One chemical element attaches or replaces a certain number of atoms of another.

The unit of valence is taken to be the valency of a hydrogen atom equal to 1, that is, hydrogen is monovalent. Therefore, the valency of an element indicates how many hydrogen atoms one atom of the element in question is connected to. For example, HCl, where chlorine is monovalent; H2O, where oxygen is divalent; NH 3, where nitrogen is trivalent.

Table of elements with constant valency.

Formulas of substances can be compiled according to the valences of their constituent elements. And vice versa, knowing the valencies of elements, you can compose a chemical formula from them.

Algorithm for compiling formulas of substances by valency.

1. Write down the symbols of the elements.

2. Determine the valency of the elements included in the formula.

3. Find the least common multiple of the numerical values ​​of valency.

4. Find the relationships between the atoms of the elements by dividing the found least common multiple by the corresponding valences of the elements.

5. Write down the indices of the elements in the chemical formula.

Example: Let's make up the chemical formula of phosphorus oxide.

1. Write down the symbols:

2. Let's determine valencies:

4. Let’s find the relationships between atoms:

5. Write down the indices:

Algorithm for determining valency using the formulas of chemical elements.

1. Write down the formula of a chemical compound.

2. Designate the known valence of elements.

3. Find the least common multiple of valence and index.

4. Find the ratio of the least common multiple to the number of atoms of the second element. This is the desired valency.

5. Check by multiplying the valence and index of each element. Their products must be equal.

Example: Let's determine the valence of hydrogen sulfide elements.

1. Let's write the formula:

H 2 S

2. Let us denote the known valency:

H 2 S

3. Find the least common multiple:

H 2 S

4. Find the ratio of the least common multiple to the number of sulfur atoms:

H 2 S

5. Let's do a check.

Valency is the ability of an atom of a given element to 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 do not have any “hands”; their role is played by the so-called. valence electrons.

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

Elements with constant valency must be remembered:

The remaining elements may exhibit different valencies.

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

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

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

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

Valence and oxidation state are not identical concepts.

These concepts are quite close, but they should not be confused! The oxidation state has a sign (+ or -), the valence does not; the oxidation state of an element in a substance can be zero, the valency 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 valency of nitrogen in N 2 is III, and the oxidation state = 0. The valence of carbon in formic acid is = IV, and the oxidation state = +2.

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

This is done quite 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 a similar product for the second element.

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

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

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

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

Solution. Oxygen has a constant valency (II); the molecule of this oxide contains five oxygen atoms and two chlorine atoms. Let the valence of chlorine = X. Let's create the equation: 5 2 = 2 X. Obviously, X = 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 valence of sulfur, it would have been impossible to solve it. Both S and Cl are elements with variable valency. Taking into account additional information, the solution is constructed according to the scheme of examples 1 and 2. Answer: Cl(I).

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

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

Example 4. Write a 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 = 1 y. As one of the solutions to this equation, we can take x = 1, y = 2. Answer: CaH 2.

“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 ​​of x and y. In the example given, these minimum (natural!) values ​​are exactly 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 formula CH 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 follow the rule: take the smallest index values.

Example 5. Write a formula for the compound of sulfur and fluorine if it is known that the valence of sulfur is six.

Solution. Let the formula of the compound be S x F y . The valence of sulfur is given (VI), the valency of fluorine is constant (I). We formulate the equation again: 6 x = 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 suggest you go through a short test on the topic "Valence".

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.

True, this method is not without flaws. Because the valence of an element can be determined in this way only in its simple combination with always monovalent hydrogen (hydride) or always divalent oxygen (oxide). For example, the valency of nitrogen in NH 3 is III, since one hydrogen atom is bonded to three nitrogen atoms. And the valency of carbon in methane (CH 4), according to the same principle, is IV.

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 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;
• lowest valence.

The following remains constant in various chemical compounds:

• valency of hydrogen, sodium, potassium, fluorine (I);
• valency 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 valency 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 valency 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

Serial number chem. element (atomic number)	Name	Chemical symbol	Valence
1	Hydrogen Helium Lithium Beryllium Carbon Nitrogen / Nitrogen Oxygen Fluorine Neon / Neon Sodium/Sodium Magnesium / Magnesium Aluminum Silicon Phosphorus / Phosphorus Sulfur/Sulfur Chlorine Argon / Argon Potassium/Potassium Calcium Scandium / Scandium Titanium Vanadium Chrome / Chromium Manganese / Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic/Arsenic Selenium Bromine Krypton / Krypton Rubidium / Rubidium Strontium / Strontium Yttrium / Yttrium Zirconium / Zirconium Niobium / Niobium Molybdenum Technetium / Technetium Ruthenium / Ruthenium Rhodium Palladium Silver Cadmium Indium Tin/Tin Antimony / Antimony Tellurium / Tellurium Iodine / Iodine Xenon / Xenon Cesium Barium / Barium Lanthanum / Lanthanum Cerium Praseodymium / Praseodymium Neodymium / Neodymium Promethium / Promethium Samarium / Samarium Europium Gadolinium / Gadolinium Terbium / Terbium Dysprosium / Dysprosium Holmium Erbium Thulium Ytterbium / Ytterbium Lutetium / Lutetium Hafnium / Hafnium Tantalum / Tantalum Tungsten/Tungsten Rhenium / Rhenium Osmium / Osmium Iridium / Iridium Platinum Gold Mercury Thalium / Thallium Lead/Lead Bismuth Polonium Astatine Radon / Radon Francium Radium / Radium Actinium Thorium Proactinium / Protactinium Uranium / Uranium	H	I (I), II, III, IV, V I, (II), III, (IV), V, VII II, (III), IV, VI, VII II, III, (IV), VI (I), II, (III), (IV) I, (III), (IV), V (II), (III), IV (II), III, (IV), V (II), III, (IV), (V), VI (II), III, IV, (VI), (VII), VIII (II), (III), IV, (VI) I, (III), (IV), V, VII (II), (III), (IV), (V), VI (I), II, (III), IV, (V), VI, VII (II), III, IV, VI, VIII (I), (II), III, IV, VI (I), II, (III), IV, VI (II), III, (IV), (V) No data No data (II), III, IV, (V), VI

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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Instructions

The table is a structure in which chemical elements are arranged according to their principles and laws. That is, we can say that it is a multi-storey “house” in which chemical elements “live”, and each of them has its own apartment under a certain number. “Floors” are located horizontally, which can be small or large. If a period consists of two rows (as indicated by numbering on the side), then such a period is called large. If it has only one row, it is called small.

The table is also divided into “entrances” - groups, of which there are eight in total. Just as in any entrance, apartments are located on the left and right, so here the chemical elements are arranged in the same way. Only in this variant their placement is uneven - on one side there are more elements and then they talk about the main group, on the other there are fewer and this indicates that the group is secondary.

Valence is the ability of elements to form chemical bonds. There is a constant, which does not change, and a variable, which has a different value depending on what substance the element is part of. When determining valency using the periodic table, you need to pay attention to the following characteristics: the group number of the elements and its type (that is, the main or secondary group). The constant valence in this case is determined by the group number of the main subgroup. To find out the value of the valence variable (if there is one, and usually y), then you need to subtract the number of the group in which the element is located from 8 (8 groups in total - hence the number).

Example No. 1. If you look at the elements of the first group of the main subgroup (alkali metals), we can conclude that they all have a valency equal to I (Li, Na, K, Rb, Cs, Fr).

Example No. 2. The elements of the second group of the main subgroup (alkaline earth metals) respectively have valency II (Be, Mg, Ca, Sr, Ba, Ra).

Example No. 3. If we talk about non-metals, then for example, P (phosphorus) is in group V of the main subgroup. Hence its valency will be equal to V. In addition, phosphorus has one more valence value, and to determine it, you must perform step 8 - element number. This means 8 – 5 (phosphorus group number) = 3. Therefore, the second valency of phosphorus is III.

Example No. 4. Halogens are in group VII of the main subgroup. This means their valency will be VII. However, given that these are non-metals, you need to perform an arithmetic operation: 8 – 7 (element group number) = 1. Therefore, the other valency of the halogens is I.

For elements of secondary subgroups (and these include only metals), the valency must be remembered, especially since in most cases it is equal to I, II, less often III. You will also have to memorize the valencies of chemical elements that have more than two meanings.

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Please note

Be careful when identifying metals and non-metals. For this purpose, symbols are usually given in the table.

Sources:

• how to correctly pronounce the elements of the periodic table
• what is the valency of phosphorus? X

From school or even earlier, everyone knows that everything around, including ourselves, consists of atoms - the smallest and indivisible particles. Thanks to the ability of atoms to connect with each other, the diversity of our world is enormous. The ability of these atoms of a chemical element to form bonds with other atoms is called the valence of the element.

Instructions

The concept entered chemistry in the nineteenth century, when the valency of the hydrogen atom was taken as its unit. The valency of another element can be defined as the number of hydrogen that one atom of another substance attaches to itself. Similarly, the valence of hydrogen is determined by the valency of oxygen, which, as a rule, is equal to two and, therefore, allows you to determine the valency of other elements in compounds with simple arithmetic operations. The oxygen valence of an element is equal to twice the number of oxygen atoms that can attach to one atom of a given element.

To determine the valence of an element, you can also use the formula. It is known that there is a certain relationship between the valence of an element, its equivalent mass and the molar mass of its atoms. The relationship between these qualities is the formula: Valency = Molar mass of atoms / Equivalent mass. Since mass is the amount necessary to replace one mole of hydrogen or to react with one mole of hydrogen, the greater the molar mass in comparison with the equivalent mass, the greater the number of hydrogen atoms that can replace or attach to an atom of the element, and therefore the higher the valence.

The connection between chemical elements has a different nature. It can be a covalent bond, ionic, metallic. To form a bond, an atom must have: an electric charge, an unpaired valence electron, an unoccupied valence orbital, or a lone pair of valence electrons. Together, these features determine the valence state and valence abilities of the atom.

Knowing the number of electrons of an atom, which is equal to the serial number of an element in the Periodic Table of Elements, guided by the principles of least energy, the Pauli principle and Hund’s rule, one can construct the electronic configuration of the atom. These constructions will allow us to analyze the valence capabilities of the atom. In all cases, the ability to form bonds is primarily realized due to the presence of unpaired valence electrons; additional valence abilities, such as a free orbital or a lone pair of valence electrons, may remain unrealized if there is not enough energy for this. And from all of the above, we can conclude that The easiest way is to determine the valency of an atom in any compound, and it is much more difficult to find out the valence abilities of atoms. However, practice will make this simple too.

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Tip 3: How to determine the valence of chemical elements

The valency of a chemical element is the ability of an atom to attach or replace a certain number of other atoms or atomic groups to form a chemical bond. It must be remembered that some atoms of the same chemical element may have different valencies in different compounds.

You will need

• periodic table

Instructions

Hydrogen is considered to be a monovalent and divalent element, respectively. A measure of valency is the number of hydrogen or oxygen atoms that an element adds to form a hydride or . Let X be the element whose valence is to be determined. Then XHn is this element, and XmOn is its oxide. Example: - NH3, here valence is 3. Sodium is monovalent in the Na2O compound.

To determine the valency of an element, you need to multiply the number of hydrogen or oxygen atoms in the compound by the valency of hydrogen and oxygen, respectively, and then divide by the number of atoms of the chemical element whose valence is found.

The valence of an element can also be determined from other atoms with known valence. In different compounds, atoms of the same element can exhibit different valences. For example, it is divalent in H2S and CuS compounds, tetravalent in SO2 and SF4 compounds, hexavalent in SO3 and SF6 compounds.

The maximum valency of an element is considered equal to the number of electrons in the outer electron shell of the atom. The maximum valency of elements of the same group of the periodic table usually corresponds to its atomic number. For example, the maximum valency of carbon atom C should be 4.

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

Instructions

Please note that the valency of the atoms of some elements is constant, while others are variable, that is, they tend to change. For example, hydrogen in all compounds is monovalent because it forms only one bond. Oxygen is capable of forming two bonds, while being divalent. But y may have II, IV or VI. It all depends on the element with which it is connected. Thus, sulfur is an element with variable valence.

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

Remember the main rule for determining valence: the product of the valence index of an atom of any element and the number of its atoms in any molecule is always equal to 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 for this equality: V1 x K1 = V2 x K2, where V is the valency 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 remaining data is known.

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

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Valency is the most important concept in chemistry. The physical meaning of this concept became clear thanks to the development of the doctrine of chemical bonds. The valency of an atom is determined by the number of covalent bonds by which it is connected to other atoms.

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 valency 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 valency.

• 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 valence 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 valence 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.