Valency (lat. valere - to have a meaning) is a measure of the "connective capacity" of a chemical element, equal to the number of individual chemical bonds that one atom can form.
Valence is determined by the number of bonds that one atom forms with others. For example, consider the molecule
To determine valency, you need to have a good idea of the graphic formulas of substances. In this article, you will see many formulas. I also inform you about chemical elements with constant valency, which are very useful to know.
In electronic theory, it is believed that the bond valency is determined by the number of unpaired (valence) electrons in the ground or excited state. We touched on the topic of valence electrons and the excited state of the atom. Using the example of phosphorus, let's combine these two topics for a complete understanding.
The vast majority of chemical elements have a variable valence value. Variable valency is characteristic of copper, iron, phosphorus, chromium, and sulfur.
Below you will see elements with variable valency and their compounds. Note that other elements help us to determine their non-permanent valency - with a constant valence.
Remember that for some simple substances, valency takes on the values: III - for nitrogen, II - for oxygen. Let's summarize the knowledge gained by writing the graphical formulas of nitrogen, oxygen, carbon dioxide and carbon monoxide, sodium carbonate, lithium phosphate, iron (II) sulfate and potassium acetate.
As you noticed, valencies are indicated by Roman numerals: I, II, III, etc. On the presented formulas, the valencies of substances are equal:
- N-III
- O-II
- H, Na, K, li - I
- S-VI
- C - II (in carbon monoxide CO), IV (in carbon dioxide CO 2 and sodium carbonate Na 2 CO 3
- Fe-II
The oxidation state (CO) is a conditional indicator that characterizes the charge of an atom in a compound and its behavior in an OVR (redox reaction). In simple substances, CO is always equal to zero, in complex substances it is determined based on the constant oxidation states of some elements.
Numerically, the oxidation state is equal to the conditional charge that can be attributed to an atom, guided by the assumption that all the electrons that form bonds have passed to a more electronegative element.
Determining the degree of oxidation, we attribute the conditional charge "+" to one element, and "-" to the other. This is due to electronegativity - the ability of an atom to attract electrons to itself. The sign "+" means a lack of electrons, and "-" - their excess. I repeat, CO is a conditional concept.
The sum of all oxidation states in a molecule is zero - this is important to remember for self-examination.
Knowing the changes in electronegativity in periods and groups of the periodic table D.I. Mendeleev, we can conclude which element takes "+" and which minus. Elements with a constant degree of oxidation also help in this matter.
Who is more electronegative, he attracts electrons to himself more strongly and "goes into the minus". Those who donate their electrons and experience a shortage of them receive the "+" sign.
Independently determine the oxidation states of atoms in the following substances: RbOH, NaCl, BaO, NaClO 3, SO 2 Cl 2, KMnO 4, Li 2 SO 3, O 2, NaH 2 PO 4. Below you will find a solution to this problem.
Compare the value of electronegativity according to the periodic table, and, of course, use your intuition :) However, as you study chemistry, accurate knowledge of oxidation states should replace even the most developed intuition ;-)
I would especially like to highlight the topic of ions. An ion is an atom or a group of atoms that, due to the loss or gain of one or more electrons, has acquired (and) a positive or negative charge.
When determining the CO of atoms in an ion, one should not strive to bring the total charge of the ion to "0", as in a molecule. Ions are given in the solubility table, they have different charges - it is necessary to bring the ion to such a charge. I'll explain with an example.
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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 | Valence 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) |
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.
-
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 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.
-
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.
-
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.
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 have a 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 state is the total number of electrons that have been removed from the atoms. The negative value of the oxidation state is equal to the number of acquired electrons. By changing the oxidation state of a chemical element, one judges 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 formed by chemical elements with the same EO value. In this case, the bonding electrons belong to all structural particles equally. Therefore, in simple substances, the oxidation state (H 0 2, O 0 2, C 0) is not characteristic of the elements. When atoms accept electrons or the general 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 higher valencies in the 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
In the formation of a sodium chloride crystal, 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:
- In simple substances, the oxidation states are zero.
- The sum of the oxidation of all atoms or ions in a neutral substance is zero.
- In a complex ion, the sum of the oxidation states of all elements must correspond to the charge of the entire particle.
- A more electronegative atom acquires a negative oxidation state, which is written with a minus sign.
- Less electronegative elements receive positive oxidation states, they are written with a plus sign.
- Oxygen generally exhibits an oxidation state of -2.
- For hydrogen, the characteristic value is: +1, in metal hydrides it occurs: H-1.
- Fluorine is the most electronegative of all elements, its oxidation state is always -4.
- 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 valency" 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 includes many of the most important processes in industry, technology, animate 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.
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 ER.
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.
With an electronegativity difference of 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.