To characterize the state of elements in compounds, the concept of the degree of oxidation has been introduced.
DEFINITION
The number of electrons displaced from an atom of a given element or to an atom of a given element in a compound is called oxidation state.
A positive oxidation state indicates the number of electrons that are displaced from a given atom, and a negative oxidation state indicates the number of electrons that are displaced towards a given atom.
From this definition it follows that in compounds with non-polar bonds, the oxidation state of the elements is zero. Molecules consisting of identical atoms (N 2 , H 2 , Cl 2) can serve as examples of such compounds.
The oxidation state of metals in the elementary state is zero, since the distribution of electron density in them is uniform.
In simple ionic compounds, the oxidation state of their constituent elements is equal to the electric charge, since during the formation of these compounds, an almost complete transfer of electrons from one atom to another occurs: Na +1 I -1, Mg +2 Cl -1 2, Al +3 F - 1 3 , Zr +4 Br -1 4 .
When determining the degree of oxidation of elements in compounds with polar covalent bonds, the values of their electronegativity are compared. Since, during the formation of a chemical bond, electrons are displaced to atoms of more electronegative elements, the latter have a negative oxidation state in compounds.
Highest oxidation state
For elements that exhibit different oxidation states in their compounds, there are concepts of higher (maximum positive) and lower (minimum negative) oxidation states. The highest oxidation state of a chemical element usually numerically coincides with the group number in the Periodic system of D. I. Mendeleev. The exceptions are fluorine (the oxidation state is -1, and the element is located in group VIIA), oxygen (the oxidation state is +2, and the element is located in group VIA), helium, neon, argon (the oxidation state is 0, and the elements are located in group VIII group), as well as elements of the cobalt and nickel subgroups (the oxidation state is +2, and the elements are located in group VIII), for which the highest oxidation state is expressed by a number whose value is lower than the number of the group to which they belong. The elements of the copper subgroup, on the contrary, have a higher oxidation state of more than one, although they belong to group I (the maximum positive oxidation state of copper and silver is +2, gold +3).
Examples of problem solving
EXAMPLE 1
- In hydrogen sulfide, the oxidation state of sulfur is (-2), and in a simple substance - sulfur - 0:
Change in the oxidation state of sulfur: -2 → 0, i.e. sixth answer.
- In a simple substance - sulfur - the oxidation state of sulfur is 0, and in SO 3 - (+6):
Change in the oxidation state of sulfur: 0 → +6, i.e. fourth answer.
- In sulfurous acid, the oxidation state of sulfur is (+4), and in a simple substance - sulfur - 0:
1×2 +x+ 3×(-2) =0;
Change in the oxidation state of sulfur: +4 → 0, i.e. third answer.
EXAMPLE 2
| Exercise | Valency III and oxidation state (-3) nitrogen shows in the compound: a) N 2 H 4; b) NH3; c) NH 4 Cl; d) N 2 O 5 |
| Solution | In order to give a correct answer to the question posed, we will alternately determine the valency and oxidation state of nitrogen in the proposed compounds. a) the valency of hydrogen is always equal to I. The total number of hydrogen valency units is 4 (1 × 4 = 4). Divide the value obtained by the number of nitrogen atoms in the molecule: 4/2 \u003d 2, therefore, the nitrogen valency is II. This answer is incorrect. b) the valency of hydrogen is always equal to I. The total number of hydrogen valence units is 3 (1 × 3 = 3). We divide the obtained value by the number of nitrogen atoms in the molecule: 3/1 \u003d 2, therefore, the nitrogen valency is III. The oxidation state of nitrogen in ammonia is (-3): This is the correct answer. |
| Answer | Option (b) |
Electronegativity (EO) is the ability of atoms to attract electrons when they bond with other atoms .
Electronegativity depends on the distance between the nucleus and valence electrons, and on how close the valence shell is to completion. The smaller the radius of an atom and the more valence electrons, the higher its EC.
Fluorine is the most electronegative element. Firstly, it has 7 electrons in the valence shell (only 1 electron is missing before an octet) and, secondly, this valence shell (…2s 2 2p 5) is located close to the nucleus.
The least electronegative atoms are alkali and alkaline earth metals. They have large radii and their outer electron shells are far from complete. It is much easier for them to give their valence electrons to another atom (then the pre-outer shell will become complete) than to “gain” electrons.
Electronegativity can be expressed quantitatively and line up the elements in ascending order. The electronegativity scale proposed by the American chemist L. Pauling is most often used.
The difference in the electronegativity of the elements in the compound ( ΔX) will allow us to judge the type of chemical bond. If the value ∆ X= 0 - connection covalent non-polar.
When the electronegativity difference is up to 2.0, the bond is called covalent polar, for example: the H-F bond in the HF hydrogen fluoride molecule: Δ X \u003d (3.98 - 2.20) \u003d 1.78
Bonds with an electronegativity difference greater than 2.0 are considered ionic. For example: the Na-Cl bond in the NaCl compound: Δ X \u003d (3.16 - 0.93) \u003d 2.23.
Oxidation state
Oxidation state (CO) is the conditional charge of an atom in a molecule, calculated on the assumption that the molecule consists of ions and is generally electrically neutral.
When an ionic bond is formed, an electron passes from a less electronegative atom to a more electronegative one, the atoms lose their electrical neutrality and turn into ions. there are integer charges. When a covalent polar bond is formed, the electron does not transfer completely, but partially, so partial charges arise (in the figure below, HCl). Let's imagine that the electron passed completely from the hydrogen atom to chlorine, and a whole positive charge +1 appeared on hydrogen, and -1 on chlorine. such conditional charges are called the oxidation state.
This figure shows the oxidation states characteristic of the first 20 elements.
Note. The highest SD is usually equal to the group number in the periodic table. Metals of the main subgroups have one characteristic CO, non-metals, as a rule, have a spread of CO. Therefore, non-metals form a large number of compounds and have more "diverse" properties compared to metals.
Examples of determining the degree of oxidation
Let's determine the oxidation states of chlorine in compounds:
The rules that we have considered do not always allow us to calculate the CO of all elements, as, for example, in a given aminopropane molecule.
Here it is convenient to use the following method:
1) We depict the structural formula of the molecule, the dash is a bond, a pair of electrons.
2) We turn the dash into an arrow directed to a more EO atom. This arrow symbolizes the transition of an electron to an atom. If two identical atoms are connected, we leave the line as it is - there is no transfer of electrons.
3) We count how many electrons "came" and "left".
For example, consider the charge on the first carbon atom. Three arrows are directed towards the atom, which means that 3 electrons have arrived, the charge is -3.
The second carbon atom: hydrogen gave it an electron, and nitrogen took one electron. The charge has not changed, it is equal to zero. Etc.
Valence
Valence(from Latin valēns "having force") - the ability of atoms to form a certain number of chemical bonds with atoms of other elements.
Basically, valency means the ability of atoms to form a certain number of covalent bonds. If an atom has n unpaired electrons and m lone electron pairs, then this atom can form n+m covalent bonds with other atoms, i.e. its valence will be n+m. When evaluating the maximum valency, one should proceed from the electronic configuration of the "excited" state. For example, the maximum valency of an atom of beryllium, boron and nitrogen is 4 (for example, in Be (OH) 4 2-, BF 4 - and NH 4 +), phosphorus - 5 (PCl 5), sulfur - 6 (H 2 SO 4) , chlorine - 7 (Cl 2 O 7).
In some cases, the valence may numerically coincide with the oxidation state, but in no way are they identical to each other. For example, in N 2 and CO molecules, a triple bond is realized (that is, the valence of each atom is 3), but the oxidation state of nitrogen is 0, carbon +2, oxygen -2.
In nitric acid, the oxidation state of nitrogen is +5, while nitrogen cannot have a valency higher than 4, because it has only 4 orbitals at the outer level (and the bond can be considered as overlapping orbitals). And in general, any element of the second period, for the same reason, cannot have a valency greater than 4.
A few more "tricky" questions in which mistakes are often made.
Table. Degrees of oxidation of chemical elements.
Table. Degrees of oxidation of chemical elements.
Oxidation state is the conditional charge of the atoms of a chemical element in a compound, calculated from the assumption that all bonds are of the ionic type. The oxidation states can have a positive, negative or zero value, therefore the algebraic sum of the oxidation states of elements in a molecule, taking into account the number of their atoms, is 0, and in an ion - the charge of the ion.
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Table: Elements with constant oxidation states. |
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Table. The oxidation states of chemical elements in alphabetical order.
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Table. The oxidation states of chemical elements by number.
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In chemical processes, the main role is played by atoms and molecules, the properties of which determine the outcome of chemical reactions. One of the important characteristics of an atom is the oxidation number, which simplifies the method of taking into account the transfer of electrons in a particle. How to determine the oxidation state or the formal charge of a particle and what rules do you need to know for this?
Any chemical reaction is due to the interaction of atoms of various substances. The reaction process and its result depend on the characteristics of the smallest particles.
The term oxidation (oxidation) in chemistry means a reaction during which a group of atoms or one of them lose electrons or gain, in the case of acquisition, the reaction is called "reduction".
The oxidation state is a quantity that is measured quantitatively and characterizes the redistributed electrons during the reaction. Those. in the process of oxidation, the electrons in the atom decrease or increase, being redistributed among other interacting particles, and the level of oxidation shows exactly how they are reorganized. This concept is closely related to the electronegativity of particles - their ability to attract and repel free ions from themselves.
Determining the level of oxidation depends on the characteristics and properties of a particular substance, so the calculation procedure cannot be unambiguously called easy or complex, but its results help to conventionally record the processes of redox reactions. It should be understood that the obtained result of calculations is the result of taking into account the transfer of electrons and has no physical meaning, and is not the true charge of the nucleus.
It is important to know! Inorganic chemistry often uses the term valency instead of the oxidation state of elements, this is not a mistake, but it should be borne in mind that the second concept is more universal.
The concepts and rules for calculating the movement of electrons are the basis for classifying chemicals (nomenclature), describing their properties and compiling communication formulas. But most often this concept is used to describe and work with redox reactions.
Rules for determining the degree of oxidation
How to find out the degree of oxidation? When working with redox reactions, it is important to know that the formal charge of a particle will always be equal to the magnitude of the electron, expressed in numerical value. This feature is connected with the assumption that the electron pairs that form a bond are always completely shifted towards more negative particles. It should be understood that we are talking about ionic bonds, and in the case of a reaction at , electrons will be divided equally between identical particles.
The oxidation number can have both positive and negative values. The thing is that during the reaction, the atom must become neutral, and for this you need to either attach a certain number of electrons to the ion, if it is positive, or take them away if it is negative. To designate this concept, when writing formulas, an Arabic numeral with the corresponding sign is usually written above the designation of the element. For example, or etc.
You should know that the formal charge of metals will always be positive, and in most cases, you can use the periodic table to determine it. There are a number of features that must be taken into account in order to determine the indicators correctly.
Degree of oxidation:
Having remembered these features, it will be quite simple to determine the oxidation number of elements, regardless of the complexity and number of atomic levels.
Useful video: determining the degree of oxidation
The periodic table of Mendeleev contains almost all the necessary information for working with chemical elements. For example, schoolchildren use only it to describe chemical reactions. So, in order to determine the maximum positive and negative values of the oxidation number, it is necessary to check the designation of the chemical element in the table:
- The maximum positive is the number of the group in which the element is located.
- The maximum negative oxidation state is the difference between the maximum positive limit and the number 8.
Thus, it is enough to simply find out the extreme boundaries of the formal charge of an element. Such an action can be performed using calculations based on the periodic table.
It is important to know! One element can have several different oxidation indices at the same time.
There are two main ways to determine the level of oxidation, examples of which are presented below. The first of these is a method that requires knowledge and skills to apply the laws of chemistry. How to arrange oxidation states using this method?
The rule for determining oxidation states
For this you need:
- Determine whether a given substance is elemental and whether it is out of bond. If yes, then its oxidation number will be equal to 0, regardless of the composition of the substance (individual atoms or multilevel atomic compounds).
- Determine whether the substance in question consists of ions. If yes, then the degree of oxidation will be equal to their charge.
- If the substance in question is a metal, then look at the indicators of other substances in the formula and calculate the metal readings by arithmetic.
- If the entire compound has one charge (in fact, this is the sum of all the particles of the elements presented), then it is enough to determine the indicators of simple substances, then subtract them from the total amount and get the metal data.
- If the relationship is neutral, then the total must be zero.
For example, consider combining with an aluminum ion whose total charge is zero. The rules of chemistry confirm the fact that the Cl ion has an oxidation number of -1, and in this case there are three of them in the compound. So the Al ion must be +3 for the entire compound to be neutral.
This method is quite good, since the correctness of the solution can always be checked by adding all the oxidation levels together.
The second method can be applied without knowledge of chemical laws:
- Find particle data for which there are no strict rules and the exact number of their electrons is unknown (possible by elimination).
- Find out the indicators of all other particles and then from the total amount by subtracting find the desired particle.
Let us consider the second method using the Na2SO4 substance as an example, in which the sulfur atom S is not defined, it is only known that it is nonzero.
To find what all oxidation states are equal to:
- Find known elements, keeping traditional rules and exceptions in mind.
- Na ion = +1 and each oxygen = -2.
- Multiply the number of particles of each substance by their electrons and get the oxidation states of all atoms except one.
- Na2SO4 consists of 2 sodium and 4 oxygen, when multiplied it turns out: 2 X +1 \u003d 2 is the oxidizing number of all sodium particles and 4 X -2 \u003d -8 - oxygen.
- Add the results 2+(-8) = -6 - this is the total charge of the compound without a sulfur particle.
- Express the chemical notation as an equation: sum of known data + unknown number = total charge.
- Na2SO4 is represented as follows: -6 + S = 0, S = 0 + 6, S = 6.
Thus, to use the second method, it is enough to know the simple laws of arithmetic.
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- 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.
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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.
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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.
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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.
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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.
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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.
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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-
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 compound serves as a good example to illustrate the algebraic method of determining the oxidation state.
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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.
- 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 type of chemical bond, can have 2 different oxidation states.
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.
