Valency and degree of oxidation of chemical elements. Electronegativity, oxidation state and valency of chemical elements

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

form a certain number with atoms of other elements.

The valency of fluorine atoms is always equal to I

Li, Na, K, F,H, Rb, Cs- monovalent;

Be, Mg, Ca, Sr, Ba, Cd, Zn,O, Ra- have a valency equal to II;

Al, BGa, In- trivalent.

The maximum valence for the atoms of a given element coincides with the number of the group in which it is located in the Periodic system. For example, for Sa it isII, for sulfur -VI, for chlorine -VII. Exceptions a lot of this rule too:

ElementVIgroup, O, has valence II (in H 3 O+ - III);
- monovalent F (instead ofVII);
- usually bi- and trivalent iron, an element of group VIII;
- N can hold only 4 atoms near itself, and not 5, as follows from the group number;
- one- and two-valent copper, located in group I.

The minimum valence value for elements in which it is variable is determined by the formula: group number in PS - 8. So, the lowest valency of sulfur 8 - 6 \u003d 2, fluorine and other halogens - (8 - 7) \u003d 1, nitrogen and phosphorus - (8 - 5)= 3 and so on.

In a compound, the sum of valency units of atoms of one element must correspond to the total valence of another (or the total number of valences of one chemical element is equal to the total number of valences of atoms of another chemical element). So, in a water molecule H-O-H, the valency of H is equal to I, there are 2 such atoms, which means that there are 2 valency units in hydrogen (1 × 2 = 2). The same value has the valency of oxygen.

When metals are combined with non-metals, the latter show a lower valence

In a compound consisting of atoms of two types, the element located in second place has the lowest valence. So, when connecting non-metals to each other, the element that is located in Mendeleev's PSCE to the right and above, and the highest, respectively, to the left and below, exhibits the lowest valency.

The valence of the acid residue coincides with the number of H atoms in the acid formula, the valency of the OH group is I.

In a compound formed by the atoms of three elements, the atom that is in the middle of the formula is called the central one. The O atoms are directly connected to it, and the rest of the atoms form bonds with oxygen.

Rules for determining the degree of oxidation of chemical elements.

The oxidation state is the conditional charge of the atoms of a chemical element in a compound, calculated from the assumption that the compounds are composed 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.
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 (I group), Cu +2 (II), from group VIII, only osmium Os and ruthenium Ru can have an oxidation state of +8.
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 -. When determining oxidation states, the following rules must be used:

The oxidation state of any element in a simple substance is 0.

The sum of the oxidation states of all the atoms that make up the particle (molecules, ions, etc.) is equal to the charge of this particle.

The sum of the oxidation states of all atoms in a neutral molecule is 0.

If the compound is formed by two elements, then the element with a higher electronegativity has an oxidation state less than zero, and the element with a lower electronegativity has an oxidation state greater than zero.

The maximum positive oxidation state of any element is equal to the group number in the Periodic Table of Elements, and the minimum negative oxidation state is N-8, where N is the group number.

The oxidation state of fluorine in compounds is -1.

The oxidation state of alkali metals (lithium, sodium, potassium, rubidium, cesium) is +1.

The oxidation state of metals of the main subgroup of group II of the periodic system (magnesium, calcium, strontium, barium) is +2.

The oxidation state of aluminum is +3.

The oxidation state of hydrogen in compounds is +1 (with the exception of compounds with metals NaH, CaH 2 , in these compounds the oxidation state of hydrogen is -1).

The oxidation state of oxygen is –2 (exceptions are peroxides H 2 O 2 , Na 2 O 2 , BaO 2 in them, the oxidation state of oxygen is -1, and in combination with fluorine - +2).

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

Example. Determine the oxidation states in compound K 2 Cr 2 O 7 .
The two chemical elements potassium and oxygen have constant oxidation states and are equal to +1 and -2, respectively. The number of oxidation states for oxygen is (-2) 7=(-14), for potassium (+1) 2=(+2). The number of positive oxidation states is equal to the number of negative ones. Therefore (-14)+(+2)=(-12). This means that the number of positive degrees of the chromium atom is 12, but there are 2 atoms, which means that there are (+12):2=(+6) per atom, we write down the oxidation states over the elementsTo + 2 Cr +6 2 O -2 7

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 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).

Categories ,

Atoms of various chemical elements can attach a different number of other atoms, i.e., exhibit different valencies.

Valency characterizes the ability of atoms to combine with other atoms. Now, having studied the structure of the atom and the types of chemical bonds, we can consider this concept in more detail.

Valency is the number of single chemical bonds that an atom forms with other atoms in a molecule. The number of chemical bonds is understood as the number of common electron pairs. Since common pairs of electrons are formed only in the case of a covalent bond, the valency of atoms can only be determined in covalent compounds.

In the structural formula of a molecule, chemical bonds are represented by dashes. The number of dashes extending from the symbol of a given element is its valence. Valence is always a positive integer value from I to VIII.

As you remember, the highest valence of a chemical element in an oxide is usually equal to the number of the group in which it is located. To determine the valence of a non-metal in a hydrogen compound, you need to subtract the group number from 8.

In the simplest cases, the valence is equal to the number of unpaired electrons in an atom, therefore, for example, oxygen (containing two unpaired electrons) has a valency II, and hydrogen (containing one unpaired electron) has an I.

In ionic and metallic crystals there are no common pairs of electrons, so for these substances the concept of valency as the number of chemical bonds does not make sense. For all classes of compounds, regardless of the type of chemical bonds, a more universal concept is applicable, which is called the degree of oxidation.

Oxidation state

is the conditional charge on an atom in a molecule or crystal. It is calculated assuming that all covalent polar bonds are ionic.

Unlike valence, the oxidation state can be positive, negative, or zero. In the simplest ionic compounds, the oxidation states coincide with the charges of the ions.

For example, in potassium chloride KCl (K + Cl - ) potassium has an oxidation state of +1, and chlorine -1, in calcium oxide CaO (Ca +2 O -2 ) calcium exhibits an oxidation state of +2, and oxygen -2. This rule applies to all basic oxides: in them, the oxidation state of the metal is equal to the charge of the metal ion (sodium +1, barium +2, aluminum +3), and the oxidation state of oxygen is -2. The oxidation state is indicated by an Arabic numeral, which is placed above the symbol of the element, like valency:

Cu +2 Cl 2 -1; Fe +2 S -2

The oxidation state of an element in a simple substance is taken equal to zero:

Na 0 , O 2 0 , S 8 0 , Cu 0

Consider how oxidation states are determined in covalent compounds.

Hydrogen chloride HCl is a substance with a polar covalent bond. The shared electron pair in the HCl molecule is shifted to the chlorine atom, which has a high electronegativity. We mentally transform the H-Cl bond into an ionic one (this really happens in an aqueous solution), completely shifting the electron pair to the chlorine atom. It will acquire a charge of -1, and hydrogen +1. Therefore, chlorine in this substance has an oxidation state of -1, and hydrogen +1:

Real charges and oxidation states of atoms in a hydrogen chloride molecule

The oxidation state and valency are related concepts. In many covalent compounds, the absolute value of the oxidation state of the elements is equal to their valency. There are, however, several cases where the valence is different from the oxidation state. This is typical, for example, for simple substances, where the oxidation state of atoms is zero, and valency is the number of common electron pairs:

O=O.

The valency of oxygen is II, and the oxidation state is 0.

In a hydrogen peroxide molecule

H-O-O-H

oxygen is divalent and hydrogen is monovalent. At the same time, the oxidation states of both elements are equal in absolute value to 1:

H 2 +1 O 2 -1

The same element in different compounds can have both positive and negative oxidation states, depending on the electronegativity of the atoms associated with it. Consider, for example, two carbon compounds, methane CH 4 and carbon(IV) fluoride CF 4 .

Carbon is more electronegative than hydrogen, so in methane the electron density of the C–H bonds is shifted from hydrogen to carbon, and each of the four hydrogen atoms has an oxidation state of +1, and the carbon atom is -4. On the contrary, in the CF4 molecule, the electrons of all bonds are shifted from the carbon atom to the fluorine atoms, the oxidation state of which is -1, therefore, carbon is in the +4 oxidation state. Remember that the oxidation state of the most electronegative atom in a compound is always negative.

Models of methane CH 4 and carbon(IV) CF 4 fluoride molecules. The polarity of the bonds is indicated by arrows.

Any molecule is electrically neutral, so the sum of the oxidation states of all atoms is zero. Using this rule, from a known oxidation state of one element in a compound, one can determine the oxidation state of another without resorting to reasoning about the displacement of electrons.

As an example, let's take chlorine(I) oxide Cl 2 O. We proceed from the electroneutrality of the particle. The oxygen atom in oxides has an oxidation state of -2, which means that both chlorine atoms carry a total charge of +2. It follows that on each of them the charge is +1, i.e. chlorine has an oxidation state of +1:

Cl 2 +1 O -2

In order to correctly place the signs of the oxidation state of different atoms, it is enough to compare their electronegativity. An atom with a higher electronegativity will have a negative oxidation state, and an atom with a lower electronegativity will have a positive one. According to the established rules, the symbol of the most electronegative element is written in the last place in the compound formula:

I +1 Cl -1, O +2 F 2 -1, P +5 Cl 5 -1

Real charges and oxidation states of atoms in a water molecule

When determining the oxidation states of elements in compounds, the following rules are observed.

The oxidation state of an element in a simple substance is zero.

Fluorine is the most electronegative chemical element, so the oxidation state of fluorine in all substances except F2 is -1.

Oxygen is the most electronegative element after fluorine, therefore the oxidation state of oxygen in all compounds except fluorides is negative: in most cases it is -2, and in hydrogen peroxide H 2 O 2 -1.

The oxidation state of hydrogen is +1 in compounds with non-metals, -1 in compounds with metals (hydrides); zero in simple matter H 2 .

The oxidation states of metals in compounds are always positive. The oxidation state of metals of the main subgroups, as a rule, is equal to the group number. The metals of the secondary subgroups often have several oxidation states.

The maximum possible positive oxidation state of a chemical element is equal to the group number (the exception is Cu +2).

The minimum oxidation state of metals is zero, and for non-metals, the group number minus eight.

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

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Solving combined problems based on the quantitative characteristics of a substance
Problem solving. The law of the constancy of the composition of substances. Calculations using the concepts of "molar mass" and "chemical amount" of a substance
Solving computational problems based on the quantitative characteristics of matter and stoichiometric laws
Solving computational problems based on the laws of the gaseous state of matter
Electronic configuration of atoms. The structure of the electron shells of atoms of the first three periods

The term is widely used in chemistry. electronegativity (EO) - the property of the atoms of a given element to pull electrons from the atoms of other elements in compounds is called electronegativity. The electronegativity of lithium is conventionally taken as unity, the EC of other elements is calculated accordingly. There is a scale of values of EO elements.

Numerical values of EO elements have approximate values: it is a dimensionless quantity. The higher the EC of an element, the more pronounced its non-metallic properties. According to the EO, the elements can be written as follows:

F > O > Cl > Br > S > P > C > H > Si > Al > Mg > Ca > Na > K > Cs

Fluorine has the highest EO value. Comparing the EO values of the elements from francium (0.86) to fluorine (4.1), it is easy to see that the EO obeys the Periodic Law. In the Periodic system of elements, EO in a period increases with an increase in the element number (from left to right), and in the main subgroups it decreases (from top to bottom). In periods, as the charges of the nuclei of atoms increase, the number of electrons on the outer layer increases, the radius of the atoms decreases, therefore, the ease of giving off electrons decreases, the EO increases, therefore, the non-metallic properties increase.

The difference in the electronegativity of the elements in the compound (ΔX) will make it possible to judge the type of chemical bond.

If the value Δ X \u003d 0 - non-polar covalent bond.

With the difference in electronegativity up to 2.0 bond is called covalent polar, for example: the H-F bond in the HF hydrogen fluoride molecule: Δ X \u003d (3.98 - 2.20) \u003d 1.78

Connections with the difference in electronegativity greater than 2.0 are considered ionic. For example: the Na-Cl bond in the NaCl compound: Δ X \u003d (3.16 - 0.93) \u003d 2.23.

Electronegativity depends on the distance between the nucleus and valence electrons, and on how close the valence shell is to being completed. The smaller the radius of an atom and the more valence electrons, the higher its ER.

Fluorine is most electronegative element. Firstly, it has 7 electrons on the valence shell (only 1 electron is missing before an octet) and, secondly, this valence shell is located close to the nucleus.

The least electronegative atoms are alkali and alkaline earth metals. They have large radii and their outer electron shells are far from complete. It is much easier for them to give their valence electrons to another atom (then the pre-outer shell will become complete) than to “gain” electrons.

Electronegativity can be expressed quantitatively and line up the elements in ascending order. Most commonly used the electronegativity scale proposed by the American chemist L. Pauling.

Oxidation state

Compounds made up of two chemical elements are called binary(from lat. bi - two), or two-element (NaCl, HCl). In the case of an ionic bond in the NaCl molecule, the sodium atom transfers its outer electron to the chlorine atom and turns into an ion with a charge of +1, while the chlorine atom accepts an electron and turns into an ion with a charge of -1. Schematically, the process of transformation of atoms into ions can be depicted as follows:

During chemical interaction in the HCl molecule, the common electron pair is shifted towards the more electronegative atom. For example, , i.e., the electron will not completely transfer from the hydrogen atom to the chlorine atom, but partially, thereby causing a partial charge of the atoms δ: H +0.18 Сl -0.18. If we imagine that in the HCl molecule, as well as in NaCl chloride, the electron completely passed from the hydrogen atom to the chlorine atom, then they would receive charges +1 and -1:

Such conditional charges are called oxidation state. When defining this concept, it is conditionally assumed that in covalent polar compounds, the binding electrons have completely transferred to a more electronegative atom, and therefore the compounds consist only of positively and negatively charged atoms.

The oxidation state is the conditional charge of the atoms of a chemical element in a compound, calculated on the basis of the assumption that all compounds (both ionic and covalently polar) consist only of ions. The oxidation state can have a negative, positive, or zero value, which is usually placed above the element symbol at the top, for example:

Those atoms that have received electrons from other atoms or to which common electron pairs are displaced have a negative value for the oxidation state, i.e. atoms of more electronegative elements. Those atoms that donate their electrons to other atoms or from which common electron pairs are drawn have a positive oxidation state, i.e., atoms of less electronegative elements. The zero value of the oxidation state has atoms in the molecules of simple substances and atoms in the free state, for example:

In compounds, the total oxidation state is always zero.

Valence

The valence of an atom of a chemical element is determined primarily by the number of unpaired electrons that take part in the formation of a chemical bond.

The valence possibilities of atoms are determined by:

The number of unpaired electrons (one-electron orbitals);

The presence of free orbitals;

The presence of lone pairs of electrons.

In organic chemistry, the concept of "valence" replaces the concept of "oxidation state", which is customary to work with in inorganic chemistry. However, they are not the same. The valence has no sign and cannot be zero, while the oxidation state is necessarily characterized by a sign and can have a value equal to zero.

Basically, valency refers to the ability of atoms to form a certain number of covalent bonds. If an atom has n unpaired electrons and m unshared electron pairs, then this atom can form n + m covalent bonds with other atoms, i.e. its valence will be equal to n + m. When evaluating the maximum valency, one should proceed from the electronic configuration of the "excited" state. For example, the maximum valence of an atom of beryllium, boron and nitrogen is 4.

Permanent valencies:

H, Na, Li, K, Rb, Cs - Oxidation state I
O, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd - Oxidation state II
B, Al, Ga, In — Oxidation state III

Valence Variables:

Cu - I and II
Fe, Co, Ni - II and III
C, Sn, Pb - II and IV
P- III and V
Cr- II, III and VI
S- II, IV and VI
Mn- II, III, IV, VI and VII
N- II, III, IV and V
Cl- I, IV, VIandVII

Using valencies, you can compose the formula of the compound.

A chemical formula is a conditional record of the composition of a substance by means of chemical signs and indices.

For example: H 2 O is the formula of water, where H and O are the chemical signs of the elements, 2 is an index that shows the number of atoms of this element that make up the water molecule.

When naming substances with variable valence, its valency must be indicated, which is placed in brackets. For example, P 2 0 5 - phosphorus oxide (V)

I. Oxidation state free atoms and atoms in molecules simple substances is equal to zero— Na 0 , R 4 0 , O 2 0

II. AT complex substance the algebraic sum of CO of all atoms, taking into account their indices, is equal to zero = 0. and in complex ion its charge.

For example:

For example, let's analyze several compounds and find out the valence chlorine:

Reference material for passing the test:

periodic table

Solubility table