Valence of chemical elements, determination of valency in binary. What is the valence of an atom

Valency of elements Until now, you have used the chemical formulas of substances given in the textbook, or those that your teacher told you. How to correctly compose chemical formulas? Chemical formulas of substances are compiled based on knowledge of the qualitative and quantitative composition of the substance. There are a huge number of substances; naturally, it is impossible to remember all the formulas. This is not necessary! It is important to know a certain pattern according to which atoms are able to combine with each other to form new chemical compounds. This ability is called valence. Valence– the property of atoms of elements to attach a certain number of atoms of other elements. Consider models of molecules of some substances, such as water, methane and carbon dioxide. It can be seen that in a water molecule, an oxygen atom attaches two hydrogen atoms. Therefore, its valency is two. In a methane molecule, a carbon atom attaches four hydrogen atoms, its valency in this substance is four. The valence of hydrogen in both cases is equal to one. Carbon exhibits the same valence in carbon dioxide, but unlike methane, the carbon atom attaches two oxygen atoms, since the valency of oxygen is two. There are elements whose valence does not change in compounds. Such elements are said to have constant valence. If the valency of an element can be different, these are elements with variable valence. Valence of some chemical elements is given in Table 2. Valency is usually denoted by Roman numerals. Table 2. Valency of some chemical elements

Element symbol	Valence	Element symbol	Valence
H, Li, Na, K, F, Ag	I	C, Si, Sn, Pb	II, IV
Be, Mg, Ca, Ba, Zn, O	II	N	I, II, III, IV
Al, B	III	P, As, Sb	III, V
S	II, IV, VI	Cl	I, II, III, IV, V, VII
Br,I	I, III, V	Ti	II, III, IV

It is worth noting that the highest valency of an element numerically coincides with the ordinal number of the group of the Periodic System in which it is located. For example, carbon is in group IV, its highest valency is IV. There are three exceptions:

• nitrogen– is in group V, but its highest valence is IV;
• oxygen– is in group VI, but its highest valency is II;
• fluorine– is in group VII, but its highest valence is I.

Based on the fact that all elements are located in eight groups of the Periodic System, valence can take values from I to VIII.

Drawing up formulas of substances using valence

To compile formulas of substances using valence, we will use a certain algorithm:

Determination of valency using the formula of a substance

To determine the valence of elements using the formula of a substance, it is necessary reverse order actions. Let's also consider it using the algorithm:

When studying this section, we considered complex substances that contain only two types of atoms of chemical elements. Formulas more complex substances are compiled differently.

Binary compounds – compounds that contain two types of atoms of elements

To determine the order of the sequence of compounds of atoms, structural (graphical) formulas of substances are used. In such formulas, the valencies of elements are indicated by valence strokes (dashes). For example, a water molecule can be represented as

N─O─N

The graphical formula depicts only the order of connection of atoms, but not the structure of molecules. In space, such molecules may look different. Thus, a water molecule has the angular structural formula:

• Valence– the ability of atoms of elements to attach a certain number of atoms of other chemical elements
• There are elements with constant and variable valence
• The highest valency of a chemical element coincides with its group number in the Periodic Table of Chemical Elements D.I. Mendeleev. Exceptions: nitrogen, oxygen, fluorine
• Binary compounds– compounds that contain two types of atoms of chemical elements
• Graphic formulas reflect the order of bonds of atoms in a molecule using valence strokes
• The structural formula reflects the actual shape of the molecule in space

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There are several definitions of the concept of “valency”. Most often, this term refers to the ability of atoms of one element to attach a certain number of atoms of other elements. Often those who are just starting to study chemistry have a question: How to determine the valency of an element? This is easy to do if you know a few rules.

Valences constant and variable

Let's consider the compounds HF, H2S and CaH2. In each of these examples, one hydrogen atom attaches to itself only one atom of another chemical element, which means its valence is equal to one. The valency value is written above the symbol of the chemical element in Roman numerals.

In the example given, the fluorine atom is bonded to only one monovalent H atom, which means its valence is also 1. The sulfur atom in H2S already attaches two H atoms to itself, so it is divalent in this compound. Calcium in its hydride CaH2 is also bound to two hydrogen atoms, which means its valency is two.

Oxygen in the vast majority of its compounds is divalent, that is, it forms two chemical bonds with other atoms.

In the first case, the sulfur atom attaches two oxygen atoms to itself, that is, it forms 4 chemical bonds in total (one oxygen forms two bonds, which means sulfur - two times 2), that is, its valency is 4.

In the SO3 compound, sulfur already attaches three O atoms, therefore its valence is 6 (three times it forms two bonds with each oxygen atom). The calcium atom attaches only one oxygen atom, forming two bonds with it, which means its valence is the same as that of O, that is, equal to 2.

Note that the H atom is monovalent in any compound. The valence of oxygen is always (except for the hydronium ion H3O(+)) equal to 2. Calcium forms two chemical bonds with both hydrogen and oxygen. These are elements with constant valence. In addition to those already mentioned, the following have constant valence:

• Li, Na, K, F - monovalent;
• Be, Mg, Ca, Zn, Cd - have a valence of II;
• B, Al and Ga are trivalent.

The sulfur atom, in contrast to the cases considered, in combination with hydrogen has a valence of II, and with oxygen it can be tetra- or hexavalent. Atoms of such elements are said to have variable valency. Moreover, its maximum value in most cases coincides with the number of the group in which the element is located in Periodic table(rule 1).

There are many exceptions to this rule. Thus, element 1 of group copper exhibits valences of both I and II. Iron, cobalt, nickel, nitrogen, fluorine, on the contrary, have a maximum valency less than the group number. So, for Fe, Co, Ni these are II and III, for N - IV, and for fluorine - I.

The minimum valency value always corresponds to the difference between the number 8 and the group number (rule 2).

It is possible to unambiguously determine what the valence of elements for which it is variable is only by the formula of a certain substance.

Determination of valency in a binary compound

Let's consider how to determine the valency of an element in a binary (of two elements) compound. There are two options here: in a compound, the valency of the atoms of one element is known exactly, or both particles have a variable valence.

Case one:

Case two:

Determination of valence using the three-element particle formula.

Not all chemicals consist of diatomic molecules. How to determine the valency of an element in a three-element particle? Let's consider this question using the example of the formulas of two compounds K2Cr2O7.

If, instead of potassium, the formula contains iron, or another element with variable valence, we will need to know what the valence of the acid residue is. For example, you need to calculate the valences of the atoms of all elements in combination with the formula FeSO4.

It should be noted that the term “valence” is more often used in organic chemistry. When compiling formulas for inorganic compounds, the concept of “oxidation state” is often used.

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

One atom of another monovalent element combines with one atom of a monovalent element(HCl) . An atom of a divalent element combines with two atoms of a monovalent element.(H2O) or one divalent atom(CaO) . This means that the valency 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 valency of an element is the number of bonds that an atom forms:

Na – monovalent (one bond)

H – monovalent (one bond)

O – divalent (two bonds for each atom)

S – hexavalent (forms six bonds with neighboring atoms)

Rules for determining valence
elements in connections

1. Valence hydrogen mistaken 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, the carbon in the compound CO 2 (carbon dioxide) has a valence of IV.

3. Higher valence equal to group number .

4. Lowest valency is equal to 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 located in “A” subgroups, the valence is equal to the group number.

6. Nonmetals generally exhibit two valences: higher and lower.

For example: sulfur has the highest valency VI and the lowest (8 – 6) equal to II; phosphorus exhibits valences V and III.

7. Valence can be constant or variable.

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

Algorithm for composing the formula of a phosphorus oxide compound

Sequence of actions	Formulating phosphorus oxide
1. Write the symbols of the elements	R O
2. Determine the valencies of elements	V II P O
3. Find the least common multiple of the numerical values ​​of valences	5 2 = 10
4. Find the relationships between atoms of elements by dividing the found smallest multiple by the corresponding valencies of the elements	10: 5 = 2, 10: 2 = 5; P:O=2:5
5. Write indices for element symbols	R 2 O 5
6. Formula of the compound (oxide)	R 2 O 5

Remember!

Features of compilation chemical formulas connections.

1) The lowest valence is shown by the element that is located to the right and above in D.I. Mendeleev’s table, and the highest valence is shown by the element located to the left and below.

For example, in combination with oxygen, sulfur exhibits the highest valency VI, and oxygen the lowest valency II. Thus, the formula for sulfur oxide will be SO 3.

In the compound of silicon with carbon, the first exhibits the highest valency IV, and the second - the lowest IV. So the formula – SiC. This is silicon carbide, the basis of refractory and abrasive materials.

2) The metal atom comes first in the formula.

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

For example,Sao – calcium oxide, NaCl – sodium chloride, PbS – lead sulfide.

Now you can write the formulas for any compounds of metals and non-metals.

Concept valence comes from the Latin word “valentia” and was known back in the mid-19th century. The first “extensive” mention of valency was in the works of J. Dalton, who argued that all substances consist of atoms connected to each other in certain proportions. Then, Frankland introduced the very concept of valency, which found further development in the works of Kekule, who spoke about the relationship between valence and chemical bond, A.M. Butlerov, who in his theory of the structure of organic compounds linked valency with the reactivity of a particular chemical compound and D.I. Mendeleev (in the Periodic Table of Chemical Elements, the highest valence of an element is determined by the group number).

DEFINITION

Valence– this is the quantity covalent bonds, which an atom is capable of forming when combined with a covalent bond.

The valence of an element is determined by the number of unpaired electrons in an atom, since they take part in the formation of chemical bonds between atoms in the molecules of compounds.

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

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

6 C 1s 2 2s 2 2p 2

The orbitals are filled as follows: first, one electron with the same spins, and then a second electron with opposite spins. Since the 2p sublevel has three orbitals with the same energy, each of the two electrons occupied one orbital. One orbital remained free.

Determination of the valence of an element using electronic graphic formulas

The valence of an element can be determined using electron graphic formulas electronic configurations electrons in an atom. Let's consider two atoms - nitrogen and phosphorus.

7 N 1s 2 2s 2 2p 3

Because The valence of an element is determined by the number of unpaired electrons, therefore, the valence of nitrogen is III. Since the nitrogen atom has no empty orbitals, an excited state is not possible for this element. However, III is not the maximum valence of nitrogen, the maximum valency of nitrogen is V and is determined by the group number. Therefore, it should be remembered that using electronic 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, in the phosphorus atom there are free d-orbitals, therefore electrons located on the 2s sublevel are able to pair up 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 valence of V.

Elements having multiple valence values

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

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

Let us consider the electronic graphic formulas of the elements and establish why the elements have different valences (Fig. 1).

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

Valence is the ability of atoms of some chemical elements to attach to themselves the exact number of atoms of other elements or atomic groups. Thanks to this concept, we can find out how many atoms of each element are included in the molecule, and also compose it graphic formula. Therefore, to successfully write formulas, reaction equations, as well as to correctly solve problems, it is important to know well how to determine the valence of an element.

Chemical elements can have constant or variable valence. It is necessary to memorize all elements with constant valency. Here is their list:

1. Hydrogen, halogens and alkali metals are always monovalent.
2. Oxygen and alkaline earth metals always exhibit a valence of two.
3. B and Al are always trivalent.

How to determine valency using the periodic table

If for some reason, for example, because you got excited during an exam, you forgot this list, you can determine the valency using the periodic table. To do this, we need to find out in which group the chemical element we are interested in is in, i.e. find out the group number, and also determine whether it is in the main or secondary group. The highest valency is always equal to the group number.

To determine the lowest variable valency, which nonmetals most often have, it is necessary to subtract the group number from 8. The result obtained will be the desired value.

To make it more clear how to determine valency using the periodic table, here are a few examples:

1. All alkali metals are included in the main subgroup of the first group and have a constant valency – I.
2. For alkaline earth metals ( main subgroup of the second group), the valency will accordingly be equal to II.
3. Most nonmetals have variable valency. Highest degree their valence is equal to the group number, and the lowest is determined, as already written above. Let's take sulfur for example. Because this element is located in group 6 - its highest valence is VI, and its lowest is II (8 – 6 = 2).
4. Unlike all other non-metals, the halogens included in the main subgroup of group eight have a constant valency equal to I.
5. For the remaining elements included in the side groups, the valency will have to be remembered. Most often, these elements are represented by metals having a valency from I to III.