What is the highest oxidation state. Rules for determining the degree of oxidation of chemical elements methodological development in chemistry (grade 8) on the topic

In many school textbooks and manuals, they teach how to write formulas for valencies, even for compounds with ionic bonds. To simplify the procedure for compiling formulas, this, in our opinion, is acceptable. But you need to understand that this is not entirely correct due to the above reasons.

A more universal concept is the concept of the degree of oxidation. By the values of the oxidation states of atoms, as well as by the values of valence, chemical formulas can be compiled and formula units can be written down.

Oxidation state is the conditional charge of an atom in a particle (molecule, ion, radical), calculated in the approximation that all bonds in the particle are ionic.

Before determining the oxidation states, it is necessary to compare the electronegativity of the bonding atoms. An atom with a higher electronegativity has a negative oxidation state, while an atom with a lower electronegativity has a positive one.

In order to objectively compare the electronegativity values of atoms when calculating oxidation states, in 2013 IUPAC recommended using the Allen scale.

* So, for example, on the Allen scale, the electronegativity of nitrogen is 3.066, and chlorine is 2.869.

Let us illustrate the above definition with examples. Let's make a structural formula of a water molecule.

Covalent polar O-H bonds are shown in blue.

Imagine that both bonds are not covalent, but ionic. If they were ionic, then one electron would pass from each hydrogen atom to the more electronegative oxygen atom. We denote these transitions with blue arrows.

*In thatexample, the arrow serves to illustrate the complete transfer of electrons, and not to illustrate the inductive effect.

It is easy to see that the number of arrows shows the number of transferred electrons, and their direction - the direction of electron transfer.

Two arrows are directed to the oxygen atom, which means that two electrons pass to the oxygen atom: 0 + (-2) = -2. An oxygen atom has a charge of -2. This is the degree of oxidation of oxygen in a water molecule.

One electron leaves each hydrogen atom: 0 - (-1) = +1. This means that hydrogen atoms have an oxidation state of +1.

The sum of the oxidation states is always equal to the total charge of the particle.

For example, the sum of oxidation states in a water molecule is: +1(2) + (-2) = 0. A molecule is an electrically neutral particle.

If we calculate the oxidation states in an ion, then the sum of the oxidation states, respectively, is equal to its charge.

The value of the oxidation state is usually indicated in the upper right corner of the element symbol. Moreover, the sign is written in front of the number. If the sign is after the number, then this is the charge of the ion.

For example, S -2 is a sulfur atom in the oxidation state -2, S 2- is a sulfur anion with a charge of -2.

S +6 O -2 4 2- - the values of the oxidation states of atoms in the sulfate anion (the charge of the ion is highlighted in green).

Now consider the case where the compound has mixed bonds: Na 2 SO 4 . The bond between the sulfate anion and sodium cations is ionic, the bonds between the sulfur atom and oxygen atoms in the sulfate ion are covalent polar. We write down the graphical formula for sodium sulfate, and the arrows indicate the direction of electron transition.

*The structural formula reflects the order of covalent bonds in a particle (molecule, ion, radical). Structural formulas are used only for particles with covalent bonds. For particles with ionic bonds, the concept of a structural formula is meaningless. If there are ionic bonds in the particle, then the graphic formula is used.

We see that six electrons leave the central sulfur atom, which means that the oxidation state of sulfur is 0 - (-6) = +6.

The terminal oxygen atoms take two electrons each, which means their oxidation states are 0 + (-2) = -2

Bridge oxygen atoms accept two electrons each, their oxidation state is -2.

It is also possible to determine the degree of oxidation by the structural-graphic formula, where the dashes indicate covalent bonds, and the ions indicate the charge.

In this formula, the bridging oxygen atoms already have unit negative charges and an additional electron comes to them from the sulfur atom -1 + (-1) = -2, which means their oxidation states are -2.

The oxidation state of sodium ions is equal to their charge, i.e. +1.

Let us determine the oxidation states of elements in potassium superoxide (superoxide). To do this, we will draw up a graphical formula for potassium superoxide, we will show the redistribution of electrons with an arrow. The O-O bond is covalent non-polar, so the redistribution of electrons is not indicated in it.

* The superoxide anion is a radical ion. The formal charge of one oxygen atom is -1, and the other, with an unpaired electron, is 0.

We see that the oxidation state of potassium is +1. The oxidation state of the oxygen atom written in the formula opposite potassium is -1. The oxidation state of the second oxygen atom is 0.

In the same way, it is possible to determine the degree of oxidation by the structural-graphic formula.

The circles indicate the formal charges of the potassium ion and one of the oxygen atoms. In this case, the values of formal charges coincide with the values of the oxidation states.

Since both oxygen atoms in the superoxide anion have different oxidation states, we can calculate arithmetic mean oxidation state oxygen.

It will be equal to / 2 \u003d - 1/2 \u003d -0.5.

The values of the arithmetic mean oxidation states are usually indicated in gross formulas or formula units to show that the sum of the oxidation states is equal to the total charge of the system.

For the case with superoxide: +1 + 2(-0.5) = 0

It is easy to determine the oxidation states using electron point formulas, in which lone electron pairs and electrons of covalent bonds are indicated by dots.

Oxygen is an element of the VIA group, therefore there are 6 valence electrons in its atom. Imagine that the bonds in the water molecule are ionic, in which case the oxygen atom would receive an octet of electrons.

The oxidation state of oxygen is respectively equal to: 6 - 8 \u003d -2.

And hydrogen atoms: 1 - 0 = +1

The ability to determine the degree of oxidation using graphic formulas is invaluable for understanding the essence of this concept, as this skill will be required in the course of organic chemistry. If we are dealing with inorganic substances, then it is necessary to be able to determine the degree of oxidation by molecular formulas and formula units.

To do this, first of all, you need to understand that the oxidation states are constant and variable. Elements that exhibit a constant oxidation state must be memorized.

Any chemical element is characterized by higher and lower oxidation states.

Lowest oxidation state is the charge that an atom acquires as a result of receiving the maximum number of electrons on the outer electron layer.

In view of this, the lowest oxidation state is negative, with the exception of metals, whose atoms never take electrons due to low electronegativity values. Metals have the lowest oxidation state of 0.

Most nonmetals of the main subgroups try to fill their outer electron layer with up to eight electrons, after which the atom acquires a stable configuration ( octet rule). Therefore, in order to determine the lowest oxidation state, it is necessary to understand how many valence electrons an atom lacks to an octet.

For example, nitrogen is an element of the VA group, which means that there are five valence electrons in the nitrogen atom. The nitrogen atom is three electrons short of an octet. So the lowest oxidation state of nitrogen is: 0 + (-3) = -3

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 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.
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.

Such a subject of the school curriculum as chemistry causes numerous difficulties for most modern schoolchildren, few people can determine the degree of oxidation in compounds. The greatest difficulties are for schoolchildren who study, that is, students of the main school (grades 8-9). Misunderstanding of the subject leads to the emergence of hostility among students to this subject.

Teachers identify a number of reasons for such a “dislike” of middle and high school students for chemistry: unwillingness to understand complex chemical terms, inability to use algorithms to consider a specific process, problems with mathematical knowledge. The Ministry of Education of the Russian Federation has made serious changes to the content of the subject. In addition, the number of hours for teaching chemistry was "cut down". This had a negative impact on the quality of knowledge in the subject, a decrease in interest in the study of the discipline.

What topics of the chemistry course are the most difficult for schoolchildren?

According to the new program, the course of the discipline "Chemistry" of the basic school includes several serious topics: the periodic table of elements of D. I. Mendeleev, classes of inorganic substances, ion exchange. The hardest thing is for eighth graders to determine the degree of oxidation of oxides.

Placement rules

First of all, students should know that oxides are complex two-element compounds that include oxygen. A prerequisite for a binary compound to belong to the class of oxides is the second position of oxygen in this compound.

Algorithm for Acid Oxides

To begin with, we note that the degrees are numerical expressions of the valency of elements. Acid oxides are formed by non-metals or metals with a valence of four to seven, the second in such oxides is necessarily oxygen.

In oxides, the valency of oxygen always corresponds to two; it can be determined from the periodic table of elements of D. I. Mendeleev. Such a typical non-metal as oxygen, being in the 6th group of the main subgroup of the periodic table, accepts two electrons in order to completely complete its external energy level. Non-metals in compounds with oxygen most often exhibit a higher valence, which corresponds to the number of the group itself. It is important to recall that the oxidation state of chemical elements is an indicator that implies a positive (negative) number.

The non-metal at the beginning of the formula has a positive oxidation state. Non-metal oxygen is stable in oxides, its index is -2. In order to check the reliability of the arrangement of values in acid oxides, you will have to multiply all the numbers you set by the indices of a particular element. Calculations are considered reliable if the total sum of all the pluses and minuses of the set degrees is 0.

Compilation of two-element formulas

The oxidation state of the atoms of the elements gives a chance to create and record compounds from two elements. When creating a formula, for starters, both symbols are written side by side, be sure to put oxygen second. Above each of the recorded signs, the values \u200b\u200bof the oxidation states are prescribed, then between the numbers found is the number that will be divisible by both digits without any remainder. This indicator must be divided separately by the numerical value of the degree of oxidation, obtaining indices for the first and second components of the two-element substance. The highest oxidation state is numerically equal to the value of the highest valence of a typical non-metal, identical to the group number where the non-metal stands in PS.

Algorithm for setting numerical values in basic oxides

Oxides of typical metals are considered to be such compounds. They in all compounds have an oxidation state index of no more than +1 or +2. In order to understand what the oxidation state of a metal will be, you can use the periodic table. For metals of the main subgroups of the first group, this parameter is always constant, it is similar to the group number, that is, +1.

Metals of the main subgroup of the second group are also characterized by a stable oxidation state, numerically +2. The oxidation states of oxides, taking into account their indices (numbers), should add up to zero, since the chemical molecule is considered to be a neutral, charge-free particle.

Arrangement of oxidation states in oxygen-containing acids

Acids are complex substances, consisting of one or more hydrogen atoms, which are associated with some kind of acidic residue. Given that oxidation states are numbers, some math skills are required to calculate them. Such an indicator for hydrogen (proton) in acids is always stable, it is +1. Next, you can specify the oxidation state for the negative oxygen ion, it is also stable, -2.

Only after these actions, it is possible to calculate the degree of oxidation of the central component of the formula. As a specific sample, consider the determination of the oxidation state of elements in sulfuric acid H2SO4. Given that the molecule of this complex substance contains two hydrogen protons, 4 oxygen atoms, we obtain an expression of this form +2+X-8=0. In order for the sum to form zero, sulfur will have an oxidation state of +6

Arrangement of oxidation states in salts

Salts are complex compounds consisting of metal ions and one or more acid residues. The procedure for determining the oxidation states of each of the constituents in a complex salt is the same as in oxygen-containing acids. Given that the oxidation state of the elements is a numerical indicator, it is important to correctly indicate the oxidation state of the metal.

If the salt-forming metal is located in the main subgroup, its oxidation state will be stable, corresponds to the group number, is a positive value. If the salt contains a metal of a similar subgroup of PS, it is possible to show different metals by the acid residue. After the oxidation state of the metal is set, put (-2), then the oxidation state of the central element is calculated using the chemical equation.

As an example, consider the determination of the oxidation states of elements in (medium salt). NaNO3. The salt is formed by a metal of the main subgroup of group 1, therefore, the oxidation state of sodium will be +1. Oxygen in nitrates has an oxidation state of -2. To determine the numerical value of the degree of oxidation is the equation +1+X-6=0. Solving this equation, we get that X should be +5, this is

Basic terms in OVR

For the oxidative as well as the reduction process, there are special terms that students are required to learn.

The oxidation state of an atom is its direct ability to attach to itself (donate to others) electrons from some ions or atoms.

An oxidizing agent is considered to be neutral atoms or charged ions that acquire electrons during a chemical reaction.

The reducing agent will be uncharged atoms or charged ions, which in the process of chemical interaction lose their own electrons.

Oxidation is presented as a procedure for donating electrons.

Reduction is associated with the acceptance of additional electrons by an uncharged atom or ion.

The redox process is characterized by a reaction during which the oxidation state of an atom necessarily changes. This definition allows you to understand how you can determine whether the reaction is OVR.

OVR Parsing Rules

Using this algorithm, you can arrange the coefficients in any chemical reaction.

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 electrical neutrality rule, 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.

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