Methods for determining valence. Elements with constant valence. Valency and acids

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In chemistry lessons, you have already become acquainted with the concept of valence of chemical elements. We have collected all useful information on this issue in one place. Use it when you prepare for the State Exam and the Unified State Exam.

Valency and chemical analysis Valence

– the ability of atoms of chemical elements to enter into chemical compounds with atoms of other elements. In other words, it is the ability of an atom to form a certain number of chemical bonds with other atoms.

From Latin the word “valency” is translated as “strength, ability.” A very correct name, right?

The concept of “valence” is one of the basic ones in chemistry. It was introduced even before scientists knew the structure of the atom (back in 1853). Therefore, as we studied the structure of the atom, it underwent some changes.

Thus, from the point of view of electronic theory, valence is directly related to the number of outer electrons of an element’s atom. This means that by “valence” we mean the number of electron pairs with which an atom is connected to other atoms.

Knowing this, scientists were able to describe the nature of the chemical bond. It lies in the fact that a pair of atoms of a substance shares a pair of valence electrons.

You may ask, how were chemists of the 19th century able to describe valence even when they believed that there were no particles smaller than an atom? This is not to say that it was so simple - they relied on chemical analysis.

Through chemical analysis, scientists of the past determined the composition of a chemical compound: how many atoms of various elements are contained in the molecule of the substance in question. To do this, it was necessary to determine what the exact mass of each element in a sample of pure (without impurities) substance was.

This method for determining valency is only suitable for simple substances. But in acids, in this way we can only determine the valency of compounds such as acidic residues, but not of all elements (except for the known valency of hydrogen) individually.

As you have already noticed, valence is indicated by Roman numerals.

Valency and acids

Since the valence of hydrogen remains unchanged and is well known to you, you can easily determine the valence of the acid residue. So, for example, in H 2 SO 3 the valency of SO 3 is I, in HСlO 3 the valency of СlO 3 is I.

In a similar way, if the valence of the acid residue is known, it is easy to write down the correct formula of the acid: NO 2 (I) - HNO 2, S 4 O 6 (II) - H 2 S 4 O 6.

Valency and formulas

The concept of valence makes sense only for substances of a molecular nature and is not very suitable for describing chemical bonds in compounds of a cluster, ionic, crystalline nature, etc.

Indices in the molecular formulas of substances reflect the number of atoms of the elements that make up them. Knowing the valence of elements helps to correctly place the indices. In the same way, by looking at the molecular formula and indices, you can tell the valences of the constituent elements.

You do tasks like this in chemistry lessons at school. For example, having the chemical formula of a substance in which the valency of one of the elements is known, you can easily determine the valence of another element.

To do this, you just need to remember that in a substance of a molecular nature, the number of valences of both elements is equal. Therefore, use the least common multiple (corresponding to the number of free valencies required for the compound) to determine the valence of an element that is unknown to you.

To make it clear, let's take the formula of iron oxide Fe 2 O 3. Here, two iron atoms with valency III and 3 oxygen atoms with valency II participate in the formation of a chemical bond. Their least common multiple is 6.

• Example: you have the formulas Mn 2 O 7. You know the valence of oxygen, it is easy to calculate that the least common multiple is 14, hence the valence of Mn is VII.

In a similar way, you can do the opposite: write down the correct chemical formula of a substance, knowing the valences of its elements.

• Example: to correctly write the formula of phosphorus oxide, we take into account the valency of oxygen (II) and phosphorus (V). This means that the least common multiple for P and O is 10. Therefore, the formula has the following form: P 2 O 5.

Knowing well the properties of elements that they exhibit in various compounds, it is possible to determine their valence even by the appearance of such compounds.

For example: copper oxides are red (Cu 2 O) and black (CuO) in color. Copper hydroxides are colored yellow (CuOH) and blue (Cu(OH) 2).

To make the covalent bonds in substances more visual and understandable for you, write their structural formulas. The lines between the elements represent the bonds (valency) that arise between their atoms:

Valency characteristics

Today, the determination of the valency of elements is based on knowledge of the structure of the outer electronic shells of their atoms.

Valency can be:

• constant (metals of the main subgroups);
• variable (non-metals and metals of secondary groups):
• higher valence;
• lower valence.

The following remains constant in various chemical compounds:

• valence of hydrogen, sodium, potassium, fluorine (I);
• valence of oxygen, magnesium, calcium, zinc (II);
• valence of aluminum (III).

But the valency of iron and copper, bromine and chlorine, as well as many other elements changes when they form various chemical compounds.

Valence and electron theory

Within the framework of electronic theory, the valence of an atom is determined based on the number of unpaired electrons that participate in the formation of electron pairs with electrons of other atoms.

Only electrons located in the outer shell of an atom participate in the formation of chemical bonds. Therefore, the maximum valence of a chemical element is the number of electrons in the outer electron shell of its atom.

The concept of valency is closely related to the Periodic Law, discovered by D. I. Mendeleev. If you look carefully at the periodic table, you can easily notice: the position of an element in the periodic system and its valency are inextricably linked. The highest valence of elements that belong to the same group corresponds to the ordinal number of the group in the periodic table.

You will find out the lowest valence when you subtract the group number of the element that interests you from the number of groups in the periodic table (there are eight of them).

For example, the valency of many metals coincides with the numbers of the groups in the table of periodic elements to which they belong.

Table of valency of chemical elements

Those valences that the elements possessing them rarely exhibit are given in parentheses.

Valency and oxidation state

Thus, speaking about the degree of oxidation, it is meant that an atom in a substance of ionic (which is important) nature has a certain conventional charge. And if valence is a neutral characteristic, then the oxidation state can be negative, positive or equal to zero.

It is interesting that for an atom of the same element, depending on the elements with which it forms a chemical compound, the valence and oxidation state can be the same (H 2 O, CH 4, etc.) or different (H 2 O 2, HNO 3 ).

Conclusion

By deepening your knowledge of the structure of atoms, you will learn more deeply and in more detail about valency. This description of chemical elements is not exhaustive. But it has great practical significance. As you yourself have seen more than once, solving problems and conducting chemical experiments in your lessons.

This article is designed to help you organize your knowledge about valence. And also remind you how it can be determined and where valence is used.

We hope you find this material useful in preparing your homework and self-preparing for tests and exams.

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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 valency:

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 valence 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:

MnO

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).

2x = 14,

x = 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.

Valency and chemical analysis– the ability of elements to attach other elements to themselves.

In simple terms, this is a number that shows how many elements a certain atom can attach to itself.

The key point in chemistry is to correctly write the formulas of compounds.

There are several rules that make it easier for us to correctly compose formulas.

1. The valence 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 valency of oxygen is two

3. The valency of hydrogen is one

4. Non-metals exhibit two types of valence:

• Lowest (8th group)
• Highest (equal to group number)

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

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

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

1) Determine the valences of elements in compounds:

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

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

2) Make up formulas for a compound consisting of:

sodium and oxygen

How to determine the valence of chemical elements? This question is faced by everyone who is just starting to get acquainted with chemistry. First, let's find out what it is. Valency can be considered as the property of atoms of one element to hold a certain number of atoms of another element.

Elements with constant and variable valency

For example, from the formula H-O-H it is clear that each H atom is connected to only one atom (in this case, oxygen). It follows that its valence is 1. The O atom in a water molecule is bonded to two monovalent H atoms, which means it is divalent. Valence values ​​are written in Roman numerals above the symbols of the elements:

The valencies of hydrogen and oxygen are constant. However, there are exceptions for oxygen. For example, in the hydronium ion H3O+, oxygen is trivalent. There are other elements with constant valence.

• Li, Na, K, F – monovalent;
• Be, Mg, Ca, Sr, Ba, Cd, Zn – have a valence of II;
• Al, B are trivalent.

Now let's determine the valency of sulfur in the compounds H2S, SO2 and SO3.

In the first case, one sulfur atom is bonded to two monovalent H atoms, which means its valency is two. In the second example, for one sulfur atom there are two oxygen atoms, which, as is known, is divalent. We obtain a valence of sulfur equal to IV. In the third case, one S atom attaches three O atoms, which means that the valence of sulfur is equal to VI (the valence of atoms of one element multiplied by their number).

As you can see, sulfur can be di-, tetra- and hexavalent:

Such elements are said to have variable valency.

Rules for determining valencies

1. The maximum valency for the atoms of a given element coincides with the number of the group in which it is located in the Periodic Table. For example, for Ca it is 2, for sulfur – 6, for chlorine – 7. There are also many exceptions to this rule:
   -element of group 6, O, has valency II (in H3O+ – III);
   - monovalent F (instead of 7);
   -usually di- and trivalent iron, an element of group VIII;
   -N can only hold 4 atoms near itself, and not 5, as follows from the group number;
   - mono- and divalent copper, located in group I.
2. The minimum value of valence for elements for which it is variable is determined by the formula: group number in PS - 8. Thus, the lowest valence of sulfur 8 - 6 = 2, fluorine and other halogens - (8 - 7) = 1, nitrogen and phosphorus - (8 – 5)= 3 and so on.
3. In a compound, the sum of the valence units of the atoms of one element must correspond to the total valency of the other.
4. In a water molecule H-O-H, the valence of H is equal to I, there are 2 such atoms, which means that hydrogen has 2 valence units in total (1×2=2). The valence of oxygen has the same meaning.
5. In a compound consisting of two types of atoms, the element located in second place has the lowest valence.
6. The valence of the acid residue coincides with the number of H atoms in the acid formula, the valence of the OH group is equal to I.
7. In a compound formed by atoms of three elements, the atom that is in the middle of the formula is called the central one. The O atoms are directly bonded to it, and the remaining atoms form bonds with oxygen.

We use these rules to complete tasks.

In order to determine the valence 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, BUT, hydrogen (H) will always be monovalent, and oxygen (O) will always be divalent. Here is a cheat sheet below that I think will help you)

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

Alkali metals, hydrogen, and halogens are considered monovalent;

But boron and aluminum are trivalent.

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

The lowest valence is determined by subtracting the group number from 8. Non-metals are endowed with a lower valence to a greater extent.

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

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

Valency can be determined using the periodic table. The highest valence of an element is always equal to the number of the group in which it is found.

Nonmetals most often have the lowest 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 valency is VI, the lowest valency will be II (86 = 2).

According to the school definition, valence is the ability of a chemical element to form a certain number of chemical bonds with other atoms.

As is known, valence can be 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 by D.I. Mendeleev will help us determine valency.

The following rules apply:

1) Maximum The valence of a chemical element is equal to the group number. For example, chlorine is in the 7th group, which means it has a maximum valency of 7. Sulfur: it is in the 6th group, which means it has a maximum valency of 6.

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

Alas, there are exceptions to both rules.

For example, copper is in group 1, but the maximum valency of copper is not 1, but 2.

Oxygen is in group 6, 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, the 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, the main subgroup) always have valence 2. For example, the valence of magnesium is always 2 because it is an alkaline earth metal.

5) Aluminum always has a valency 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 definitions of valency may differ in different sources.

More or less accurately, valency can be defined as the number of shared electron pairs through 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 there would be 10 electrons circling the nitrogen atom. But this cannot happen, because the maximum number 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 valence, which changes depending on which element it (this element) is in combination with or enters into.

Valencies of some chemical elements:

Let's now move on to how the valency of an element is determined from the table.

So, valence can be determined by periodic table:

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





From the school chemistry course we know that all chemical elements can have a constant or variable valence. Elements that have a constant valency just need to be remembered (for example, hydrogen, oxygen, alkali metals and other elements). Valency can be easily determined 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 valence of any element can be determined from 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.

The story with non-metals is a little different: their highest valence (in compounds with oxygen) is also equal to the group number, but the lowest valency (in compounds with hydrogen and metals) must be determined using the following formula: 8 - group number.

The more you work with chemical elements, the better you remember their valency. To get started, this cheat sheet will suffice:



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

Valency is the ability of atoms of some chemical elements to attach to themselves atoms of other elements. To successfully write formulas and correctly solve problems, 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 (divalent); 3. B and Al (trivalent). To determine valency using the periodic table, you need to find out which group the chemical element is in and determine whether it is in the main group or a secondary group.

An element can have one or more valencies.

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

Valence is indicated by a Roman numeral and is typically 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.

Determining valency using the periodic table is simple. As a rule, it corresponds to the number of the group in which the element is located. But there are elements that can have different valencies in different compounds. In this case we are talking about constant and variable valence. 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 valence of elements if one element in a compound has a constant valence, for example, oxygen or hydrogen. On the left is a reducing agent, that is, an element with a positive valence, on the right is an oxidizing agent, that is, an element with a negative valence. The index of an element with a constant valence is multiplied by that valence and divided by the index of an element with an unknown valence.

Example: silicon oxides. The valence of oxygen is -2. Let's find the valency of silicon.

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

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