In school, chemistry is still one of the most difficult subjects, which, due to the fact that it hides many difficulties, arouses in students (usually in the period from 8 to 9 classes) more hatred and indifference to study than interest. All this reduces the quality and quantity of knowledge on the subject, although many areas still require specialists in this field. Yes, sometimes there are even more difficult moments and incomprehensible rules in chemistry than it seems. One of the questions that concern most students is what is the oxidation state and how to determine the oxidation states of elements.

An important rule is the placement rule, algorithms

There is much talk here about compounds such as oxides. To begin with, every student must learn determination of oxides- These are complex compounds of two elements, they contain oxygen. Oxides are classified as binary compounds because oxygen is second in line in the algorithm. When determining the indicator, it is important to know the placement rules and calculate the algorithm.

Algorithms for Acid Oxides

Oxidation states - these are numerical expressions of the valency of the elements. For example, acid oxides are formed according to a certain algorithm: non-metals or metals come first (their valency is usually from 4 to 7), and then oxygen comes, as it should be, second in order, its valency is two. It is determined easily - according to the periodic table of chemical elements of Mendeleev. It is also important to know that the oxidation state of elements is an indicator that suggests either positive or negative number.

At the beginning of the algorithm, as a rule, a non-metal, and its oxidation state is positive. Non-metal oxygen in oxide compounds has a stable value, which is -2. To determine the correctness of the arrangement of all values, you need to multiply all the available numbers by the indices of one particular element, if the product, taking into account all the minuses and pluses, is 0, then the arrangement is reliable.

Arrangement in acids containing oxygen

Acids are complex substances, they are associated with some acidic residue and contain one or more hydrogen atoms. Here, to calculate the degree, skills in mathematics are required, since the indicators necessary for the calculation are digital. For hydrogen or a proton, it is always the same - +1. The negative oxygen ion has a negative oxidation state of -2.

After carrying out all these actions, you can determine the degree of oxidation and the central element of the formula. The expression for its calculation is a formula in the form of an equation. For example, for sulfuric acid, the equation will be with one unknown.

Basic terms in OVR

ORR is a reduction-oxidation reaction.

• The oxidation state of any atom - characterizes the ability of this atom to attach or give electrons to other atoms of ions (or atoms);
• It is customary to consider either charged atoms or uncharged ions as oxidizing agents;
• The reducing agent in this case will be charged ions or, on the contrary, uncharged atoms that lose their electrons in the process of chemical interaction;
• Oxidation is the donation of electrons.

How to arrange the oxidation state in salts

Salts are composed of one metal and one or more acid residues. The determination procedure is the same as in acid-containing acids.

The metal that directly forms a salt is located in the main subgroup, its degree will be equal to the number of its group, that is, it will always remain a stable, positive indicator.

As an example, consider the arrangement of oxidation states in sodium nitrate. Salt is formed using an element of the main subgroup of group 1, respectively, the oxidation state will be positive and equal to one. In nitrates, oxygen has the same value - -2. In order to get a numerical value, first an equation is drawn up with one unknown, taking into account all the minuses and pluses of the values: +1+X-6=0. By solving the equation, you can come to the fact that the numerical indicator is positive and equal to + 5. This is the indicator of nitrogen. An important key to calculate the degree of oxidation - table.

Arrangement rule in basic oxides

• Oxides of typical metals in any compounds have a stable oxidation index, it is always no more than +1, or in other cases +2;
• The digital indicator of the metal is calculated using the periodic table. If the element is contained in the main subgroup of group 1, then its value will be +1;
• The value of oxides, taking into account their indices, after multiplication, summed up should be equal to zero, because the molecule in them is neutral, a particle devoid of charge;
• Metals of the main subgroup of group 2 also have a stable positive indicator, which is +2.

The oxidation state is the conditional charge of an atom in a molecule, it receives an atom as a result of the complete acceptance of electrons, it is calculated from the assumption that all bonds are ionic in nature. How to determine the degree of oxidation?

Determination of the degree of oxidation

There are charged particles, ions, whose positive charge is equal to the number of electrons received from one atom. The negative charge of an ion is equal to the number of electrons accepted by one atom of a chemical element. For example, the entry of such an element as Ca2 + means that the atoms of the elements have lost one, two or three elements. To find the composition of ionic compounds and compounds of molecules, we need to know how to determine the oxidation state of elements. The oxidation states are negative, positive and zero. If we take into account the number of atoms, then the algebraic oxidation state in the molecule is zero.

To determine the oxidation state of an element, you need to be guided by certain knowledge. For example, in metal compounds, the oxidation state is positive. And the highest oxidation state corresponds to the group number of the periodic system, where the element is located. In metals, oxidation states can be positive or negative. This will depend on the factor by which atom the metal is connected. For example, if it is connected to a metal atom, then the degree will be negative, but if it is connected to a non-metal, then the degree will be positive.

The negative highest oxidation state of the metal can be determined by subtracting the number of the group where the necessary element is located from the number eight. As a rule, it is equal to the number of electrons located on the outer layer. The number of these electrons also corresponds to the group number.

How to Calculate the Oxidation State

In most cases, the oxidation state of an atom of a particular element does not match the number of bonds that it forms, that is, it is not equal to the valence of this element. This can be clearly seen in the example of organic compounds.

Let me remind you that the valency of carbon in organic compounds is 4 (that is, it forms 4 bonds), but the oxidation state of carbon, for example, in methanol CH 3 OH is -2, in CO 2 +4, in CH4 -4, in formic acid HCOOH + 2. Valency is measured by the number of covalent chemical bonds, including those formed by the donor-acceptor mechanism.

When determining the oxidation state of atoms in molecules, an electronegative atom, when one electron pair is displaced in its direction, acquires a charge of -1, but if there are two electron pairs, then -2 will be a charge. The degree of oxidation is not affected by the bond between the same atoms. For example:

• The bond of C-C atoms is equal to their zero oxidation state.
• The C-H bond - here, carbon as the most electronegative atom will correspond to a charge of -1.
• The C-O bond, the charge of carbon, being less electronegative, will be +1.

Examples of determining the degree of oxidation

1. In a molecule such as CH 3 Cl, there are three C-HC bonds). Thus, the oxidation state of the carbon atom in this compound will be equal to: -3 + 1 = -2.
2. Let's find the oxidation state of carbon atoms in the acetaldehyde molecule Cˉ³H3-C¹O-H. In this compound, three C-H bonds will give a total charge on the C atom, which is (Cº+3e→Cˉ³)-3. The double bond C = O (here oxygen will take electrons from the carbon atom, because oxygen is more electronegative) gives a charge on the C atom, it is +2 (Cº-2e → C²), while the C-H bond has a charge of -1, which means the total the charge on atom C is: (2-1=1)+1.
3. Now let's find the oxidation state in the ethanol molecule: Cˉ³H-Cˉ¹H2-OH. Here, three C-H bonds will give a total charge on the C atom, which is (Cº+3e→Cˉ³)-3. Two C-H bonds will give a charge on the C atom, which will be equal to -2, while the C→O bond will give a charge of +1, which means the total charge on the C atom: (-2+1=-1)-1.

Now you know how to determine the oxidation state of an element. If you have at least basic knowledge of chemistry, then this task will not be a problem for you.

In chemistry, the description of various redox processes is not complete without oxidation states - special conditional values ​​with which you can determine the charge of an atom of any chemical element.

If we represent the oxidation state (do not confuse it with valence, since in many cases they do not match) as an entry in a notebook, then we will see just numbers with zero signs (0 - in a simple substance), plus (+) or minus (-) above substance of interest to us. Be that as it may, they play a huge role in chemistry, and the ability to determine CO (oxidation state) is a necessary base in the study of this subject, without which further actions make no sense.

We use CO to describe the chemical properties of a substance (or an individual element), the correct spelling of its international name (understandable for any country and nation, regardless of the language used) and formula, as well as for classification by features.

The degree can be of three types: the highest (to determine it, you need to know which group the element is in), intermediate and lowest (it is necessary to subtract the number of the group in which the element is located from the number 8; naturally, the number 8 is taken because the total in the periodic system D. Mendeleev 8 groups). Details on determining the degree of oxidation and its correct placement will be discussed below.

How oxidation state is determined: constant CO

First, CO can be variable or constant.

Determining the constant oxidation state is not difficult, so it is better to start the lesson with it: for this, you only need the ability to use the PS (periodic system). So, there are a number of certain rules:

1. Zero degree. It was mentioned above that only simple substances have it: S, O2, Al, K, and so on.
2. If the molecules are neutral (in other words, they have no electrical charge), then the sum of their oxidation states is zero. However, in the case of ions, the sum must equal the charge of the ion itself.
3. In I, II, III groups of the periodic table are located mainly metals. The elements of these groups have a positive charge, the number of which corresponds to the group number (+1, +2, or +3). Perhaps the big exception is iron (Fe) - its CO can be both +2 and +3.
4. Hydrogen CO (H) is most often +1 (when interacting with non-metals: HCl, H2S), but in some cases we set -1 (when hydrides are formed in compounds with metals: KH, MgH2).
5. CO oxygen (O) +2. Compounds with this element form oxides (MgO, Na2O, H20 - water). However, there are also cases when oxygen has an oxidation state of -1 (in the formation of peroxides) or even acts as a reducing agent (in combination with fluorine F, because the oxidizing properties of oxygen are weaker).

Based on this information, the oxidation states are placed in a variety of complex substances, redox reactions are described, and so on, but more on that later.

CO variable

Some chemical elements differ in that they have more than one oxidation state and change it depending on which formula they are in. According to the rules, the sum of all powers must also be equal to zero, but to find it, you need to do some calculations. In the written version, it looks like just an algebraic equation, but over time we “fill our hand”, and it is not difficult to compose and quickly execute the entire algorithm of actions mentally.

It will not be so easy to understand the words, and it is better to immediately go to practice:

HNO3 - in this formula, determine the oxidation state of nitrogen (N). In chemistry, we read the names of the elements, and we approach the arrangement of the oxidation states also from the end. So, it is known that CO2 of oxygen is -2. We must multiply the oxidation state by the coefficient on the right (if any): -2*3=-6. Next, we move on to hydrogen (H): its CO in the equation will be +1. This means that in order for the total CO to give zero, you need to add 6. Check: +1+6-7=-0.

Additional exercises can be found at the end, but first of all we need to determine which elements have a variable oxidation state. In principle, all elements, except for the first three groups, change their degrees. The most striking examples are the halogens (elements of group VII, not counting fluorine F), group IV, and the noble gases. Below you will see a list of some metals and non-metals with a variable degree:

• H(+1, -1);
• Be(-3, +1, +2);
• B (-1, +1, +2, +3);
• C (-4, -2, +2, +4);
• N (-3, -1, +1, +3, +5);
• O(-2, -1);
• Mg (+1, +2);
• Si (-4, -3, -2, -1, +2, +4);
• P(-3, -2, -1, +1, +3, +5);
• S (-2, +2, +4, +6);
• Cl (-1, +1, +3, +5, +7).

This is just a small number of items. It takes study and practice to learn how to determine SD, but this does not mean that you need to memorize all the constants and variables of SD: just remember that the latter are much more common. Often, the coefficient and what substance is represented play a significant role - for example, sulfur (S) takes a negative degree in sulfides, oxygen (O) in oxides, and chlorine (Cl) in chlorides. Therefore, in these salts, another element takes a positive degree (and is called a reducing agent in this situation).

Solving problems for determining the degree of oxidation

Now we come to the most important thing - practice. Try the following tasks yourself, and then watch the breakdown of the solution and check the answers:

1. K2Cr2O7 - find the degree of chromium.
   CO for oxygen is -2, for potassium +1, and for chromium we denote for now as an unknown variable x. The total value is 0. Therefore, we will make the equation: +1*2+2*x-2*7=0. After the decision, we get the answer 6. Let's check - everything coincided, which means that the task is solved.
2. H2SO4 - find the degree of sulfur.
   Using the same concept, we make an equation: +2*1+x-2*4=0. Next: 2+x-8=0.x=8-2; x=6.

Brief conclusion

To learn how to determine the oxidation state on your own, you need not only to be able to write equations, but also to thoroughly study the properties of elements of various groups, remember algebra lessons, composing and solving equations with an unknown variable.
Do not forget that the rules have their exceptions and they should not be forgotten: we are talking about elements with a CO variable. Also, to solve many problems and equations, it is necessary to be able to set the coefficients (and to know for what purpose this is done).

Editorial "website"

Instruction

As a result, a complex compound is formed - hydrogen tetrachloraurate. The complexing agent in it is a gold ion, the ligands are chlorine ions, and the outer sphere is a hydrogen ion. How to determine the degree oxidation elements in this complex connection?

First of all, determine which of the elements that make up the molecule is the most electronegative, that is, which will pull the total electron density towards itself. This is chlorine, because it is in the upper right part of the periodic table, and second only to fluorine and oxygen. Therefore, his degree oxidation will be with a minus sign. What is the degree oxidation chlorine?

Chlorine, like all other halogens, is located in the 7th group of the periodic table, there are 7 electrons in its outer electronic level. By dragging another electron to this level, it will move to a stable position. Thus, his degree oxidation will be equal to -1. And since in this complex connection four chloride ions, then the total charge will be -4.

But the sum of the powers oxidation elements that make up the molecule must be equal to zero, because any molecule is electrically neutral. Thus, -4 must be balanced with a positive charge of +4, at the expense of hydrogen and gold.

You will need

• A school textbook in chemistry for grades 8-9 of any author, the periodic table, a table of electronegativity of elements (printed in school textbooks in chemistry).

Instruction

To begin with, it is necessary to indicate that the degree is a concept that takes connections for, that is, does not go deep into the structure. If the element is in a free state, then this is the simplest case - a simple substance is formed, which means that the degree oxidation its equal to zero. For example, hydrogen, oxygen, nitrogen, fluorine, etc.

In complex substances, everything is different: electrons are distributed unevenly between atoms, and it is the degree oxidation helps to determine the number of donated or received electrons. Degree oxidation may be positive or negative. With a plus, electrons are given away, with a minus they are received. Some elements of their degree oxidation are stored in various compounds, but many do not differ in this feature. It is necessary to remember an important rule - the sum of degrees oxidation is always zero. The simplest example, CO gas: knowing that the degree oxidation oxygen in the vast majority of cases is -2 and using the above rule, you can calculate the degree oxidation for C. In sum with -2, zero gives only +2, which means the degree oxidation carbon +2. Let's complicate the problem and take CO2 gas for calculations: the degree oxidation oxygen still remains -2, but in this case there are two molecules of it. Therefore, (-2) * 2 = (-4). A number that adds up to -4 to zero, +4, that is, in this gas it has a degree oxidation+4. A more complicated example: H2SO4 - hydrogen has a degree oxidation+1, oxygen has -2. In the given compound, there are 2 hydrogens and 4 oxygens, i.e. will be, respectively, +2 and -8. In order to get a total of zero, you need to add 6 pluses. So the degree oxidation sulfur +6.

When it is difficult to determine in a compound where the plus is, where the minus is, electronegativity is needed (it is easy to find in a general textbook). Metals often have a positive degree oxidation, while non-metals are negative. But for example, PI3 - both elements are non-metals. The table indicates that the electronegativity of iodine is 2.6, and 2.2. When compared, it turns out that 2.6 is greater than 2.2, that is, electrons are pulled towards iodine (iodine has a negative degree oxidation). Following the given simple examples, it is easy to determine the degree oxidation any element in the connections.

note

No need to confuse metals and non-metals, then the oxidation state will be easier to find and not get confused.

Degree oxidation called the conditional charge of an atom in a molecule. It is assumed that all bonds are ionic. In other words, oxidation characterizes the ability of an element to form an ionic bond.

You will need

• - periodic table.

Instruction

In a compound, the sum of the powers of the atoms is equal to the charge of that compound. This means that in a simple substance, for example, Na or H2, the degree oxidation element is zero.

Degree oxidation oxygen in compounds is usually -2. For example, H2O water has two hydrogen atoms and one oxygen atom. Indeed, -2+1+1 = 0 - on the left side of the expression is the sum of the powers oxidation all the atoms in the compound. In CaO, calcium has a degree oxidation+2, and - -2. Exceptions to this are OF2 and H2O2 compounds.
Y degree oxidation is always -1.

Usually the maximum positive degree oxidation element matches the number of its group in Mendeleev's periodic table of elements. Max Degree oxidation is equal to the element minus eight. An example is chlorine in the seventh group. 7-8 = -1 - degree oxidation. The exception to this rule is fluorine, oxygen and iron - the highest degree oxidation below their group number. The elements of the copper subgroup have the highest degree oxidation more than 1.

Sources:

• The oxidation state of elements in 2018

Degree oxidation element is the conditional charge of the atoms of a chemical element in a compound, calculated from the assumption that the compounds consist only of ions. They can have positive, negative, zero values. Metals have positive oxidation states, while non-metals can have both positive and negative oxidation states. It depends on which atom the nonmetal atom is connected to.

Instruction

note

The oxidation state can have fractional values, for example, in magnetic iron ore, Fe2O3 is +8/3.

Sources:

• "Manual in Chemistry", G.P. Khomchenko, 2005.

The degree of oxidation is a characteristic of elements often found in chemistry textbooks. There are a large number of tasks aimed at determining this degree, and many of them cause difficulties for schoolchildren and students. But by following a certain algorithm, these difficulties can be avoided.

You will need

• - periodic system of chemical elements (table D.I. Mendeleev).

Instruction

Remember one general rule: any element in a simple substance is equal to zero (simple substances: Na, Mg, Al, - ie substances consisting of one element). To determine a substance, first simply write it down without losing the indices - the numbers in the lower right part next to the symbol of the element. An example would be sulfuric - H2SO4.

Next, open the table D.I. Mendeleev and find the degree of the leftmost element in your substance - in the case of this example. According to the existing rule, its oxidation state will always be positive, and it is written with a “+” sign, since it occupies the extreme left position in the formula of a substance. To determine the numerical value of the oxidation state, pay attention to the location of the element relative to the groups. Hydrogen is in the first group, therefore, its oxidation state is +1, but since there are two hydrogen atoms in sulfuric acid (this is shown to us by the index), write +2 above its symbol.

After that, determine the oxidation state of the rightmost element in the record - oxygen in this case. Its conditional (or oxidation state) will always be negative, since it occupies the right position in the substance notation. This rule is true in all cases. The numerical value of the right element is found by subtracting the number 8 from its group number. In this case, the oxidation state of oxygen is -2 (6-8=-2), taking into account the index - -8.

To find the conditional charge of an atom of the third element, use the rule - the sum of the oxidation states of all elements must be equal to zero. Hence, the conditional charge of the oxygen atom in the substance will be equal to +6: (+2)+(+6)+(-8)=0. After that, write +6 above the sulfur symbol.

Sources:

• as the oxidation states of chemical elements

Phosphorus is a chemical element that has the 15th serial number in the periodic table. It is located in her V group. A classic non-metal discovered by the alchemist Brand in 1669. There are three main modifications of phosphorus: red (which is part of the mixture for lighting matches), white and black. At very high pressures (of the order of 8.3 * 10^10Pa), black phosphorus passes into another allotropic state (“metallic phosphorus”) and begins to conduct current. phosphorus in various substances?

Instruction

Remember degree. This is the value corresponding to the charge of the ion in the molecule, provided that the electron pairs that carry out the bond are shifted towards the more electronegative element (located to the right and above in the Periodic Table).

It is also necessary to know the main condition: the sum of the electric charges of all the ions that make up the molecule, taking into account the coefficients, must always be equal to zero.

The oxidation state does not always quantitatively coincide with the valency. The best example is carbon, which in organics always has , equal to 4, and the oxidation state can be equal to -4, and 0, and +2, and +4.

What is the oxidation state in a phosphine PH3 molecule, for example? With all that said, this question is very easy to answer. Since hydrogen is the very first element in the Periodic Table, it, by definition, cannot be located there "more to the right and higher" than. Therefore, it is phosphorus that will attract hydrogen electrons to itself.

Each hydrogen atom, having lost an electron, will turn into a positively charged oxidation ion +1. Therefore, the total positive charge is +3. Hence, taking into account the rule that the total charge of the molecule is zero, the oxidation state of phosphorus in the phosphine molecule is -3.

Well, what is the oxidation state of phosphorus in P2O5 oxide? Take the periodic table. Oxygen is located in group VI, to the right of phosphorus, and also higher, therefore, it is definitely more electronegative. That is, the oxidation state of oxygen in this compound will be with a minus sign, and phosphorus with a plus sign. What are these degrees so that the molecule as a whole is neutral? It can be easily seen that the least common multiple of the numbers 2 and 5 is 10. Therefore, the oxidation state of oxygen is -2, and that of phosphorus is +5.

When studying ionic and covalent polar chemical bonds, you got acquainted with complex substances consisting of two chemical elements. Such substances are called bi-pair (from Latin bi - “two”) or two-element.

Let us recall the typical binary compounds that we cited as an example to consider the mechanisms for the formation of ionic and covalent polar chemical bonds: NaHl - sodium chloride and HCl - hydrogen chloride. In the first case, the bond is ionic: the sodium atom transferred its outer electron to the chlorine atom and turned into an ion with a charge of -1. and the chlorine atom accepted an electron and turned into an ion with a charge of -1. Schematically, the process of transformation of atoms into ions can be depicted as follows:

In the HCl molecule, the bond is formed due to the pairing of unpaired outer electrons and the formation of a common electron pair of hydrogen and chlorine atoms.

It is more correct to represent the formation of a covalent bond in a hydrogen chloride molecule as an overlap of a one-electron s-cloud of a hydrogen atom with a one-electron p-cloud of a chlorine atom:

During chemical interaction, the common electron pair is shifted towards the more electronegative chlorine atom:

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

is the conditional charge of the atoms of a chemical element in a compound, calculated on the basis of the assumption that all compounds (both ionic and covalently polar) consist only of ions.

The oxidation state can have a negative, positive, or zero value, which is usually placed above the element symbol at the top, for example:

Those atoms that have received electrons from other atoms or to which common electron pairs are displaced, that is, atoms of more electronegative elements, have a negative value for the degree of oxidation. Fluorine always has an oxidation state of -1 in all compounds. Oxygen, the second most electronegative element after fluorine, almost always has an oxidation state of -2, except for compounds with fluorine, for example:

Those atoms that donate their electrons to other atoms or from which common electron pairs are drawn, that is, atoms of less electronegative elements, have a positive oxidation state. Metals always have a positive oxidation state. For metals of the main subgroups:

Group I in all compounds, the oxidation state is +1,
Group II is equal to +2. Group III - +3, for example:

In compounds, the total oxidation state is always zero. Knowing this and the oxidation state of one of the elements, you can always find the oxidation state of another element using the formula of a binary compound. For example, let's find the oxidation state of chlorine in the compound Cl2O2. Let's denote the oxidation state -2
oxygen: Cl2O2. Therefore, seven oxygen atoms will have a total negative charge (-2) 7 =14. Then the total charge of two chlorine atoms will be +14, and one chlorine atom:
(+14):2 = +7.

Similarly, knowing the oxidation states of the elements, one can formulate the formula of a compound, for example, aluminum carbide (a compound of aluminum and carbon). Let's write the signs of aluminum and carbon next to AlC, and first the sign of aluminum, since it is a metal. We determine the number of external electrons from the periodic table of elements: Al has 3 electrons, C has 4. An aluminum atom will give up its 3 external electrons to carbon and receive an oxidation state of +3, equal to the charge of the ion. The carbon atom, on the contrary, will take the 4 electrons missing to the "cherished eight" and will receive an oxidation state of -4.

Let's write these values ​​in the formula: AlС, and find the least common multiple for them, it is equal to 12. Then we calculate the indices:

Knowing the oxidation states of elements is also necessary in order to be able to correctly name a chemical compound.

Names of binary compounds consist of two words - the names of the chemical elements that form them. The first word denotes the electronegative part of the compound - non-metal, its Latin name with the suffix -id is always in the nominative case. The second word denotes the electropositive part - a metal or a less electronegative element, its name is always in the genitive case. If the electropositive element exhibits different degrees of oxidation, then this is reflected in the name, indicating the degree of oxidation with a Roman numeral, which is placed at the end.

In order for chemists from different countries to understand each other, it was necessary to create a unified terminology and nomenclature of substances. The principles of chemical nomenclature were first developed by French chemists A. Lavoisier, A. Fourctua, L. Giton and C. Berthollet in 1785. At present, the International Union of Pure and Applied Chemistry (IUPAC) coordinates the activities of scientists from several countries and issues recommendations on the nomenclature of substances and terminology used in chemistry.