2 x K2, where V is the valence of the atoms of the elements, and K is the number of atoms in the molecule. With its help, it is easy to determine the valence index of any element if the rest of the data is known.

Consider the example of the sulfur oxide molecule SO2. Oxygen in all compounds is divalent, therefore, substituting the values ​​in the proportion: Voxygen x Oxygen \u003d Vsulfur x Kser, we get: 2 x 2 \u003d Vsulfur x 2. From here, Vsulfur \u003d 4/2 \u003d 2. Thus, the valence of sulfur in this molecule is 2.

Related videos

Valence- one of the main terms used in the theory of chemical structure. This concept defines the ability of an atom to form chemical bonds and quantitatively represents the number of bonds in which it participates.

Instruction

Valence(from Latin valentia - “strength”) - an indicator of the ability of an atom to attach other atoms to itself, forming chemical bonds with them inside the molecule. The total number of bonds in which an atom can participate is equal to the number of its unpaired electrons. Such bonds are called covalent.

Unpaired electrons are free electrons in the outer shell of an atom that pair with the outer electrons of another atom. Moreover, each such pair is called an electron pair, and such electrons are called valence. Based on this, valencies can sound like this: this is the number of electron pairs along which a given atom is connected to other atoms.

The maximum valence index of chemical elements of one group of the periodic system, as a rule, is equal to the serial number of the group. Different atoms of the same element can have different valencies. The polarity of the resulting is not taken into account, so the valence has no sign. It cannot be zero or negative.

The quantity of any chemical element is considered to be the number of univalent hydrogen atoms or divalent oxygen atoms. However, when determining valence, other elements can be used, the valency of which is precisely known.

Sometimes the concept of valence is identified with the concept of "oxidation state", but this is not true, although in some cases these indicators coincide. Oxidation state is a formal term that means the possible charge that an atom would receive if its electrons in electrons were transferred to more electronegative atoms. In this case, the oxidation state is expressed in units of charge and may have a sign, in contrast to valence. This term has become widespread in the inorganic, since in inorganic compounds one judges valency. Valence It is also used in organic chemistry, since most organic compounds have a molecular structure.

Related videos

This is the ability of an atom to interact with other atoms, forming chemical bonds with them. Many scientists, first of all, the German Kekule and our compatriot Butlerov, made a great contribution to the creation of the theory of valency. Electrons, which take part in the formation of a chemical bond, are called valence.

You will need

• Mendeleev table.

Instruction

Remember the atom. It is our solar system: in the center there is a massive core (“star”), and electrons (“”) revolve around it. The size of the nucleus, although almost the entire mass of the atom is concentrated in it, is negligible compared to the distance to the electron orbits. Which of the electrons of an atom will most easily enter into interactions with the electrons of other atoms? It is not difficult to understand that those that are farthest from the nucleus are on the outer electron shell.

One of the important topics in the study of school topics is the course on valency. This will be discussed in the article.

Valency - what is it?

Valency in chemistry means the property of the atoms of a chemical element to bind to itself the atoms of another element. 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 valency of this element will be equal to two.

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

How to determine the valence of chemical elements according to the table of Dmitry Mendeleev

Looking at the periodic table of elements, you can see the vertical rows. They are called groups of elements. Valency also depends on the group. The elements of the first group have the first valence. The second is the second. Third - third. And so on.

There are also elements with a constant valency index. For example, hydrogen, halogen group, silver and so on. They must be learned.

How to determine the valence of chemical elements by formulas?

Sometimes it is difficult to determine the valence from the periodic table. Then you need to look at the specific chemical formula. Take the oxide FeO. Here, iron, like oxygen, has a valency index of two. But in Fe2O3 oxide it is different. Iron will be trivalent.

It is always necessary to remember different ways of determining valency and not to 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 represent them in a 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.