DEFINITION
The ability of an atom to form chemical bonds is called valence. A quantitative measure of valency is considered to be the number of different atoms in a molecule with which a given element forms bonds.
According to the exchange mechanism of the method of valence bonds, the valence of chemical elements is determined by the number of unpaired electrons contained in the atom. For s- and p-elements, these are the electrons of the outer level, for d-elements, these are the outer and pre-outer levels.
The values \u200b\u200bof the highest and lowest valencies of a chemical element can be determined using the Periodic Table of D.I. Mendeleev. The highest valence of an element coincides with the number of the group in which it is located, and the lowest is the difference between the number 8 and the group number. For example, bromine is located in the VIIA group, which means that its highest valency is VII, and the lowest is I.
Paired (located two in atomic orbitals) electrons, when excited, can be separated in the presence of free cells of the same level (electron separation into any level is impossible). Consider the example of elements of groups I and II. For example, the valency of the elements of the main subgroup of group I is equal to one, since at the external level the atoms of these elements have one electron:
3 Li 1s 2 2s 1
The valency of the elements of the main subgroup of group II in the ground (unexcited) state is zero, since there are no unpaired electrons at the external energy level:
4 Be 1s 2 2 s 2
When these atoms are excited, the paired s-electrons separate into free cells of the p-sublevel of the same level and the valence becomes equal to two (II):
Oxidation state
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.
The concept of the oxidation state for most compounds is conditional, since it does not reflect the real charge of the atom. However, this concept is very widely used in chemistry.
Most elements can exhibit different oxidation states in compounds. When determining their oxidation state, they use the rule according to which the sum of the oxidation states of elements in electrically neutral molecules is zero, and in complex ions, the charge of these ions. As an example, we calculate the degree of nitrogen oxidation in compounds of the composition KNO 2 and HNO 3 . The oxidation state of hydrogen and alkali metals in compounds is (+), and the oxidation state of oxygen is (-2). Accordingly, the oxidation state of nitrogen is:
KNO 2 1+ x + 2 × (-2) = 0, x=+3.
HNO 3 1+x+ x + 3 × (-2) = 0, x=+5.
Examples of problem solving
EXAMPLE 1
| Exercise | Valency IV is typical for: a) Ca; b) P; c) O; d) Si? |
| Solution | In order to give a correct answer to the question posed, we will consider each of the proposed options separately. a) Calcium is a metal. It is characterized by the only possible valency value that matches the group number in the Periodic Table of D.I. Mendeleev, in which it is located, i.e. the valency of calcium is II. The answer is incorrect. b) Phosphorus is a non-metal. Refers to a group of chemical elements with variable valence: the highest is determined by the group number in the Periodic Table of D.I. Mendeleev, in which it is located, i.e. is equal to V, and the lowest is the difference between the number 8 and the group number, i.e. is equal to III. The answer is incorrect. c) Oxygen is a non-metal. It is characterized by the only possible valence value equal to II. The answer is incorrect. d) Silicon is a non-metal. It is characterized by the only possible valency value that matches the group number in the Periodic Table of D.I. Mendeleev, in which it is located, i.e. the valency of silicon is IV. This is the correct answer. |
| Answer | Option (d) |
EXAMPLE 2
| Exercise | What valency does iron have in the compound that is formed when it interacts with hydrochloric acid: a) I; b) II; c) III; d) VIII? |
| Solution | We write the equation for the interaction of iron with hydrochloric acid: Fe + HCl \u003d FeCl 2 + H 2. As a result of the interaction, iron chloride is formed and hydrogen is released. To determine the valence of iron by the chemical formula, we first count the number of chlorine atoms: Calculate the total number of chlorine valency units: We determine the number of iron atoms: it is equal to 1. Then the valence of iron in its chloride will be equal to: |
| Answer | The valency of iron in the compound formed during its interaction with hydrochloric acid is II. |
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.
Electronegativity, oxidation state and valency of chemical elements
Electronegativity
The term is widely used in chemistry. electronegativity (EO).
The property of atoms of a given element to attract electrons from atoms of other elements in compounds is called electronegativity.
The electronegativity of lithium is conventionally taken as unity, the EC of other elements is calculated accordingly. There is a scale of values of EO elements.
The numerical values of the EO elements have approximate values: this is a dimensionless quantity. The higher the EC of an element, the more pronounced its non-metallic properties. According to the EO, the elements can be written as follows:
$F > O > Cl > Br > S > P > C > H > Si > Al > Mg > Ca > Na > K > Cs$. Fluorine has the highest EO value.
Comparing the EO values of elements from francium $(0.86)$ to fluorine $(4.1)$, it is easy to see that the EO obeys the Periodic Law.
In the Periodic system of elements, EO in a period increases with an increase in the number of the element (from left to right), and in the main subgroups it decreases (from top to bottom).
In periods, as the charges of the nuclei of atoms increase, the number of electrons on the outer layer increases, the radius of the atoms decreases, therefore, the ease of giving off electrons decreases, the EO increases, therefore, the non-metallic properties increase.
Oxidation state
Compounds made up of two chemical elements are called binary(from lat. bi - two), or two-element.
Let us recall the typical binary compounds that were cited as an example to consider the mechanisms of formation of ionic and covalent polar bonds: $NaCl$ - 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 $+1$, while 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:
$(Na)↖(0)+(Cl)↖(0)→(Na)↖(+1)(Cl)↖(-1)$.
In the $HCl$ molecule, however, 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: $(H)↖(δ+)→(Cl)↖(δ−)$, i.e. the electron will not completely transfer from the hydrogen atom to the chlorine atom, but partially, thus causing the partial charge of the atoms $δ$: $H^(+0.18)Cl^(-0.18)$. If we imagine that in the $HCl$ molecule, as well as in the chloride $NaCl$, the electron completely passed from the hydrogen atom to the chlorine atom, then they would receive charges $+1$ and $-1$: $(H)↖ (+1)(Cl)↖(−1). Such conditional charges are called degree of oxidation. 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 atoms.
The oxidation state 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:
$(Na_2)↖(+1)(S)↖(-2), (Mg_3)↖(+2)(N_2)↖(-3), (H_3)↖(-1)(N)↖(-3 ), (Cl_2)↖(0)$.
Those atoms that have received electrons from other atoms or to which common electron pairs are displaced have a negative value of the oxidation state, i.e. atoms of more electronegative elements.
Those atoms that donate their electrons to other atoms or from which common electron pairs are drawn have a positive value of the oxidation state, i.e. atoms of less electronegative elements.
The zero value of the degree of oxidation have atoms in the molecules of simple substances and atoms in the free state.
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: $Cl_2O_7$. Let us denote the oxidation state of oxygen: $(Cl_2)(O_7)↖(-2)$. Therefore, seven oxygen atoms will have a total negative charge $(-2)·7=-14$. Then the total charge of two chlorine atoms is $+14$, and that of one chlorine atom is $(+14):2=+7$.
Similarly, knowing the oxidation states of the elements, one can formulate a compound formula, for example, aluminum carbide (a compound of aluminum and carbon). Let's write the signs of aluminum and carbon side by side - $AlC$, and first - the sign of aluminum, because it's metal. Let us determine the number of external electrons from the periodic table of elements: $Al$ has $3$ electrons, $C$ has $4$. An aluminum atom will donate its three outer electrons to carbon and, in doing so, will receive an oxidation state of $+3$ equal to the charge of the ion. The carbon atom, on the contrary, will accept the $4$ electrons missing up to the "cherished eight" and will receive an oxidation state of $-4$. Let's write these values in the formula $((Al)↖(+3)(C)↖(-4))$ and find the least common multiple for them, it is equal to $12$. Then we calculate the indices:
Valence
Very important in the description of the chemical structure of organic compounds is the concept valency.
Valency characterizes the ability of atoms of chemical elements to form chemical bonds; it determines the number of chemical bonds by which a given atom is connected to other atoms in a molecule.
The valence of an atom of a chemical element is determined, first of all, by the number of unpaired electrons that take part in the formation of a chemical bond.
The valence possibilities of atoms are determined by:
- the number of unpaired electrons (one-electron orbitals);
- the presence of free orbitals;
- the presence of lone pairs of electrons.
In organic chemistry, the concept of "valence" replaces the concept of "oxidation state", which is customary to work with in inorganic chemistry. However, they are not the same. The valence has no sign and cannot be zero, while the oxidation state is necessarily characterized by a sign and can have a value equal to zero.
Valency and oxidation state are concepts often used in inorganic chemistry. In many chemical compounds, the valence value and the oxidation state of the element are the same, which is why schoolchildren and students often get confused. These concepts do have something in common, but the differences are more significant. To understand how these two concepts differ, it is worth learning more about them.
Information about the degree of oxidation
The oxidation state is an auxiliary value attributed to an atom of a chemical element or a group of atoms, which shows how common pairs of electrons are distributed between interacting elements.
This is an auxiliary quantity that has no physical meaning as such. Its essence is quite simple to explain with the help of examples:
food salt molecule NaCl It is made up of two atoms, a chlorine atom and a sodium atom. The bond between these atoms is ionic. Sodium has 1 electron at the valence level, which means that it has one common electron pair with the chlorine atom. Of these two elements, chlorine is more electronegative (has the property of mixing electron pairs towards itself), then the only common pair of electrons will shift towards it. In a compound, an element with a higher electronegativity has a negative oxidation state, a less electronegative one, respectively, a positive one, and its value is equal to the number of common pairs of electrons. For the NaCl molecule under consideration, the oxidation states of sodium and chlorine will look like this:
Chlorine, with an electron pair displaced to it, is now considered as an anion, that is, an atom that has attached an additional electron to itself, and sodium as a cation, that is, an atom that has donated an electron. But when recording the degree of oxidation, the sign is in the first place, and the numerical value is in the second, and vice versa when recording the ionic charge.
The oxidation state can be defined as the number of electrons that a positive ion lacks to make an electrically neutral atom, or that need to be taken from a negative ion in order to be oxidized to an atom. In this example, it is obvious that the positive sodium ion lacks an electron due to the displacement of the electron pair, and the chlorine ion has one extra electron.
The oxidation state of a simple (pure) substance, regardless of its physical and chemical properties, is zero. The O 2 molecule, for example, consists of two oxygen atoms. They have the same electronegativity values, so the shared electrons are not displaced towards either of them. This means that the electron pair is strictly between the atoms, so the oxidation state will be zero.
For some molecules, it can be difficult to determine where the electrons are moving, especially if there are three or more elements in it. To calculate the oxidation states in such molecules, you need to use a few simple rules:
- The hydrogen atom almost always has a constant oxidation state of +1..
- For oxygen, this indicator is -2. The only exception to this rule is fluorine oxides.
OF 2 and O 2 F 2,
Since fluorine is the element with the highest electronegativity, therefore, it always shifts interacting electrons towards itself. According to international rules, the element with the lower electronegativity value is written first, therefore, in these oxides, oxygen is in the first place.
- If you sum up all the oxidation states in a molecule, you get zero.
- Metal atoms are characterized by a positive oxidation state.
When calculating the oxidation states, you need to remember that the highest oxidation state of an element is equal to its group number, and the minimum is the group number minus 8. For chlorine, the maximum possible oxidation state is +7, because it is in the 7th group, and the minimum 7-8 = -one. 
General information about valence
Valency is the number of covalent bonds that an element can form in different compounds.
Unlike the oxidation state, the concept of valence has a real physical meaning.
The highest valency is equal to the group number in the periodic table. Sulfur S is located in the 6th group, that is, its maximum valence is 6. But it can also be 2 (H 2 S) or 4 (SO 2).
Almost all elements are characterized by variable valency. However, there are atoms for which this value is constant. These include alkali metals, silver, hydrogen (their valency is always 1), zinc (valence is always 2), lanthanum (valence is 3). 
What do valency and oxidation state have in common?
- To designate both of these quantities, positive integers are used, which are written above the Latin designation of the element.
- The highest valence, as well as the highest oxidation state, coincides with the group number of the element.
- The oxidation state of any element in a complex compound coincides with the numerical value of one of the valence indicators. For example, chlorine, being in the 7th group, can have a valency of 1, 3, 4, 5, 6, or 7, which means that the possible oxidation states are ±1, +3, +4, +5, +6, +7.
The main differences between these concepts
- The concept of "valence" has a physical meaning, and the degree of oxidation is an auxiliary term that has no real physical meaning.
- The oxidation state can be zero, greater than or less than zero. The valency is strictly greater than zero.
- Valency displays the number of covalent bonds, and the oxidation state - the distribution of electrons in the compound.
Chapter 3. CHEMICAL BOND
The ability of an atom of a chemical element to attach or replace a certain number of atoms of another element to form a chemical bond is called element valency.
Valency is expressed as a positive integer ranging from I to VIII. There is no valency equal to 0 or more than VIII. Permanent valency is shown by hydrogen (I), oxygen (II), alkali metals - elements of the first group of the main subgroup (I), alkaline earth elements - elements of the second group of the main subgroup (II). Atoms of other chemical elements exhibit variable valency. So, transition metals - elements of all side subgroups - show from I to III. For example, iron in compounds can be divalent or trivalent, copper can be monovalent or divalent. Atoms of other elements can show in compounds a valence equal to the group number and intermediate valences. For example, the highest valency of sulfur is IV, the lowest is II, and the intermediate ones are I, III and IV.
Valence is equal to the number of chemical bonds by which an atom of a chemical element is connected to the atoms of other elements in a chemical compound. A chemical bond is indicated by a dash (–). Formulas that show the order of connection of atoms in a molecule and the valency of each element are called graphic.
Oxidation state is the conditional charge of an atom in a molecule, calculated on the assumption that all bonds are ionic in nature. This means that a more electronegative atom, by displacing one electron pair completely towards itself, acquires a charge of 1–. A non-polar covalent bond between like atoms does not contribute to the oxidation state.
To calculate the oxidation state of an element in a compound, one should proceed from the following provisions:
1) the degree of oxidation of elements in simple substances is taken equal to zero (Na 0; O 2 0);
2) the algebraic sum of the oxidation states of all the atoms that make up the molecule is equal to zero, and in a complex ion this sum is equal to the charge of the ion;
3) atoms have a constant oxidation state: alkali metals (+1), alkaline earth metals, zinc, cadmium (+2);
4) the degree of oxidation of hydrogen in compounds +1, except for metal hydrides (NaH, etc.), where the degree of oxidation of hydrogen is –1;
5) the degree of oxidation of oxygen in compounds -2, except for peroxides (-1) and oxygen fluoride OF 2 (+2).
The maximum positive oxidation state of an element is usually the same as its group number in the periodic table. The maximum negative oxidation state of an element is equal to the maximum positive oxidation state minus eight.
The exceptions are fluorine, oxygen, iron: their highest oxidation state is expressed by a number whose value is lower than the number of the group to which they belong. For elements of the copper subgroup, on the contrary, the highest oxidation state is greater than one, although they belong to group I.
Atoms of chemical elements (except for noble gases) can interact with each other or with atoms of other elements forming b.m. complex particles - molecules, molecular ions and free radicals. The chemical bond is due electrostatic forces between atoms , those. forces of interaction of electrons and atomic nuclei. In the formation of a chemical bond between atoms, the main role is played by valence electrons, i.e. electrons in the outer shell.
