Theory of oxidation state. Qualitative characteristics of redox reactions

DEFINITION

The ability of an atom to form chemical bonds is called valence. A quantitative measure of valency is considered to be the number of different atoms in a molecule with which a given element forms bonds.

According to the exchange mechanism of the method of valence bonds, the valency of chemical elements is determined by the number of unpaired electrons contained in the atom. For s- and p-elements, these are the electrons of the outer level, for d-elements, these are the outer and pre-outer levels.

The values \u200b\u200bof the highest and lowest valencies of a chemical element can be determined using the Periodic Table of D.I. Mendeleev. The highest valence of an element coincides with the number of the group in which it is located, and the lowest is the difference between the number 8 and the group number. For example, bromine is located in the VIIA group, which means that its highest valency is VII, and the lowest is I.

Paired (located two in atomic orbitals) electrons, when excited, can be separated in the presence of free cells of the same level (electron separation into any level is impossible). Consider the example of elements of groups I and II. For example, the valency of the elements of the main subgroup of group I is equal to one, since at the external level the atoms of these elements have one electron:

3 Li 1s 2 2s 1

The valency of the elements of the main subgroup of group II in the ground (unexcited) state is zero, since there are no unpaired electrons at the external energy level:

4 Be 1s 2 2 s 2

When these atoms are excited, the paired s-electrons separate into free cells of the p-sublevel of the same level and the valence becomes equal to two (II):

Oxidation state

To characterize the state of elements in compounds, the concept of the degree of oxidation has been introduced.

DEFINITION

The number of electrons displaced from an atom of a given element or to an atom of a given element in a compound is called oxidation state.

A positive oxidation state indicates the number of electrons that are displaced from a given atom, and a negative oxidation state indicates the number of electrons that are displaced towards a given atom.

From this definition it follows that in compounds with non-polar bonds, the oxidation state of the elements is zero. Molecules consisting of identical atoms (N 2 , H 2 , Cl 2) can serve as examples of such compounds.

The oxidation state of metals in the elementary state is zero, since the distribution of electron density in them is uniform.

In simple ionic compounds, the oxidation state of their constituent elements is equal to the electric charge, since during the formation of these compounds, an almost complete transfer of electrons from one atom to another occurs: Na +1 I -1, Mg +2 Cl -1 2, Al +3 F - 1 3 , Zr +4 Br -1 4 .

When determining the degree of oxidation of elements in compounds with polar covalent bonds, the values of their electronegativity are compared. Since, during the formation of a chemical bond, electrons are displaced to atoms of more electronegative elements, the latter have a negative oxidation state in compounds.

The concept of the oxidation state for most compounds is conditional, since it does not reflect the real charge of the atom. However, this concept is very widely used in chemistry.

Most elements can exhibit different oxidation states in compounds. When determining their oxidation state, they use the rule according to which the sum of the oxidation states of elements in electrically neutral molecules is zero, and in complex ions, the charge of these ions. As an example, we calculate the degree of nitrogen oxidation in compounds of the composition KNO 2 and HNO 3 . The oxidation state of hydrogen and alkali metals in compounds is (+), and the oxidation state of oxygen is (-2). Accordingly, the oxidation state of nitrogen is:

KNO 2 1+ x + 2 × (-2) = 0, x=+3.

HNO 3 1+x+ x + 3 × (-2) = 0, x=+5.

Examples of problem solving

EXAMPLE 1

Exercise

Valency IV is typical for: a) Ca; b) P; c) O; d) Si?

Decision

In order to give a correct answer to the question posed, we will consider each of the proposed options separately.

a) Calcium is a metal. It is characterized by the only possible valency value that matches the group number in the Periodic Table of D.I. Mendeleev, in which it is located, i.e. the valency of calcium is II. The answer is incorrect.

b) Phosphorus is a non-metal. Refers to a group of chemical elements with variable valency: the highest is determined by the group number in the Periodic Table of D.I. Mendeleev, in which it is located, i.e. is equal to V, and the lowest is the difference between the number 8 and the group number, i.e. is equal to III. The answer is incorrect.

c) Oxygen is a non-metal. It is characterized by the only possible valence value equal to II. The answer is incorrect.

d) Silicon is a non-metal. It is characterized by the only possible valency value that matches the group number in the Periodic Table of D.I. Mendeleev, in which it is located, i.e. the valency of silicon is IV. This is the correct answer.

Answer

Option (d)

EXAMPLE 2

Exercise

What valency does iron have in the compound that is formed when it interacts with hydrochloric acid: a) I; b) II; c) III; d) VIII?

Decision

We write the equation for the interaction of iron with hydrochloric acid:

Fe + HCl \u003d FeCl 2 + H 2.

As a result of the interaction, iron chloride is formed and hydrogen is released. To determine the valence of iron by the chemical formula, we first count the number of chlorine atoms:

Calculate the total number of chlorine valency units:

We determine the number of iron atoms: it is equal to 1. Then the valence of iron in its chloride will be equal to:

Answer

The valency of iron in the compound formed during its interaction with hydrochloric acid is II.

08. Electronegativity, oxidation state, oxidation and reduction

Let's discuss the meaning of extremely interesting concepts that exist in chemistry, and, as is often the case in science, quite confusing and used upside down. We will talk about "electronegativity", "oxidation states" and "redox reactions".

What does it mean - the concept is used upside down?

We will try to gradually talk about it.

Electronegativity shows us the redox properties of a chemical element. That is, its ability to take or give away free photons. And also whether this element is a source or absorber of energy (ether). Yang or Yin.

Oxidation state is a concept analogous to the concept of "electronegativity". It also characterizes the redox properties of the element. But there is a difference between them.

Electronegativity characterizes a single element. By itself, without being in the composition of any chemical compound. While the degree of oxidation characterizes its redox abilities precisely when the element is part of a molecule.

Let's talk a little about what is the ability to oxidize, and what is the ability to reduce.

Oxidation is the process of transferring free photons (electrons) to another element. Oxidation is not at all the removal of electrons, as it is now considered in science. . When an element oxidizes another element, it acts like an acid or oxygen (hence the name "oxidation"). To oxidize means to contribute to the destruction, decay, combustion of elements. . The ability to oxidize is the ability to cause the destruction of molecules by the energy transferred to them (free photons). Remember that energy always destroys matter.

It's amazing how long there are contradictions in logic in science that no one notices.

Here, for example: "Now we know that an oxidizing agent is a substance that acquires electrons, and a reducing agent is a substance that gives them away" (Encyclopedia of a young chemist, article "Redox reactions)".

And then, two paragraphs below: “The strongest oxidizing agent is an electric current (a stream of negatively charged electrons)” (ibid.).

Those. the first quote says that an oxidizing agent is something that accepts electrons, and the second quote says that an oxidizing agent is something that gives it away.

And such erroneous, contradictory conclusions are forced to memorize in schools and institutes!

It is known that the best oxidizing agents are non-metals. Moreover, the smaller the period number and the larger the group number, the more pronounced the properties of the oxidizing agent. This is not surprising. We analyzed the reasons for this in the article devoted to the analysis of the periodic system, in the second part, where we talked about the color of nucleons. From group 1 to 8, the color of nucleons in the elements gradually changes from violet to red (if we also take into account the blue color of the d- and f-elements). The combination of yellow and red particles facilitates the return of accumulated free photons. Yellow ones accumulate, but hold weakly. And the reds contribute to the return. Giving off photons is the process of oxidation. But when some are red, there are no particles capable of accumulating photons. That is why the elements of group 8, the noble gases, are not oxidizing agents, unlike their neighbors, the halogens.

Recovery is a process opposite to oxidation. Now, in science, it is believed that when a chemical element receives electrons, it is restored. This point of view is quite understandable (but not accepted). When studying the structure of chemical elements, it was found that they emit electrons. They concluded that electrons are part of the elements. This means that the transfer of electrons to an element is, in a way, the restoration of its lost structure.

However, in reality, everything is not so.

Electrons are free photons. They are not nucleons. They are not part of the element's body. They are attracted, coming from outside, and accumulate on the surface of nucleons and between them. But their accumulation does not at all lead to the restoration of the structure of the element or molecule. On the contrary, these photons, emitted by their ether (energy), weaken and destroy bonds between elements. And this is an oxidation process, but not reduction.

Restoring a molecule is really taking energy (in this case, free photons) from it, not imparting it. Selecting photons, the reducing element compacts the substance - restores it.

The best reducing agents are metals. This property naturally follows from their qualitative and quantitative composition - their Fields of Attraction are the largest and there are necessarily many or enough blue particles on the surface.

One can even derive the following definition of metals.

Metal - This is a chemical element, in the composition of the surface layers of which there are necessarily blue particles.

BUT non-metal - this is an element in the composition of the surface layers of which there are no or almost no blue photons, and there are always red ones.

Metals are excellent at taking electrons with their strong attraction. And so they are restorers.

Let's give a definition of the concepts of "electronegativity", "oxidation state", "redox reactions", which can be found in textbooks on chemistry.

« Oxidation state is the conditional charge of an atom in a compound, calculated on the assumption that it consists only of ions. When defining this concept, it is conditionally assumed that the binding (valence) electrons pass to more electronegative atoms, and therefore the compounds consist, as it were, of positively and negatively charged ions. The oxidation state can have zero, negative, and positive values, which are usually placed above the element symbol at the top.

The zero value of the oxidation state is attributed to the atoms of the elements that are in the free state ... The negative value of the oxidation state are those atoms towards which the binding electron cloud (electron pair) is shifted. For fluorine in all its compounds, it is -1. Atoms that donate valence electrons to other atoms have a positive oxidation state. For example, for alkali and alkaline earth metals, it is respectively +1 and +2. In simple ions, it is equal to the charge of the ion. In most compounds, the oxidation state of hydrogen atoms is +1, but in metal hydrides (their compounds with hydrogen) and others, it is -1. For oxygen, the oxidation state is -2, but, for example, in combination with fluorine it will be +2, and in peroxide compounds -1. …

The algebraic sum of the oxidation states of atoms in a compound is zero, and in a complex ion it is the charge of the ion. …

The highest oxidation state is its highest positive value. For most elements, it is equal to the group number in the periodic system and is an important quantitative characteristic of the element in its compounds. The lowest value of the oxidation state of an element that occurs in its compounds is commonly called the lowest oxidation state; all the rest are intermediate” (Encyclopedic Dictionary of a Young Chemist, article “Oxidation State”).

Here is the basic information about this concept. It is closely related to another term - "electronegativity".

« Electronegativity is the ability of an atom in a molecule to attract electrons involved in the formation of a chemical bond to itself ”(Encyclopedic Dictionary of a Young Chemist, article“ Electronegativity ”).

“Redox reactions are accompanied by a change in the oxidation state of the atoms that make up the reactants, as a result of the movement of electrons from an atom of one of the reagents (reductant) to an atom of another. In redox reactions, oxidation (electron donation) and reduction (electron gain) occur simultaneously” (Chemical Encyclopedic Dictionary, edited by I.L. Knunyants, article “Redox Reactions”).

In our opinion, there are many errors hidden in these three concepts.

First of all , we believe that the formation of a chemical bond between two elements is not at all the process of sharing their electrons. A chemical bond is a gravitational bond. The electrons allegedly flying around the nucleus are free photons that accumulate on the surface of nucleons in the composition of the body of the element and between them. In order for two elements to bond, their free photons do not need to travel between the elements. This is not happening. In fact, a heavier element removes (attracts) free photons from a lighter one, and leaves them on itself (more precisely, on itself). And the zone of the lighter element, from which these photons were taken, is more or less exposed. Because of what, the attraction in this zone is manifested to a greater extent. And the lighter element is attracted to the heavier one. This is how a chemical bond occurs.

Secondly , modern chemistry sees the ability of elements to attract electrons to themselves distorted - inverted. It is believed that the greater the electronegativity of an element, the more it is able to attract electrons to itself. And fluorine and oxygen allegedly do this best of all - they attract foreign electrons to themselves. As well as other elements of the 6th and 7th groups.

In fact, this opinion is nothing more than a delusion. It is based on the misconception that the larger the group number, the heavier the elements. And also, the greater the positive charge of the nucleus. This is bullshit. Scientists still do not even bother to explain what, from their point of view, is a “charge”. Simply, as in numerology, they counted all the elements in order, and put down the amount of charge in accordance with the number. Great hike!

It is clear even to a child that gas is lighter than dense metal. How is it that in chemistry it is believed that gases are better at attracting electrons to themselves?

Dense metals, of course, they are better at attracting electrons.

Chemical scientists, of course, can keep the concept of "electronegativity" in use, since it is so commonly used. However, they will have to change its meaning to the exact opposite.

Electronegativity is the ability of a chemical element in a molecule to attract electrons to itself. And, of course, this ability is better expressed in metals than in non-metals.

As for the electric poles in the molecule, then, indeed, negative pole - these are non-metal elements that donate electrons, with smaller Attractive Fields. BUT positive - these are always elements with more pronounced metallic properties, with large Fields of Attraction.

Let's smile together.

Electronegativity - this is another, another attempt to describe the quality of a chemical element, along with the already existing mass and charge. As often happens, scientists from another field of science, in this case, chemistry, seem to distrust their fellow physicists, but rather simply because any person, making discoveries, goes his own way, and not just exploring the experience of others.

So it happened this time.

Mass and charge did not help chemists in any way to understand what happens in atoms when they interact with each other - and electronegativity was introduced - the ability of an element to attract electrons involved in the formation of a chemical bond. It should be recognized that the idea of this concept is very true. With the only amendment that it reflects reality upside down. As we have already said, metals, and not non-metals, attract electrons best of all - due to the color features of surface nucleons. Metals are the best reducing agents. Nonmetals are oxidizers. Metals are taken, non-metals are given away. Metals are Yin, non-metals are Yang.

Esotericism comes to the aid of science in matters of comprehending the secrets of Nature.

Concerning oxidation states , then this is a good attempt to understand how the distribution of free electrons occurs within a chemical compound - a molecule.

If a chemical compound is homogeneous - that is, it is simple, its structure consists of elements of the same type - then everything is correct, indeed the oxidation state of any element in the compound is zero. Since there are no oxidizing agents and no reducing agents in this compound. And all elements are equal in quality. No one takes electrons, no one gives. Whether it is a dense substance, or a liquid, or a gas, it does not matter.

The oxidation state, like electronegativity, shows the quality of a chemical element - only within the framework of a chemical element. The oxidation state is designed to compare the quality of the chemical elements in the compound. In our opinion, the idea is good, but its implementation is not entirely satisfactory.

We are categorically against the whole theory and concept of the structure of chemical elements and the bonds between them. Well, at least because the number of groups, according to our ideas, should be more than 8. This means that the whole system is collapsing. And not only that. In general, counting the number of electrons in atoms “on the fingers” is somehow not serious.

In accordance with the current concept, it turns out that the smallest conditional charges are assigned to the strongest oxidizing agents - fluorine has a charge of -1 in all compounds, oxygen almost everywhere -2. And for very active metals - alkali and alkaline earth - these charges are +1 and +2, respectively. After all, this is completely illogical. Although, again, we understand very well the general scheme in accordance with which this was done - all for the sake of 8 groups in the table and 8 electrons in the outer energy level.

Already, at a minimum, the value of these charges for halogens and oxygen should have been the largest with a minus sign. And for alkali and alkaline earth metals, it is also large, only with a plus sign.

In any chemical compound there are elements that donate electrons - oxidizing agents, non-metals, a negative charge, and elements that take away electrons - reducing agents, metals, a positive charge. It is in this way that elements are compared, correlated with each other, and they try to determine their oxidation state.

However, to find out the degree of oxidation in this way, in our opinion, does not accurately reflect reality. It would be more correct to compare the electronegativity of elements in a molecule. After all, electronegativity is almost the same as the degree of oxidation (it characterizes the quality of only a single element).

You can take the scale of electronegativity and put down its values in the formula for each element. And then it will be immediately clear which elements donate electrons and which ones take away. The element with the highest electronegativity in the compound, the negative pole, donates electrons. And the one whose electronegativity is the smallest - the positive pole - takes electrons.

If there are, say, 3 or 4 elements in a molecule, nothing changes. We also set the electronegativity values and compare.

Although you should not forget to draw a model of the structure of the molecule. Indeed, in any compound, if it is not simple, that is, it does not consist of one type of elements, metals and non-metals are connected with each other, first of all. Metals take electrons from non-metals and bond with them. And from one non-metal element, 2 or more elements with more pronounced metallic properties can simultaneously take away electrons. So there is a complex, complex molecule. But this does not mean that in such a molecule the metal elements will enter into a strong bond with each other. Perhaps they will be located on opposite sides of each other. If nearby, they will be attracted. But a strong bond is formed only if one element is more metallic than the other. It is imperative that one element selects electrons - removes them. Otherwise, there will be no exposure of the element - liberation from free photons on the surface. The Field of Attraction will not fully manifest, and there will be no strong connection. This is a complex topic - the formation of chemical bonds, and we will not go into detail about this in this article.

We believe that we have covered in sufficient detail the topic devoted to the analysis of the concepts of "electronegativity", "oxidation state", "oxidation" and "reduction", and provided you with a lot of interesting information.

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Part 1. Task A5.

Checked items: Electronegativity. Oxidation state and

valency of chemical elements.

Electronegativity-value characterizing the ability of an atom to polarize covalent bonds. If in a diatomic molecule A - B, the bonding electrons are attracted to atom B more strongly than to atom A, then atom B is considered to be more electronegative than A.

The electronegativity of an atom is the ability of an atom in a molecule (compound) to attract electrons that bind it to other atoms.

The concept of electronegativity (EO) was introduced by L. Pauling (USA, 1932). The quantitative characteristic of the electronegativity of an atom is very conditional and cannot be expressed in units of any physical quantities; therefore, several scales have been proposed for the quantitative determination of EO. The scale of relative EC has received the greatest recognition and distribution:

Electronegativity values of elements according to Pauling

Electronegativity χ (Greek chi) - the ability of an atom to hold external (valence) electrons. It is determined by the degree of attraction of these electrons to a positively charged nucleus.

This property manifests itself in chemical bonds as a shift of bond electrons towards a more electronegative atom.

The electronegativity of the atoms involved in the formation of a chemical bond is one of the main factors that determines not only the TYPE, but also the PROPERTIES of this bond, and thus affects the nature of the interaction between atoms during a chemical reaction.

In the scale of relative electronegativity of elements by L. Pauling (compiled on the basis of the bond energies of diatomic molecules), metals and organogen elements are arranged in the following row:

The electronegativity of elements obeys the periodic law: it grows from left to right in periods and from bottom to top in the main subgroups of D.I. Mendeleev.

Electronegativity is not an absolute constant of an element. It depends on the effective charge of the atomic nucleus, which can change under the influence of neighboring atoms or groups of atoms, the type of atomic orbitals and the nature of their hybridization.

Oxidation state is the conditional charge of the atoms of a chemical element in a compound, calculated from the assumption that the compounds consist only of ions.

The oxidation states can have a positive, negative or zero value, and the sign is placed before the number: -1, -2, +3, in contrast to the charge of the ion, where the sign is placed after the number.

In molecules, the algebraic sum of the oxidation states of elements, taking into account the number of their atoms, is 0.

The oxidation states of metals in compounds are always positive, the highest oxidation state corresponds to the group number of the periodic system where this element is located (excluding some elements: gold Au + 3 (group I), Cu + 2 (II), from group VIII, the oxidation state +8 can be only in osmium Os and ruthenium Ru.

The degrees of non-metals can be both positive and negative, depending on which atom it is connected to: if with a metal atom, then it is always negative, if with a non-metal, then it can be both + and - (you will learn about this when studying a number of electronegativity) . The highest negative oxidation state of non-metals can be found by subtracting from 8 the number of the group in which this element is located, the highest positive is equal to the number of electrons in the outer layer (the number of electrons corresponds to the group number).

The oxidation states of simple substances are 0, regardless of whether it is a metal or a non-metal.

Table showing the constant degrees for the most commonly used elements:

The degree of oxidation (oxidation number, formal charge) is an auxiliary conditional value for recording the processes of oxidation, reduction and redox reactions, the numerical value of the electric charge attributed to an atom in a molecule on the assumption that the electron pairs that carry out the bond are completely shifted towards more electronegative atoms.

Ideas about the degree of oxidation form the basis for the classification and nomenclature of inorganic compounds.

The degree of oxidation is a purely conditional value that has no physical meaning, but characterizes the formation of a chemical bond of interatomic interaction in a molecule.

Valency of chemical elements -(from Latin valens - having power) - the ability of atoms of chemical elements to form a certain number of chemical bonds with atoms of other elements. In compounds formed with the help of ionic bonds, the valence of atoms is determined by the number of attached or donated electrons. In compounds with covalent bonds, the valency of atoms is determined by the number of socialized electron pairs formed.

Permanent valence:

Remember:

The oxidation state is the conditional charge of the atoms of a chemical element in a compound, calculated from the assumption that all bonds are ionic in nature.

1. An element in a simple substance has a zero oxidation state. (Cu, H2)

2. The sum of the oxidation states of all atoms in a substance molecule is zero.

3. All metals have a positive oxidation state.

4. Boron and silicon in compounds have positive oxidation states.

5. Hydrogen has an oxidation state (+1) in compounds. Excluding hydrides

(hydrogen compounds with metals of the main subgroup of the first and second groups, oxidation state -1, for example Na + H -)

6. Oxygen has an oxidation state (-2), except for the combination of oxygen with fluorine OF2, the oxidation state of oxygen (+2), the oxidation state of fluorine (-1) . And in peroxides H 2 O 2 - the degree of oxidation of oxygen (-1);

7. Fluorine has an oxidation state (-1).

Electronegativity is the property of HeMe atoms to attract shared electron pairs. Electronegativity has the same dependence as that of non-metallic properties: it increases in the period (from left to right), and weakens in the group (top).

The most electronegative element is Fluorine, followed by Oxygen, Nitrogen…etc….

The algorithm for completing the task in the demo version:

Exercise:

The chlorine atom is located in group 7, so it can have a maximum oxidation state of +7.

The chlorine atom exhibits this degree of oxidation in the HClO4 substance.

Let's check it: The two chemical elements hydrogen and oxygen have constant oxidation states and are equal to +1 and -2, respectively. The number of oxidation states for oxygen is (-2) 4=(-8), for hydrogen (+1) 1=(+1). The number of positive oxidation states is equal to the number of negative ones. Therefore (-8)+(+1)=(-7). This means that the number of positive degrees of the chromium atom is 7, we write down the oxidation states of the elements. The oxidation state of chlorine is +7 in the HClO4 compound.

Answer: Option 4. The oxidation state of chlorine is +7 in the HClO4 compound.

Different formulations of task A5:

3. The oxidation state of chlorine in Ca (ClO 2) 2

1) 0 2) -3 3) +3 4) +5

4. The element has the least electronegativity

5. The lowest oxidation state of manganese is in the compound

1) MnSO 4 2) MnO 2 3) K 2 MnO 4 4) Mn 2 O 3

6. Nitrogen exhibits an oxidation state of +3 in each of the two compounds

1) N 2 O 3 NH 3 2) NH 4 Cl N 2 O 3) HNO 2 N 2 H 4 4) NaNO 2 N 2 O 3

7. The valency of the element is

1) the number of σ bonds formed by it

2) the number of bonds formed by it

3) the number of covalent bonds formed by it

4) oxidation states with the opposite sign

8. Nitrogen shows its maximum oxidation state in the compound

1) NH 4 Cl 2) NO 2 3) NH 4 NO 3 4) NOF

Video lesson 2: The degree of oxidation of chemical elements

Video lesson 3: Valence. Definition of valence

Lecture: Electronegativity. The oxidation state and valency of chemical elements

Electronegativity

Electronegativity- this is the ability of atoms to attract the electrons of other atoms to themselves to connect with them.

It is easy to judge the electronegativity of a chemical element from the table. Remember, in one of our lessons it was said that it increases when moving from left to right through periods in the periodic table and moving from bottom to top in groups.

For example, given the task to determine which element from the proposed series is the most electronegative: C (carbon), N (nitrogen), O (oxygen), S (sulfur)? We look at the table and find that this is O, because it is to the right and above the rest.

What factors affect electronegativity? This is:

The radius of an atom, the smaller it is, the higher the electronegativity.
The filling of the valence shell with electrons, the more of them, the higher the electronegativity.

Of all the chemical elements, fluorine is the most electronegative, because it has a small atomic radius and 7 electrons in the valence shell.

Elements with low electronegativity include alkali and alkaline earth metals. They have large radii and very few electrons in the outer shell.

The values of the electronegativity of an atom cannot be constant, because it depends on many factors, including those listed above, as well as the degree of oxidation, which can be different for the same element. Therefore, it is customary to talk about the relativity of electronegativity values. You can use the following scales:

You will need electronegativity values when writing formulas for binary compounds consisting of two elements. For example, the formula for copper oxide is Cu 2 O - the first element should be the one whose electronegativity is lower.

At the moment of formation of a chemical bond, if the difference in electronegativity between the elements is greater than 2.0, a covalent polar bond is formed, if less, an ionic one.

Oxidation state

Oxidation state (CO)- this is the conditional or real charge of the atom in the compound: conditional - if the bond is covalent polar, real - if the bond is ionic.

An atom acquires a positive charge when it donates electrons, and a negative charge when it receives electrons.

The oxidation states are written above the signed symbols «+»/«-» . There are also intermediate COs. The maximum CO of the element is positive and equal to the group number, and the minimum negative for metals is zero, for non-metals = (group number - 8). Elements with a maximum CO only accept electrons, and with a minimum, they only give them away. Elements that have intermediate COs can both donate and accept electrons.

Consider some of the rules that should be followed to determine the CO:

CO of all simple substances is equal to zero.

The sum of all CO atoms in the molecule is also equal to zero, since any molecule is electrically neutral.

In compounds with a covalent non-polar bond, CO is zero (O 2 0), and with an ionic bond it is equal to the charges of the ions (Na + Cl - CO sodium +1, chlorine -1). CO elements of compounds with a covalent polar bond are considered as with an ionic bond (H:Cl \u003d H + Cl -, hence H +1 Cl -1).

The elements in a compound that have the highest electronegativity have negative oxidation states if the least are positive. Based on this, we can conclude that metals have only a “+” oxidation state.

Constant oxidation states:

Alkali metals +1.

All metals of the second group +2. Exception: Hg +1, +2.

Aluminum +3.

Hydrogen +1. Exception: active metal hydrides NaH, CaH 2, etc., where the oxidation state of hydrogen is –1.
Oxygen -2. Exception: F 2 -1 O +2 and peroxides that contain the –О–О– group, in which the oxidation state of oxygen is –1.

When an ionic bond is formed, there is a certain transition of an electron, from a less electronegative atom to an atom of greater electronegativity. Also, in this process, atoms always lose their electrical neutrality and subsequently turn into ions. Integer charges are formed in the same way. When a covalent polar bond is formed, the electron transfers only partially, so partial charges arise.

Valence

Valence- this is the ability of atoms to form n - the number of chemical bonds with atoms of other elements.

And valency is the ability of an atom to keep other atoms near it. As you know from the school chemistry course, different atoms are connected to each other by electrons of the outer energy level. An unpaired electron seeks a pair for itself from another atom. These outer level electrons are called valence electrons. This means that valence can also be defined as the number of electron pairs that bind atoms to each other. Look at the structural formula of water: H - O - N. Each dash is an electron pair, which means it shows valence, i.e. oxygen here has two dashes, which means it is divalent, one dash comes from hydrogen molecules, which means hydrogen is monovalent. When writing, valency is indicated by Roman numerals: O (II), H (I). It can also be placed above an element.

Valence is either constant or variable. For example, in alkali metals, it is constant and equals I. But chlorine in various compounds exhibits valences I, III, V, VII.

How to determine the valency of an element?

Let's go back to the Periodic Table. The metals of the main subgroups have a constant valence, so the metals of the first group have a valence of I, the second of II. And for metals of secondary subgroups, the valency is variable. It is also variable for non-metals. The highest valence of an atom is equal to the group number, the lowest is = group number - 8. A familiar wording. Does this mean that the valency coincides with the oxidation state. Remember, valence may coincide with the degree of oxidation, but these indicators are not identical to each other. Valency cannot have the =/- sign, and also cannot be zero.

The second way to determine the valence by the chemical formula, if the constant valency of one of the elements is known. For example, take the formula for copper oxide: CuO. Oxygen valency II. We see that there is one copper atom per oxygen atom in this formula, which means that the valency of copper is II. Now let's take a more complicated formula: Fe 2 O 3. The valency of the oxygen atom is II. There are three such atoms here, we multiply 2 * 3 \u003d 6. We found that there are 6 valences for two iron atoms. Let's find out the valency of one iron atom: 6:2=3. So the valency of iron is III.

In addition, when it is necessary to evaluate the "maximum valence", one should always proceed from the electronic configuration that exists in the "excited" state.

Among chemical reactions, including those in nature, redox reactions are the most common. These include, for example, photosynthesis, metabolism, biological processes, as well as fuel combustion, metal production, and many other reactions. Redox reactions have long been successfully used by mankind for various purposes, but the electronic theory of redox processes itself appeared quite recently - at the beginning of the 20th century.

In order to move on to the modern theory of redox, it is necessary to introduce several concepts - these are valency, oxidation state and structure of electron shells of atoms. Studying such sections as , elements and , we have already come across these concepts. Next, let's look at them in more detail.

Valency and oxidation state

Valence- a complex concept that arose along with the concept of a chemical bond and is defined as the property of atoms to attach or replace a certain number of atoms of another element, i.e. is the ability of atoms to form chemical bonds in compounds. Initially, the valency was determined by hydrogen (its valence was taken equal to 1) or oxygen (valence equal to 2). Later, they began to distinguish between positive and negative valency. Quantitatively, positive valence is characterized by the number of electrons donated by the atom, and negative valency is the number of electrons that must be attached to the atom to implement the octet rule (i.e., complete the external energy level). Later, the concept of valency began to combine the nature of the chemical bonds that arise between atoms in their combination.

As a rule, the highest valency of the elements corresponds to the group number in the periodic system. But, as with all rules, there are exceptions: for example, copper and gold are in the first group of the periodic system and their valence must be equal to the group number, i.e. 1, but in reality the highest valency of copper is 2, and gold - 3.

Oxidation state sometimes called the oxidation number, electrochemical valence or oxidation state and is a conditional concept. Thus, when calculating the degree of oxidation, it is assumed that only ions make up the molecule, although most compounds are not ionic at all. Quantitatively, the oxidation state of the atoms of an element in a compound is determined by the number of electrons attached to the atom or displaced from the atom. Thus, in the absence of displacement of electrons, the oxidation state will be zero, with a displacement of electrons towards a given atom, it will be negative, and with a displacement from a given atom, it will be positive.

Defining oxidation state of atoms you must follow the following rules:

In the molecules of simple substances and metals, the oxidation state of atoms is 0.
Hydrogen in almost all compounds has an oxidation state equal to +1 (and only in hydrides of active metals equal to -1).
For oxygen atoms in its compounds, the oxidation state is -2 (exceptions: OF 2 and metal peroxides, the oxidation state of oxygen is +2 and -1, respectively).
The atoms of alkali (+1) and alkaline earth (+2) metals, as well as fluorine (-1) also have a constant oxidation state
In simple ionic compounds, the oxidation state is equal in magnitude and sign to its electrical charge.
For a covalent compound, the more electronegative atom has an oxidation state with the "-" sign, and the less electronegative one has the "+" sign.
For complex compounds indicate the degree of oxidation of the central atom.
The sum of the oxidation states of atoms in a molecule is zero.

For example, let's determine the oxidation state of Se in the compound H 2 SeO 3

So, the oxidation state of hydrogen is +1, oxygen -2, and the sum of all oxidation states is 0, we will make an expression, taking into account the number of atoms in the H 2 + Se x O 3 -2 compound:

(+1)2+x+(-2)3=0, whence

those. H 2 + Se +4 O 3 -2

Knowing what value the oxidation state of an element in a compound has, it is possible to predict its chemical properties and reactivity with respect to other compounds, as well as whether this compound is reducing agent or oxidizing agent. These concepts are fully developed in redox theories:

Oxidation- is the process of loss of electrons by an atom, ion or molecule, which leads to an increase in the degree of oxidation.

Al 0 -3e - = Al +3;

2O -2 -4e - \u003d O 2;

2Cl - -2e - \u003d Cl 2

Recovery - is the process by which an atom, ion, or molecule acquires electrons, resulting in a decrease in oxidation state.

Ca +2 +2e - = Ca 0;

2H + +2e - \u003d H 2

Oxidizers- compounds that accept electrons during a chemical reaction, and reducing agents are electron-donating compounds. Reducing agents are oxidized during the reaction, and oxidizing agents are reduced.
The essence of redox reactions- the movement of electrons (or the displacement of electron pairs) from one substance to another, accompanied by a change in the oxidation states of atoms or ions. In such reactions, one element cannot be oxidized without reducing the other, because. the transfer of electrons always causes both oxidation and reduction. Thus, the total number of electrons taken from one element during oxidation coincides with the number of electrons received by another element during reduction.

So, if the elements in the compounds are in their highest oxidation states, then they will only exhibit oxidizing properties, due to the fact that they can no longer donate electrons. On the contrary, if the elements in the compounds are in their lowest oxidation states, then they exhibit only reducing properties, because they can no longer add electrons. Atoms of elements in an intermediate oxidation state, depending on the reaction conditions, can be both oxidizing and reducing agents. Let us give an example: sulfur in its highest oxidation state +6 in the H 2 SO 4 compound can only exhibit oxidizing properties, in the H 2 S compound - sulfur is in its lowest oxidation state -2 and will only exhibit reducing properties, and in the H 2 SO 3 being in an intermediate oxidation state of +4, sulfur can be both an oxidizing agent and a reducing agent.

Based on the values of the oxidation states of the elements, it is possible to predict the probability of a reaction between substances. It is clear that if both elements in their compounds are in higher or lower oxidation states, then the reaction between them is impossible. A reaction is possible if one of the compounds can exhibit oxidizing properties, while the other can exhibit reducing properties. For example, in HI and H 2 S, both iodine and sulfur are in their lowest oxidation states (-1 and -2) and can only be reducing agents, therefore, they will not react with each other. But they will perfectly interact with H 2 SO 4, which is characterized by reducing properties, tk. sulfur here is in its highest oxidation state.

The most important reducing agents and oxidizing agents are presented in the following table.

Restorers
Neutral atoms	General scheme M-n →Mn+ All metals, as well as hydrogen and carbon. The most powerful reducing agents are alkali and alkaline earth metals, as well as lanthanides and actinides. Weak reducing agents - noble metals - Au, Ag, Pt, Ir, Os, Pd, Ru, Rh. In the main subgroups of the periodic system, the reducing ability of neutral atoms increases with increasing serial number.
negatively charged non-metal ions	General scheme E +ne - → En- Negatively charged ions are strong reducing agents due to the fact that they can donate both their excess electrons and their outer electrons. Restorative capacity, with the same charge, increases with increasing radius of the atom. For example, I is a stronger reducing agent than Br - and Cl -. S 2-, Se 2-, Te 2- and others can also be reducing agents.
positively charged metal ions of the lowest oxidation state	Metal ions of the lowest oxidation state can exhibit reducing properties if they are characterized by states with a higher oxidation state. For example, Sn 2+ -2e - → Sn 4+ Cr 2+ -e - → Cr 3+ Cu + -e - → Cu 2+
Complex ions and molecules containing atoms in an intermediate oxidation state	Complex or complex ions, as well as molecules, can exhibit reducing properties if the atoms that make up them are in an intermediate oxidation state. For example, SO 3 2-, NO 2 -, AsO 3 3-, 4-, SO 2, CO, NO and others.
	Carbon, Carbon monoxide (II), Iron, Zinc, Aluminum, Tin, Sulphurous acid, Sodium sulfite and bisulfite, Sodium sulfide, Sodium thiosulfate, Hydrogen, Electric current
Oxidizers
Neutral atoms	General scheme E + ne- → E n- The oxidizing agents are p-element atoms. Typical non-metals are fluorine, oxygen, chlorine. The strongest oxidizing agents are halogens and oxygen. In the main subgroups of groups 7, 6, 5 and 4, from top to bottom, the oxidative activity of atoms decreases
positively charged metal ions	All positively charged metal ions exhibit oxidizing properties to varying degrees. Of these, the strongest oxidizing agents are ions in a high degree of oxidation, for example, Sn 4+, Fe 3+, Cu 2+. Noble metal ions, even in a low oxidation state, are strong oxidizing agents.
Complex ions and molecules containing metal atoms in the highest oxidation state	Typical oxidizing agents are substances that contain metal atoms in the state of the highest oxidation state. For example, KMnO4, K2Cr2O7, K2CrO4, HAuCl4.
Complex ions and molecules containing non-metal atoms in a state of positive oxidation state	These are mainly oxygen-containing acids, as well as their corresponding oxides and salts. For example, SO 3 , H 2 SO 4 , HClO, HClO 3 , NaOBr and others. In a row H2SO4 →H2SeO4 →H6Teo 6 oxidizing activity increases from sulfuric to telluric acid. In a row HClO-HClO 2 -HClO 3 -HClO 4 HBrO - HBrO 3 - HIO - HIO 3 - HIO 4 , H5IO 6 oxidative activity increases from right to left, while acidity increases from left to right.
The most important reducing agents in engineering and laboratory practice	Oxygen, Ozone, Potassium permanganate, Chromic and Dichromic acids, Nitric acid, Nitrous acid, Sulfuric acid (conc), Hydrogen peroxide, Electric current, Perchloric acid, Manganese dioxide, Lead dioxide, Chlorine, Potassium and sodium hypochlorite solutions, Potassium hypobromide , Potassium hexacyanoferrate (III).

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