How to determine valency in binary compounds. Valence

There are elements whose valence is always constant, and there are very few of them. But all other elements exhibit variable valence.

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One atom of another monovalent element is combined 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 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 shaft 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 valency
elements in connections

1. Shaft 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 valency II. Therefore, the carbon in the compound CO 2 (carbon dioxide) has a valence of IV.

3. Supreme shaft equal to group number .

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

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

Figuratively speaking, a shaft is the number of “arms” with which an atom clings to other atoms. Naturally, atoms do not have any “hands”; their role is played by the so-called. valence electrons.

You can say it differently: 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 (of which there are relatively few) and elements with variable valence (of which the majority are).

Elements with constant valence must be remembered.

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 valence 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 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 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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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 valence 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 valency 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 valence 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 valency. In addition to those already indicated, 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 valence. Moreover, its maximum value in most cases coincides with the number of the group in which the element is located in the 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 valency using the three-element particle formula.

Not all chemical substances consist of diatomic molecules. How to determine the valence 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.

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

One atom of another monovalent element is combined 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 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 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 valency
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 valency 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 valence 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 valence 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

Sequencing	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 compiling chemical formulas of compounds.

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.

Different chemical elements differ in their ability to form chemical bonds, that is, to combine with other atoms. Therefore, in complex substances they can only be present in certain proportions. Let's figure out how to determine valency using 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 quantity is always only positive and is denoted 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 a given element can combine with. For oxygen, this value is always equal to II.

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

This concept originated in chemistry in the 19th century. Frankland started a theory explaining the combination of atoms in various proportions, but his ideas about the “binding force” were not very widespread. The decisive role in the development of the theory belonged to Kekula. He called the property of forming a certain number of bonds basicity. Kekulé believed that this was a fundamental and unchanging property of every type of atom. Butlerov made important additions to the theory. With the development of this theory, it became possible to visually depict molecules. This was very helpful in studying the structure of various substances.

How can the periodic table help?

You can find valency 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.

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

How to determine the valence of chemical elements if it changes? It can coincide with the group number or be equal to the group number minus eight, and also take 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.

The way valence 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 of sulfur, two unpaired electrons are located in 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 move to the free 3d sublevel, and there will be 4 unpaired electrons.

Sulfur will become tetravalent. If you give it even more energy, then another 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 may change. It depends on which compound the element is included 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 is called the lowest. The highest and lowest valencies according to the periodic table can be established as follows: the highest coincides with the group number, and the lowest is equal to 8 minus the group number.

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

Metals of the main subgroups have a constant ability to form chemical bonds.
For metals of secondary subgroups - variable.
For non-metals it is also variable. In most cases, it takes two meanings - higher and lower, but sometimes there can be a greater number of options. Examples are sulfur, chlorine, bromine, iodine, chromium and others.

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

Often the ability to form chemical bonds takes on more than two meanings. 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 valence of an element in the formula of a compound? If it is known for other components of the substance, this is not difficult. For example, you need to calculate this property for chlorine in NaCl. Sodium is an element of the main subgroup of the first group, so it is monovalent. Consequently, 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, we can only establish that ClO4 is a monovalent residue.

How else can you find out this value?

The ability to form a certain number of connections does not always coincide with the group number, and in some cases it will simply have to be learned. Here the table of valency of chemical elements will come to the rescue, which shows the values of this value. The 8th grade chemistry textbook provides values for the ability to combine with other atoms of the most common types of atoms.

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 currently hardly use the concept of valency according to the periodic table. Instead, the concept of oxidation degree is used for the ability of a substance to form a certain number of relationships, for substances with a structure - covalence, and for substances with an ionic structure - the charge of the ion.

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

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

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Let's sum it up

Using the periodic table, it is not possible to determine the ability to form chemical bonds for all elements. For those that exhibit 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.