The concept is widely used in chemistry electronegativity (EO) — the property of atoms of a given element to attract electrons from atoms of other elements in compounds is called electronegativity. The electronegativity of lithium is conventionally taken as unity, the EO of other elements is calculated accordingly. There is a scale of values of EO elements.
The numerical values of EO elements have approximate values: this is a dimensionless quantity. The higher the EO of an element, the brighter its non-metallic properties. According to 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 greatest EO value. Comparing the EO values of elements from francium (0.86) to fluorine (4.1), it is easy to notice that EO obeys the Periodic Law. In the Periodic Table of Elements, EO in a period increases with the element number (from left to right), and in the main subgroups it decreases (from top to bottom). In periods, as the charges of atomic nuclei increase, the number of electrons per outer layer increases, the radius of the atoms decreases, therefore the ease of electron loss decreases, the EO increases, therefore, the non-metallic properties increase.
The difference in electronegativity of the elements in a compound (ΔX) will allow us to judge the type of chemical bond.
If the value Δ X = 0 – covalent nonpolar bond.
With a difference in electronegativity up to 2.0 the bond is called polar covalent, For example: H-F connection in a hydrogen fluoride molecule HF: Δ X = (3.98 – 2.20) = 1.78
Connections with electronegativity differences greater than 2.0 are considered ionic. For example: Na-Cl bond in NaCl compound: Δ X = (3.16 – 0.93) = 2.23.
Electronegativity depends on the distance between the nucleus and the valence electrons, and on how close the valence shell is to completion. The smaller the radius of an atom and the more valence electrons, the higher its EO.
Fluorine is most electronegative element. Firstly, it has 7 electrons in its valence shell (only 1 electron is missing from the octet) and, secondly, this valence shell is located close to the nucleus.
The atoms of alkali and alkaline earth metals are the least electronegative. They have large radii and their outer electron shells are far from complete. It is much easier for them to give up their valence electrons to another atom (then the outer shell will become complete) than to “gain” electrons.
Electronegativity can be expressed quantitatively and the elements can be ranked in increasing order. Most often used electronegativity scale proposed by the American chemist L. Pauling.
Oxidation state
Complex substances consisting of two chemical elements, called binary(from Latin bi - two), or two-element (NaCl, HCl). In the case of an ionic bond in a NaCl molecule, the sodium atom transfers its outer electron to the chlorine atom and becomes an ion with a charge of +1, and the chlorine atom accepts an electron and becomes an ion with a charge of -1. Schematically, the process of converting atoms into ions can be depicted as follows:
At chemical interaction in the HCl molecule, the shared 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 determining the partial charge of the atoms δ:
H +0.18 Cl -0.18 . If we imagine that in the HCl molecule, as well as in the NaCl chloride, the electron has completely transferred 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 conventionally assumed that in covalent polar compounds the bonding electrons are 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 number 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 accepted electrons from other atoms or to which common electron pairs are displaced have a negative oxidation state value. i.e. atoms of more electronegative elements. A positive oxidation state is given to those atoms that donate their electrons to other atoms or from which shared electron pairs are drawn, i.e. atoms of less electronegative elements. Atoms in molecules have a zero oxidation state simple substances and atoms in a free state, for example:
In compounds, the total oxidation state is always zero.
Valence
The valency of an atom of a chemical element is determined primarily by the number of unpaired electrons participating in the formation of a chemical bond.
The valence capabilities of atoms are determined:
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”, with which it is customary to work in inorganic chemistry. However, this is not the same thing. Valency 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 covalent bonds. If an atom has n unpaired electrons and m lone 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 estimating the maximum valency, one should proceed from the electronic configuration of the “excited” state. For example, the maximum valency of a beryllium, boron and nitrogen atom is 4.
Constant valences:
- 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 create a formula for a compound.
A chemical formula is a conventional recording of the composition of a substance using chemical symbols and indices.
For example: H 2 O is the formula of water, where H and O-chemical signs elements, 2 is an index that shows the number of atoms of a given element that make up a water molecule.
When naming substances with variable valence, its valence 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 equal to zero—Na 0 , R 4 0 , ABOUT 2 0
II. IN complex substancealgebraic sum The CO of all atoms, taking into account their indices, is equal to zero = 0. and in complex ion its charge.
For example:
Let's look at several compounds as an example and find out the valence chlorine:
Reference material for taking the test:
Mendeleev table
Solubility table
I.Valence (repetition)
Valency is the ability of atoms to attach to themselves a certain number of other atoms.
Rules for determining valence
elements in connections
1. Valence hydrogen mistaken for I(unit). Then, in accordance with the formula of water H 2 O, two hydrogen atoms are attached to one oxygen atom.
2. Oxygen in its compounds always exhibits valency II. Therefore, the carbon in the compound CO 2 (carbon dioxide) has a valence of IV.
3. Higher valence equal to group number .
4. Lowest valency is equal to the difference between the number 8 (the number of groups in the table) and the number of the group in which this element is located, i.e. 8 - N groups .
5. For metals located in “A” subgroups, the valence is equal to the group number.
6. Nonmetals generally exhibit two valences: higher and lower.
For example: sulfur has the highest valency VI and the lowest (8 – 6) equal to II; phosphorus exhibits valences V and III.
7. Valence can be constant or variable.
The valency of elements must be known in order to compose chemical formulas of compounds.
Remember!
Features of compilation chemical formulas connections.
1) The lowest valence is shown by the element that is located to the right and above in D.I. Mendeleev’s table, and the highest valence is shown by the element located to the left and below.
For example, in combination with oxygen, sulfur exhibits the highest valency VI, and oxygen the lowest valency II. Thus, the formula for sulfur oxide will be SO 3.
In the compound of silicon with carbon, the first exhibits the highest valency IV, and the second - the lowest IV. So the formula– SiC. This is silicon carbide, the basis of refractory and abrasive materials.
2) The metal atom comes first in the formula.
2) In the formulas of compounds, the non-metal atom exhibiting the lowest valency always comes in second place, and the name of such a compound ends in “id”.
For example, Sao – calcium oxide, NaCl - sodium chloride, PbS – lead sulfide.
Now you can write the formulas for any compounds of metals and non-metals.
3) The metal atom is placed first in the formula.
II. Oxidation state (new material)
Oxidation state- this is a conditional charge that an atom receives as a result of the complete donation (acceptance) of electrons, based on the condition that all bonds in the compound are ionic.
Let's consider the structure of fluorine and sodium atoms:
F +9)2)7
Na +11)2)8)1
- What can be said about the completeness of the external level of fluorine and sodium atoms?
- Which atom is easier to accept, and which is easier to give away valence electrons in order to complete the outer level?
Do both atoms have an incomplete outer level?
It is easier for a sodium atom to give up electrons, and for a fluorine atom to accept electrons before completing the outer level.
F 0 + 1ē → F -1 (a neutral atom accepts one negative electron and acquires an oxidation state of “-1”, turning into negatively charged ion - anion )
Na 0 – 1ē → Na +1 (a neutral atom gives up one negative electron and acquires an oxidation state of “+1”, turning into positively charged ion - cation )
How to determine the oxidation state of an atom in PSHE D.I. Mendeleev?
Determination rules oxidation state of an atom in PSHE D.I. Mendeleev:
1. Hydrogen usually exhibits oxidation number (CO) +1 (exception, compounds with metals (hydrides) - in hydrogen, CO is equal to (-1) Me + n H n -1)
2. Oxygen usually exhibits SO -2 (exceptions: O +2 F 2, H 2 O 2 -1 - hydrogen peroxide)
3. Metals only show + n positive CO
4. Fluorine always exhibits CO equal -1 (F -1)
5. For elements main subgroups:
Higher CO (+) = group number N groups
Lowest CO (-) = N groups – 8
Rules for determining the oxidation state of an atom in a compound:
I. Oxidation state free atoms and atoms in molecules simple substances equal to zero - Na 0 , P 4 0 , O 2 0
II. IN complex substance the algebraic sum of the COs of all atoms, taking into account their indices, is equal to zero = 0 , and in complex ion its charge.
For example, H +1 N +5 O 3 -2 : (+1)*1+(+5)*1+(-2)*3 = 0
2- : (+6)*1+(-2)*4 = -2
Exercise 1 – determine the oxidation states of all atoms in the formula of sulfuric acid H 2 SO 4?
1. Let’s put the known oxidation states of hydrogen and oxygen, and take CO of sulfur as “x”
H +1 S x O 4 -2
(+1)*1+(x)*1+(-2)*4=0
X = 6 or (+6), therefore, sulfur has C O +6, i.e. S+6
Task 2 – determine the oxidation states of all atoms in the formula of phosphoric acid H 3 PO 4?
1. Let’s put the known oxidation states of hydrogen and oxygen, and take the CO of phosphorus as “x”
H 3 +1 P x O 4 -2
2. Let’s compose and solve the equation according to rule (II):
(+1)*3+(x)*1+(-2)*4=0
X = 5 or (+5), therefore, phosphorus has C O +5, i.e. P+5
Task 3 – determine the oxidation states of all atoms in the formula of ammonium ion (NH 4) +?
1. Let’s put the known oxidation state of hydrogen, and take CO2 of nitrogen as “x”
(N x H 4 +1) +
2. Let’s compose and solve the equation according to rule (II):
(x)*1+(+1)*4=+1
X = -3, therefore, nitrogen has C O -3, i.e. N-3
form a definite number with atoms of other elements.The valence 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 Table. For example, for Sa it isII, for sulfur -VI, for chlorine -VII. Exceptions There is also a lot from this rule:
ElementVIgroup, O, has valency II (in H 3
O+ - III);
- monovalent F (instead ofVII);
- usually di- 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;
- mono- and divalent copper, located in group I.
The minimum value of valency for elements for which it is variable is determined by the formula: group number in PS - 8. Thus, the lowest valency of sulfur is 8 - 6 = 2, fluorine and other halogens - (8 - 7) = 1, nitrogen and phosphorus - (8 - 5)= 3 and so on.
In a compound, the sum of the valence units of the atoms of one element must correspond to the total valency of the other (or total number valencies of one chemical element is equal to the total number of valences of atoms of another chemical element). Yes, in a molecule water N-O-N 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 valency of oxygen has the same meaning.
When metals combine with nonmetals, the latter exhibit lower valence
In a compound consisting of two types of atoms, the element located in second place has the lowest valency. So, when non-metals combine with each other, the element that is located to the right and above in Mendeleev’s PSHE exhibits the lowest valence, and the highest, respectively, to the left and below.
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.
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.
Rules for determining the degree of oxidation of chemical elements.
The oxidation state is the nominal charge of the atoms of a chemical element in a compound, calculated from the assumption that the compounds consist only of ions.
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 periodic table, where this element is located (excluding some elements: gold Au +3
(I group), Cu +2
(II), from group VIII the oxidation state +8 can only be found 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 it is always negative, if with a non-metal 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 atoms that make up a 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 equal to 0.
If a compound is formed by two elements, then the element with greater electronegativity has an oxidation state less than zero, and the element with less 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 is equal to N– 8, where N is the group number.
The oxidation state of fluorine in the compounds is -1.
Oxidation state alkali metals(lithium, sodium, potassium, rubidium, cesium) is +1.
The oxidation state of metals of the main subgroup of group II of the periodic table (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 H peroxide 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 equal to 0.
Example.
Determine the oxidation states in compound K 2
Cr 2
O 7
.
For two chemical elements, potassium and oxygen, the oxidation states are constant and 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 chromium atom has 12 positive degrees, but there are 2 atoms, which means there are (+12) per atom: 2=(+6), we write down the oxidation states over the elementsTO
+
2
Cr
+6
2
O
-2
7
Among chemical reactions, including in nature, redox reactions are the most common. These include, for example, photosynthesis, metabolism, biological processes, as well as the combustion of fuel, the production of metals and many other reactions. Redox reactions have long been successfully used by humanity for various purposes, but the electronic theory of redox processes itself appeared quite recently - at the beginning of the 20th century.
In order to go to modern theory oxidation-reduction, it is necessary to introduce several concepts - these are valency, oxidation state and structure electronic shells atoms. While studying sections such as , elements and , we have already encountered these concepts. Next, let's look at them in more detail.
Valency and oxidation state
Valence- a complex concept that arose together 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, valency was determined by hydrogen (its valency was taken to be 1) or oxygen (valency was taken to be 2). Later they began to distinguish between positive and negative valence. Quantitatively, positive valency is characterized by the number of electrons donated by an atom, and negative valency is characterized by the number of electrons that must be added 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 chemical bonds, arising between atoms in their connection.
As a rule, the highest valence of elements corresponds to the group number in the periodic table. But, as with all rules, there are exceptions: for example, copper and gold are in the first group of the periodic table and their valency must be equal to the group number, i.e. 1, but in reality the highest valence of copper is 2, and gold is 3.
Oxidation state sometimes called oxidation number, electrochemical valence or oxidation state and is a relative concept. Thus, when calculating the oxidation state, it is assumed that the molecule consists only of ions, although most compounds are not ionic at all. Quantitatively, the degree of oxidation 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 electron displacement, the oxidation state will be zero, when electrons are displaced towards a given atom, it will be negative, and when electrons are displaced from a given atom, it will be positive.
Defining oxidation state of atoms the following rules must be followed:
- In 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 typical 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 electric charge.
- For covalent compound, a more electronegative atom has an oxidation state with a “-” sign, and a less electronegative one has a “+” sign.
- For complex compounds, the oxidation state of the central atom is indicated.
- 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, let’s create an expression, taking into account the number of atoms in the compound H 2 + Se x O 3 -2:
(+1)2+x+(-2)3=0, whence
those. H 2 + Se +4 O 3 -2
Knowing what the oxidation state of an element in a compound is, it is possible to predict it Chemical properties and reactivity towards other compounds, as well as whether the compound is reducing agent or oxidizing agent. These concepts are fully revealed in oxidation-reduction theories:
- Oxidation is the process of loss of electrons by an atom, ion or molecule, which leads to an increase in the oxidation state.
Al 0 -3e - = Al +3 ;
2O -2 -4e - = O 2 ;
2Cl - -2e - = Cl 2
- Recovery - This is the process by which an atom, ion or molecule gains electrons, resulting in a decrease in oxidation state.
Ca +2 +2e - = Ca 0 ;
2H + +2e - =H 2
- Oxidizing agents– compounds that accept electrons during chemical reaction, A reducing agents– electron donating compounds. Reducing agents are oxidized during a reaction, and oxidizing agents are reduced.
- The essence of redox reactions– movement of electrons (or 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 away from one element during oxidation is the same as the number of electrons gained by another element during reduction.
So, if the elements in compounds are in their highest oxidation states, then they will exhibit only oxidizing properties, due to the fact that they can no longer give up 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 intermediate degree oxidations, depending on the reaction conditions, can be both oxidizing and reducing agents. Let's give an example: sulfur in its highest oxidation state +6 in the compound H 2 SO 4 can exhibit only oxidizing properties, in the compound H 2 S - sulfur is in its lowest oxidation state -2 and will exhibit only reducing properties, and in the compound H 2 SO 3 being in the intermediate oxidation state +4, sulfur can be both an oxidizing agent and a reducing agent.
Based on the oxidation states of elements, the likelihood of a reaction between substances can be predicted. It is clear that if both elements in their compounds are in higher or lower oxidation states, then a reaction between them is impossible. A reaction is possible if one of the compounds can exhibit oxidizing properties, and the other – 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 interact well with H 2 SO 4, which is characterized by reducing properties, because sulfur here is in its highest state of oxidation.
The most important reducing and oxidizing agents are presented in the following table.
| Restorers | |
| Neutral atoms | General scheme M—ne →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 are noble metals - Au, Ag, Pt, Ir, Os, Pd, Ru, Rh. In the main subgroups of the periodic table, the reducing ability of neutral atoms increases with increasing atomic number. |
| negatively charged nonmetal 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. The reducing power, with the same charge, increases with increasing atomic radius. For example, I is a stronger reducing agent than Br - and Cl -. Reducing agents can also be S 2-, Se 2-, Te 2- and others. |
| positively charged metal ions of the lowest oxidation state | Metal ions of lower oxidation states 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 intermediate oxidation states | Complex or complex ions, and also molecules can exhibit reducing properties if the atoms included in their composition 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, Sulfurous Acid, Sodium Sulfite and Bisulfite, Sodium Sulfide, Sodium Thiosulfate, Hydrogen, Electricity | |
| Oxidizing agents | |
| Neutral atoms | General scheme E + ne- → E n- Oxidizing agents are atoms of p-elements. Typical nonmetals are fluorine, oxygen, chlorine. The strongest oxidizing agents are halogens and oxygen. In the main subgroups of groups 7, 6, 5 and 4, the oxidative activity of atoms decreases from top to bottom |
| positively charged metal ions | All positively charged metal ions in varying degrees exhibit oxidizing properties. Of these, the most powerful oxidizing agents are ions with a high oxidation state, for example, Sn 4+, Fe 3+, Cu 2+. Noble metal ions, even in low oxidation states, 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 highest degree oxidation. 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 H 2SO4 →H 2SeO4 →H 6TeO6 oxidizing activity increases from sulfuric to telluric acid. In a row HClO -HClO 2 -HClO 3 -HClO4 HBrO - HBrO 3 - HIO - HIO 3 - HIO 4 , H5IO 6 oxidative activity increases from right to left, and increased acidic properties happens from left to right. |
| The most important reducing agents in technology and laboratory practice | Oxygen, Ozone, Potassium permanganate, Chromic and Dichromic acids, Nitric acid, Nitrous acid, Sulfuric acid(conc), Hydrogen peroxide, Electric current, Hypochlorous acid, Manganese dioxide, Lead dioxide, Bleach, Solutions of potassium and sodium hypochlorites, Potassium hypobromide, Potassium hexacyanoferrate (III). |
