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 the value attributed to one or more atoms. numerical value characterizing the number of redistributed electrons and showing 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.

  1. 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.

  2. 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 the compound contains 3 such ions, 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.

  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.

  4. 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.

  5. 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 explained by 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.

  6. The sum of the oxidation states in a compound is equal to its charge. The oxidation states of all the atoms in 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 good method checks - if the sum of the oxidation states is not equal to the total charge of the compound, then you made a mistake 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 fall under 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 connection serves good example to illustrate the algebraic method for determining the degree of oxidation.

    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. Find the unknown oxidation state from the charge of the compound. Now you have all the data for a simple calculation of the desired oxidation state. Write down an equation, on the left side of which there will be the sum of the number obtained in the previous calculation step and the unknown oxidation state, and on the right side - the total charge of the compound. In other words, (Sum of known oxidation states) + (desired oxidation state) = (compound charge).

      • In our case Na 2 SO 4 the solution looks like this:
        • (Sum of known oxidation states) + (desired oxidation state) = (compound charge)
        • -6+S=0
        • S=0+6
        • S = 6. In Na 2 SO 4, sulfur has an oxidation state 6 .
    • 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.
    • 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.
    • 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 species chemical bond, may have 2 different meanings degree of oxidation.

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When defining this concept, it is conditionally assumed that the binding (valence) electrons pass to more electronegative atoms (see Electronegativity), and therefore the compounds consist, as it were, of positively and negatively charged ions. The oxidation state can be zero, negative and positive value, which are usually placed above the element symbol at the top.

The zero value of the oxidation state is assigned to the atoms of the elements in the free state, for example: Cu, H 2 , N 2 , P 4 , S 6 . Negative meaning oxidation states are those atoms towards which the binding electron cloud (electron pair) is displaced. For fluorine in all its compounds, it is -1. Atoms that donate valence electrons to other atoms have a positive oxidation state. For example, for alkali and alkaline earth metals, it is respectively +1 and +2. In simple ions like Cl − , S 2− , K + , Cu 2+ , Al 3+ , it is equal to the charge of the ion. In most compounds, the oxidation state of hydrogen atoms is +1, but in metal hydrides (their compounds with hydrogen) - NaH, CaH 2 and others - it is -1. For oxygen, the oxidation state is -2, but, for example, in combination with fluorine OF 2 it will be +2, and in peroxide compounds (BaO 2, etc.) -1. In some cases, this value can also be expressed as a fractional number: for iron in iron oxide (II, III) Fe 3 O 4 it is equal to +8/3.

The algebraic sum of the oxidation states of atoms in a compound is zero, and in a complex ion it is the charge of the ion. Using this rule, we calculate, for example, the oxidation state of phosphorus in phosphoric acid H 3 PO 4 . Denoting it by x and multiplying the oxidation state for hydrogen (+1) and oxygen (−2) by the number of their atoms in the compound, we get the equation: (+1) 3+x+(−2) 4=0, whence x=+5 . Similarly, we calculate the oxidation state of chromium in the Cr 2 O 7 2− ion: 2x+(−2) 7=−2; x=+6. In the compounds MnO, Mn 2 O 3, MnO 2, Mn 3 O 4, K 2 MnO 4, KMnO 4, the oxidation state of manganese will be +2, +3, +4, +8/3, +6, +7, respectively.

The highest oxidation state is its highest positive value. For most elements, it is equal to the group number in the periodic system and is an important quantitative characteristic of the element in its compounds. Lowest value the oxidation state of an element that occurs in its compounds is commonly called the lowest oxidation state; all others are intermediate. Yes, for sulfur highest degree oxidation is +6, the lowest is -2, the intermediate is +4.

Change in the oxidation states of elements by groups periodic system reflects the frequency of their change chemical properties with increasing serial number.

The concept of the oxidation state of elements is used in the classification of substances, describing their properties, formulating compounds and their international names. But it is especially widely used in the study of redox reactions. The concept of "oxidation state" is often used in inorganic chemistry instead of the concept of "valency" (cf.

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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 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 number is total electrons that have been removed from atoms. The negative value of the oxidation state is equal to the number of acquired electrons. By change in oxidation state chemical element judge 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 made up of chemical elements with the same value EO. In this case, the bonding electrons belong to all structural particles equally. Therefore, in simple substances the elements do not have an oxidation state (H 0 2, O 0 2, C 0). When atoms accept electrons or common 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 highest valency in an 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

When a crystal is formed table salt 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:

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