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

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

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

In redox reactions, the concept of the 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 valence 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 common electron pairs. The oxidation state is used to describe reactions that involve the donation or attachment of electrons.

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

How to determine the oxidation state of a specific chemical element
Determination of the oxidation state for a specific 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 oxidation state in the products of chemical reactions, one proceeds from the electroneutrality rule, according to which the sum of the oxidation states of various elements that make up a substance must be zero.
Aluminum in all compounds exhibits an oxidation state equal to +3.

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

Distinguish between higher, lower and intermediate oxidation states. The highest oxidation state, like the valence, corresponds to the group number of a chemical element in the periodic table, but at the same time 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 present to your attention the following auxiliary table. Select the element you are interested in and you will get the values \u200b\u200bof its possible oxidation states. Rare values \u200b\u200bwill be indicated in brackets.

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

Oxidation and reduction

In the past, chemists have 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 magnitude 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". Its use is based on the assumption that the bonding electrons in molecules or ions belong to an atom with a higher electronegativity (EO) value. The ability to hold onto its electrons and attract them from other atoms is well expressed in strong non-metals (halogens, oxygen). Strong metals (sodium, potassium, lithium, calcium, other alkaline and alkaline earth elements) have opposite properties.

Determination of the oxidation state

The oxidation state is the charge that an atom would acquire if the electrons participating in the formation of a bond were completely displaced 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. 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 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 can judge what happens to its atoms during the reaction (and vice versa). The color of the substance determines what changes have occurred in the oxidation state. Compounds of chromium, iron and a number of other elements, in which they exhibit different valences, are colored differently.

Negative, zero and positive oxidation states

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 (Н 0 2, О 0 2, С 0) is unusual for the elements. When atoms take electrons or the general cloud shifts in their direction, charges are usually written with a minus sign. For example, F -1, O -2, C -4. By donating electrons, atoms acquire a real or formal positive charge. In oxide OF 2, the oxygen atom donates one electron to two fluorine atoms and is in the O +2 oxidation state. It is believed that in a molecule or polyatomic ion, more electronegative atoms receive all of the bonding electrons.

Sulfur is an element that exhibits different valencies and oxidation states

Chemical elements of the main subgroups often show the lowest valency equal to VIII. For example, the valence of sulfur in hydrogen sulfide and metal sulfides is II. The element is characterized by intermediate and higher valences in an excited state, when the atom donates one, two, four or all six electrons and exhibits valencies I, II, IV, VI, respectively. The same values, only with the "minus" or "plus" signs, 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 the lowest degree it exhibits strong reducing properties. The S +4 atoms can function as reducing or oxidizing agents in compounds, depending on the conditions.

Transition of electrons in chemical reactions

When a crystal of sodium chloride forms, 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 socializing and shifting electron pairs to a more electronegative atom, only the concept of a formal charge applies. But we can assume that all compounds are composed of ions. Then the atoms, attracting electrons, acquire a conditional negative charge, and when they donate, they acquire a positive one. The reactions indicate how many electrons are shifted. For example, in a molecule of carbon dioxide C +4 O - 2 2, the index in the upper right corner at the chemical symbol of carbon reflects the number of electrons removed from the atom. For oxygen in this substance, the oxidation state is -2. The corresponding index at 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 attached by atoms can be time-consuming. The following rules facilitate this task:

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 get 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 occurs: H-1.
Fluorine is the most electronegative of all elements, and its oxidation state is always -4.
For most metals, oxidation numbers and valences are the same.

Oxidation state and valence

Most of the 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 valence. These values \u200b\u200boften coincide. As a synonym for the term "oxidation state" you can use the phrase "electrochemical valence". 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 -3 oxidation state. 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, living and inanimate nature: combustion, corrosion, fermentation, intracellular respiration, photosynthesis, and other phenomena.

When drawing up the OVR equations, the coefficients are selected using the electronic balance method, in which they operate in the following categories:

oxidation state;
the reducing agent donates electrons and is oxidized;
the oxidant accepts electrons and is reduced;
the number of electrons donated must be equal to the number of electrons attached.

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 oxidative number of an element as a result of reactions. For ORR flowing between ions of strong electrolytes in aqueous solutions, not electronic balance is often used, but the half-reaction method.

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

What is the oxidation state

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

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

Figure: 1. A series of electronegativity.

The oxidation state can have three meanings:

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

How to determine

There is a table listing 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, rarely
		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, rarely +2
		3, +4, +6, rarely +1, +2
		2, +4, +6, rarely +1, +3
		1, +3, rarely +2

		1, +3, rarely +2

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

Or use this version of the table in the lessons.

Figure: 2. Table of oxidation states.

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

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

Figure: 3. Mendeleev's 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 compounds of non-metals (SO 3, CO 2, SiC), the oxidizing agent is a non-metal with a negative oxidation state or with a high value of electronegativity. For example, in compound PCl 3, phosphorus has an oxidation state of +3, chlorine -1. Electronegativity of phosphorus - 2.19, chlorine - 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. 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, they learned about the oxidation state. This is a conditional value that shows how many electrons an atom can give or receive during a chemical reaction. The meaning is associated with electronegativity. Oxidants take electrons and have a negative oxidation state, reducing agents donate electrons and exhibit a positive oxidation state. Most metals are constant or variable oxidation reducing agents. Non-metals can exhibit oxidizing and reducing agent properties 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 a reaction. Determination of this value can be either simple or rather complicated, depending on the atoms and the molecules they consist of. Moreover, the atoms of some elements can have several oxidation states. Fortunately, there are simple unambiguous rules for determining the oxidation state, for the confident use of which a knowledge of the basics of chemistry and algebra is enough.

Steps

Part 1

Determination of the oxidation state 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 separate 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, since both are in a chemically unbound elemental state.
Note that the allotropic form of sulfur S 8, or octacer, despite its atypical structure, is also characterized by a zero oxidation state.

Determine if the substance in question is composed 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 composition of 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.
Please note that metal ions can have several oxidation states. The atoms of many metallic elements can ionize to different amounts. For example, the charge of ions of a metal such as iron (Fe) is +2 or +3. The charge of metal ions (and their oxidation state) 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 denoted 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 the compound contains 3 such ions, for the general neutrality of the substance under consideration, 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 in the case of other elemental substances.
- If oxygen is part of peroxide, its oxidation state is -1. Peroxides are a group of compounds containing a simple oxygen-oxygen bond (ie the anion of peroxide O 2 -2). For example, in the composition of the H 2 O 2 (hydrogen peroxide) molecule, oxygen has a charge and an oxidation state of -1.
- When combined with fluorine, oxygen has an oxidation state of +2, read the rule for fluorine below.
Hydrogen has an oxidation state of +1, with a few exceptions. As with oxygen, there are also exceptions. Typically, 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 because the oxygen atom has a charge of -2, and two +1 charges are required for overall neutrality. Nevertheless, 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 the general electroneutrality the charge of the hydrogen atom (and thus its oxidation state) should be -1.
Fluorine is always has an oxidation state of -1. As already noted, the oxidation state 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 greatest electronegativity - in other words, fluorine atoms are the least willing to part with their own electrons and most actively attract foreign 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 should add up to the charge of this compound. For example, if a compound is neutral, the sum of the oxidation states of all its atoms should 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 test method - if the sum of the oxidation states does not equal the total charge of the compound, then you are wrong somewhere.
Part 2
Determination of the oxidation state without using the laws of chemistry
1. Find atoms that don't have strict rules about oxidation states. For some elements, there are no firmly established rules for finding the oxidation state. If an atom does not fit any of the rules listed above, and you do not know its charge (for example, an atom is a part of a complex, and its charge is not specified), you can determine the oxidation state of such an atom by exclusion. First, determine the charge of all other atoms in the compound, and then, from the known total charge of the compound, calculate the oxidation state of that atom.
  - For example, in the compound Na 2 SO 4, the charge of the sulfur atom (S) is unknown - we only know that it is not zero, since sulfur is not in an elemental state. This compound serves as a good example to illustrate an algebraic method for determining the oxidation state.
2. Find the oxidation states of the remaining 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 for 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 equal the total ionic charge.
4. It is very useful to be able to use the periodic table and to know where the metallic and non-metallic elements are located in it.
5. The oxidation state of atoms in 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 is +2 in elemental form. 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 valence of fluorine atoms is always equal to I

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

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

Al, B Ga, In - are trivalent.

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

ElementVI group, О, has valency II (in H 3 O + - III);
- monovalent F (instead ofVii);
- usually bivalent and trivalent iron, an element of group VIII;
- N can keep only 4 atoms near itself, and not 5, as follows from the group number;
- mono- and bivalent 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 valence of sulfur is 8 - 6 \u003d 2, fluorine and other halogens - (8 - 7) \u003d 1, nitrogen and phosphorus - (8 - 5) \u003d 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 the total number of valencies of one chemical element is equal to the total number of valencies of the atoms of another chemical element). So, in a water molecule H-O-H, the valence of H is equal to I, there are 2 such atoms, which means that there are 2 valence units for hydrogen (1 × 2 \u003d 2). The oxygen valence is of the same importance.

When metals are combined with non-metals, the latter exhibit 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 with each other, the element that is located in Mendeleev's PSHE to the right and above, and the highest, respectively, to the left and below, shows the lowest valency.

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

In the compound formed by the atoms of the three elements, the atom 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 oxidation state 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 ion charge, 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 number of the group of the periodic system 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 for 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 it is with a metal atom, then it is always negative, if with a non-metal, there can be both + and -. When determining oxidation states, the following rules should 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 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 0.

If the compound is formed by two elements, then the element with a higher electronegativity has an oxidation state less than zero, and an element with a lower electronegativity is 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 one is equal to N– 8, where N is the group number.

The oxidation state of fluorine in the 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 - peroxide 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 equal to 0.

Example. Determine the oxidation state in compound K 2 Cr 2 O 7 .
The oxidation states of the two chemical elements potassium and oxygen are constant and equal to +1 and -2, respectively. The number of oxidation states for oxygen is (-2) 7 \u003d (- 14), for potassium (+1) 2 \u003d (+ 2). The number of positive oxidation states is equal to the number of negative ones. Hence (-14) + (+ 2) \u003d (- 12). This means that the chromium atom has 12 positive powers, but there are 2 atoms, which means that there is (+12) per atom: 2 \u003d (+ 6), we write down the oxidation states over the elementsTO + 2 Cr +6 2 O -2 7