The degree of oxidation of atoms of chemical elements in compounds. How to determine the degree of oxidation

The ability to find the degree of oxidation of chemical elements is a necessary condition for the successful solution of chemical equations describing redox reactions. Without it, you will not be able to draw up an exact formula for a substance resulting from a reaction between various chemical elements. As a result, the solution of chemical problems based on such equations will either be impossible or erroneous.

The concept of the oxidation state of a chemical element
Oxidation state- this is a conditional value, with the help of which it is customary to describe redox reactions. Numerically, it is equal to the number of electrons that an atom acquires a positive charge, or the number of electrons that an atom acquires a negative charge attaches to itself.

In redox reactions, the concept of oxidation state is used to determine the chemical formulas of compounds of elements resulting from the interaction of several substances.

At first glance, it may seem that the oxidation state is equivalent to the concept of the valency of a chemical element, but this is not so. concept valence used to quantify the electronic interaction in covalent compounds, that is, in compounds formed by the formation of shared electron pairs. The oxidation state is used to describe reactions that are accompanied by the donation or gain of electrons.

Unlike valency, which is a neutral characteristic, the oxidation state can have a positive, negative, or zero value. A positive value corresponds to the number of donated electrons, and a negative value corresponds to the number of attached ones. A value of zero means that the element is either in the form of a simple substance, or it was reduced to 0 after oxidation, or oxidized to zero after a previous reduction.

How to determine the oxidation state of a particular chemical element
The determination of the oxidation state for a particular chemical element is subject to the following rules:

The oxidation state of simple substances is always zero.
Alkali metals, which are in the first group of the periodic table, have an oxidation state of +1.
Alkaline earth metals, which occupy the second group in the periodic table, have an oxidation state of +2.
Hydrogen in compounds with various non-metals always exhibits an oxidation state of +1, and in compounds with metals +1.
The oxidation state of molecular oxygen in all compounds considered in the school course of inorganic chemistry is -2. Fluorine -1.
When determining the degree of oxidation in the products of chemical reactions, they proceed from the rule of electrical neutrality, according to which the sum of the oxidation states of the various elements that make up the substance must be equal to zero.
Aluminum in all compounds exhibits an oxidation state of +3.

Further, as a rule, difficulties begin, since the remaining chemical elements show and exhibit a variable oxidation state depending on the types of atoms of other substances involved in the compound.

There are higher, lower and intermediate oxidation states. The highest oxidation state, like valence, corresponds to the group number of the chemical element in the periodic table, but it has a positive value. The lowest oxidation state is numerically equal to the difference between the number 8 of the element group. The intermediate oxidation state will be any number in the range from the lowest oxidation state to the highest.

To help you navigate the variety of oxidation states of chemical elements, we bring to your attention the following auxiliary table. Select the element you are interested in and you will get the values of its possible oxidation states. Rarely occurring values will be indicated in brackets.

The formal charge of an atom in compounds is an auxiliary quantity, it is usually used in descriptions of the properties of elements in chemistry. This conditional electric charge is the degree of oxidation. Its value changes as a result of many chemical processes. Although the charge is formal, it vividly characterizes the properties and behavior of atoms in redox reactions (ORDs).

Oxidation and reduction

In the past, chemists used the term "oxidation" to describe the interaction of oxygen with other elements. The name of the reactions comes from the Latin name for oxygen - Oxygenium. Later it turned out that other elements also oxidize. In this case, they are restored - they attach electrons. Each atom during the formation of a molecule changes the structure of its valence electron shell. In this case, a formal charge appears, the value of which depends on the number of conditionally given or received electrons. To characterize this value, the English chemical term "oxidation number" was previously used, which means "oxidation number" in translation. Its use is based on the assumption that the bonding electrons in molecules or ions belong to the atom with the higher electronegativity (EO). The ability to retain their electrons and attract them from other atoms is well expressed in strong non-metals (halogens, oxygen). Strong metals (sodium, potassium, lithium, calcium, other alkali and alkaline earth elements) have opposite properties.

Determination of the degree of oxidation

The oxidation state is the charge that an atom would acquire if the electrons involved in the formation of the bond were completely shifted to a more electronegative element. There are substances that do not have a molecular structure (alkali metal halides and other compounds). In these cases, the oxidation state coincides with the charge of the ion. The conditional or real charge shows what process took place before the atoms acquired their current state. A positive oxidation state is the total number of electrons that have been removed from the atoms. The negative value of the oxidation state is equal to the number of acquired electrons. By changing the oxidation state of a chemical element, one judges what happens to its atoms during the reaction (and vice versa). The color of the substance determines what changes in the state of oxidation have occurred. Compounds of chromium, iron and a number of other elements in which they exhibit different valences are colored differently.

Negative, zero and positive oxidation state values

Simple substances are formed by chemical elements with the same EO value. In this case, the bonding electrons belong to all structural particles equally. Therefore, in simple substances, the oxidation state (H 0 2, O 0 2, C 0) is not characteristic of the elements. When atoms accept electrons or the general cloud shifts in their direction, it is customary to write charges with a minus sign. For example, F -1, O -2, C -4. By donating electrons, atoms acquire a real or formal positive charge. In OF 2 oxide, the oxygen atom donates one electron each to two fluorine atoms and is in the O +2 oxidation state. It is believed that in a molecule or a polyatomic ion, the more electronegative atoms receive all the binding electrons.

Sulfur is an element that exhibits different valencies and oxidation states.

Chemical elements of the main subgroups often exhibit a lower valence equal to VIII. For example, the valency of sulfur in hydrogen sulfide and metal sulfides is II. The element is characterized by intermediate and higher valencies in the excited state, when the atom gives up one, two, four or all six electrons and exhibits valences I, II, IV, VI, respectively. The same values, only with a minus or plus sign, have the oxidation states of sulfur:

in fluorine sulfide gives one electron: -1;
in hydrogen sulfide, the lowest value: -2;
in dioxide intermediate state: +4;
in trioxide, sulfuric acid and sulfates: +6.

In its highest oxidation state, sulfur only accepts electrons; in its lowest state, it exhibits strong reducing properties. The S +4 atoms can act as reducing or oxidizing agents in compounds, depending on the conditions.

Transfer of electrons in chemical reactions

In the formation of a sodium chloride crystal, sodium donates electrons to the more electronegative chlorine. The oxidation states of the elements coincide with the charges of the ions: Na +1 Cl -1 . For molecules created by the socialization and displacement of electron pairs to a more electronegative atom, only the concept of a formal charge is applicable. But it can be assumed that all compounds are composed of ions. Then the atoms, by attracting electrons, acquire a conditional negative charge, and by giving away, they acquire a positive one. In reactions, indicate how many electrons are displaced. For example, in the carbon dioxide molecule C +4 O - 2 2, the index indicated in the upper right corner of the chemical symbol for carbon displays the number of electrons removed from the atom. Oxygen in this substance has an oxidation state of -2. The corresponding index with the chemical sign O is the number of added electrons in the atom.

How to calculate oxidation states

Counting the number of electrons donated and added by atoms can be time consuming. The following rules make this task easier:

In simple substances, the oxidation states are zero.
The sum of the oxidation of all atoms or ions in a neutral substance is zero.
In a complex ion, the sum of the oxidation states of all elements must correspond to the charge of the entire particle.
A more electronegative atom acquires a negative oxidation state, which is written with a minus sign.
Less electronegative elements receive positive oxidation states, they are written with a plus sign.
Oxygen generally exhibits an oxidation state of -2.
For hydrogen, the characteristic value is: +1, in metal hydrides it occurs: H-1.
Fluorine is the most electronegative of all elements, its oxidation state is always -4.
For most metals, oxidation numbers and valences are the same.

Oxidation state and valence

Most compounds are formed as a result of redox processes. The transition or displacement of electrons from one element to another leads to a change in their oxidation state and valency. Often these values coincide. As a synonym for the term "oxidation state", the phrase "electrochemical valence" can be used. But there are exceptions, for example, in the ammonium ion, nitrogen is tetravalent. At the same time, the atom of this element is in the oxidation state -3. In organic substances, carbon is always tetravalent, but the oxidation states of the C atom in methane CH 4, formic alcohol CH 3 OH and acid HCOOH have different values: -4, -2 and +2.

Redox reactions

Redox processes include many of the most important processes in industry, technology, animate and inanimate nature: combustion, corrosion, fermentation, intracellular respiration, photosynthesis, and other phenomena.

When compiling the OVR equations, the coefficients are selected using the electronic balance method, in which the following categories are operated:

oxidation states;
the reducing agent donates electrons and is oxidized;
the oxidizing agent accepts electrons and is reduced;
the number of given electrons must be equal to the number of attached ones.

The acquisition of electrons by an atom leads to a decrease in its oxidation state (reduction). The loss of one or more electrons by an atom is accompanied by an increase in the oxidation number of the element as a result of reactions. For OVR, flowing between ions of strong electrolytes in aqueous solutions, not the electronic balance, but the method of half-reactions is more often used.

To determine the conditional charge of atoms in redox reactions, use the table of oxidation of chemical elements. Depending on the properties of the atom, an element can exhibit a positive or negative oxidation state.

What is oxidation state

The conditional charge of the atoms of elements in complex substances is called the degree of oxidation. The charge value of atoms is recorded in redox reactions to understand which element is the reducing agent and which is the oxidizing agent.

The oxidation state is related to electronegativity, which indicates the ability of atoms to accept or donate electrons. The higher the electronegativity value, the greater the atom's ability to take electrons in reactions.

Rice. 1. Series of electronegativity.

The oxidation state can have three values:

zero- the atom is at rest (all simple substances have an oxidation state of 0);
positive- an atom donates electrons and is a reducing agent (all metals, some non-metals);
negative- an atom accepts electrons and is an oxidizing agent (most non-metals).

For example, the oxidation states in the reaction of sodium with chlorine are as follows:

2Na 0 + Cl 2 0 → 2Na +1 Cl -1

In the reaction of metals with non-metals, the metal is always the reducing agent and the non-metal is the oxidizing agent.

How to determine

There is a table that lists all the possible oxidation states of the elements.

*Name*	*Symbol*	*Oxidation state*



Beryllium
		1, 0, +1, +2, +3
		4, -3, -2, -1, 0, +2, +4
		3, -2, -1, 0, +1, +2, +3, +4, +5
Oxygen		2, -1, 0, +1, +2




Aluminum



		1, 0, +1, +3, +5, +7, rarely +2 and +4







Manganese		2, +3, +4, +6, +7
		2, +3, rarely +4 and +6
		2, +3, rarely +4
		2, rarely +1, +3, +4
		1, +2, rarely +3

		3, rarely +2
Germanium
		3, +3, +5, rarely +2
		2, +4, +6, rarely +2
		1, +1, +5, rarely +3, +4


Strontium

Zirconium		4, rarely +2, +3
		3, +5, rarely +2, +4
Molybdenum		3, +6, rarely +2, +3, +5
Technetium
		3, +4, +8, rarely +2, +6, +7
		4, rarely +2, +3, +6
Palladium		2, +4, rarely +6
		1, rarely +2, +3
		2, rarely +1
		3, rarely +1, +2

		3, +3, +5, rarely +4
		2, +4, +6, rare
		1, +1, +5, +7, rarely +3, +4





Praseodymium

Promethium
		3, rarely +2
		3, rarely +2
Gadolinium

Dysprosium


		3, rarely +2
Ytterbium		3, rarely +2


		5, rarely +3, +4
Tungsten		6, rarely +2, +3, +4, +5
		2, +4, +6, +7, rarely -1, +1, +3, +5
		3, +4, +6, +8, rare +2
		3, +4, +6, rarely +1, +2
		2, +4, +6, rarely +1, +3
		1, +3, rare +2

		1, +3, rare +2

		3, rarely +3, +2, +4, +5
		2, +4, rarely -2, +6

Or use this version of the table in the lessons.

Rice. 2. Table of oxidation states.

In addition, the oxidation states of chemical elements can be determined from the periodic table of Mendeleev:

the highest degree (maximum positive) coincides with the group number;
to determine the minimum oxidation state value, eight is subtracted from the group number.

Rice. 3. Periodic table.

Most non-metals have positive and negative oxidation states. For example, silicon is in group IV, which means that its maximum oxidation state is +4, and the minimum is -4. In non-metal compounds (SO 3 , CO 2 , SiC), the oxidizing agent is a non-metal with a negative oxidation state or with a high electronegativity value. For example, in the compound PCl 3, phosphorus has an oxidation state of +3, chlorine -1. The electronegativity of phosphorus is 2.19, of chlorine is 3.16.

The second rule does not work for alkali and alkaline earth metals, which always have one positive oxidation state equal to the group number. The exceptions are magnesium and beryllium (+1, +2). Also have a constant oxidation state:

aluminum (+3);
zinc (+2);
cadmium (+2).

The rest of the metals have a variable oxidation state. In most reactions, they act as a reducing agent. In rare cases, they can be oxidizing agents with a negative oxidation state.

Fluorine is the most powerful oxidizing agent. Its oxidation state is always -1.

What have we learned?

From the 8th grade lesson, we learned about the degree of oxidation. This is a conditional value showing how many electrons an atom can give or take during a chemical reaction. The value is related to electronegativity. Oxidizing agents accept electrons and have a negative oxidation state, while reducing agents donate electrons and exhibit a positive oxidation state. Most metals are reducing agents with a constant or variable oxidation state. Non-metals can exhibit the properties of an oxidizing and reducing agent, depending on the substance with which they react.

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In chemistry, the terms "oxidation" and "reduction" mean reactions in which an atom or a group of atoms lose or, respectively, gain electrons. The oxidation state is a numerical value attributed to one or more atoms that characterizes the number of redistributed electrons and shows how these electrons are distributed between atoms during the reaction. Determining this quantity can be both a simple and quite complex procedure, depending on the atoms and the molecules consisting of them. Moreover, the atoms of some elements can have several oxidation states. Fortunately, there are simple unambiguous rules for determining the degree of oxidation, for the confident use of which it is enough to know the basics of chemistry and algebra.

Steps

Part 1

Determination of the degree of oxidation according to the laws of chemistry

Determine if the substance in question is elemental. The oxidation state of atoms outside a chemical compound is zero. This rule is true both for substances formed from individual free atoms, and for those that consist of two or polyatomic molecules of one element.

For example, Al(s) and Cl 2 have an oxidation state of 0 because both are in a chemically uncombined elemental state.
Please note that the allotropic form of sulfur S 8, or octasulfur, despite its atypical structure, is also characterized by a zero oxidation state.

Determine if the substance in question consists of ions. The oxidation state of ions is equal to their charge. This is true both for free ions and for those that are part of chemical compounds.
- For example, the oxidation state of the Cl ion is -1.
- The oxidation state of the Cl ion in the chemical compound NaCl is also -1. Since the Na ion, by definition, has a charge of +1, we conclude that the charge of the Cl ion is -1, and thus its oxidation state is -1.
Note that metal ions can have several oxidation states. Atoms of many metallic elements can be ionized to different extents. For example, the charge of ions of a metal such as iron (Fe) is +2 or +3. The charge of metal ions (and their degree of oxidation) can be determined by the charges of ions of other elements with which this metal is part of a chemical compound; in the text, this charge is indicated by Roman numerals: for example, iron (III) has an oxidation state of +3.
- As an example, consider a compound containing an aluminum ion. The total charge of the AlCl 3 compound is zero. Since we know that Cl - ions have a charge of -1, and there are 3 such ions in the compound, for the total neutrality of the substance in question, the Al ion must have a charge of +3. Thus, in this case, the oxidation state of aluminum is +3.
The oxidation state of oxygen is -2 (with some exceptions). In almost all cases, oxygen atoms have an oxidation state of -2. There are several exceptions to this rule:
- If oxygen is in the elemental state (O 2 ), its oxidation state is 0, as is the case for other elemental substances.
- If oxygen is included peroxides, its oxidation state is -1. Peroxides are a group of compounds containing a single oxygen-oxygen bond (ie the peroxide anion O 2 -2). For example, in the composition of the H 2 O 2 molecule (hydrogen peroxide), oxygen has a charge and an oxidation state of -1.
- In combination with fluorine, oxygen has an oxidation state of +2, see the rule for fluorine below.
Hydrogen has an oxidation state of +1, with a few exceptions. As with oxygen, there are also exceptions. As a rule, the oxidation state of hydrogen is +1 (unless it is in the elemental state H 2). However, in compounds called hydrides, the oxidation state of hydrogen is -1.
- For example, in H 2 O, the oxidation state of hydrogen is +1, since the oxygen atom has a charge of -2, and two +1 charges are needed for overall neutrality. However, in the composition of sodium hydride, the oxidation state of hydrogen is already -1, since the Na ion carries a charge of +1, and for total electroneutrality, the charge of the hydrogen atom (and thus its oxidation state) must be -1.
Fluorine always has an oxidation state of -1. As already noted, the degree of oxidation of some elements (metal ions, oxygen atoms in peroxides, and so on) can vary depending on a number of factors. The oxidation state of fluorine, however, is invariably -1. This is due to the fact that this element has the highest electronegativity - in other words, fluorine atoms are the least willing to part with their own electrons and most actively attract other people's electrons. Thus, their charge remains unchanged.
The sum of the oxidation states in a compound is equal to its charge. The oxidation states of all the atoms that make up a chemical compound, in total, should give the charge of this compound. For example, if a compound is neutral, the sum of the oxidation states of all its atoms must be zero; if the compound is a polyatomic ion with a charge of -1, the sum of the oxidation states is -1, and so on.
- This is a good method of checking - if the sum of the oxidation states does not equal the total charge of the compound, then you are wrong somewhere.
Part 2
Determining the oxidation state without using the laws of chemistry
1. Find atoms that do not have strict rules regarding oxidation state. In relation to some elements, there are no firmly established rules for finding the degree of oxidation. If an atom does not fit any of the rules listed above, and you do not know its charge (for example, the atom is part of a complex, and its charge is not indicated), you can determine the oxidation state of such an atom by elimination. First, determine the charge of all other atoms of the compound, and then from the known total charge of the compound, calculate the oxidation state of this atom.
  - For example, in the Na 2 SO 4 compound, the charge of the sulfur atom (S) is unknown - we only know that it is not zero, since sulfur is not in the elementary state. This compound serves as a good example to illustrate the algebraic method of determining the oxidation state.
2. Find the oxidation states of the rest of the elements in the compound. Using the rules described above, determine the oxidation states of the remaining atoms of the compound. Don't forget about the exceptions to the rule in the case of O, H, and so on.
  - For Na 2 SO 4 , using our rules, we find that the charge (and hence the oxidation state) of the Na ion is +1, and for each of the oxygen atoms it is -2.
3. In compounds, the sum of all oxidation states must equal the charge. For example, if the compound is a diatomic ion, the sum of the oxidation states of the atoms must be equal to the total ionic charge.
4. It is very useful to be able to use the periodic table of Mendeleev and know where the metallic and non-metallic elements are located in it.
5. The oxidation state of atoms in the elementary form is always zero. The oxidation state of a single ion is equal to its charge. Elements of group 1A of the periodic table, such as hydrogen, lithium, sodium, in elemental form have an oxidation state of +1; the oxidation state of group 2A metals, such as magnesium and calcium, in its elemental form is +2. Oxygen and hydrogen, depending on the type of chemical bond, can have 2 different oxidation states.

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. O atoms are directly connected 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 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 a 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