Electronegativity, like other properties of atoms of chemical elements, changes with increasing sequence number of the element periodically:
The graph above shows the frequency of changes in the electronegativity of the elements of the main subgroups depending on the serial number of the element.
When moving down a subgroup of the periodic table, the electronegativity of chemical elements decreases, while moving to the right along the period increases.
Electronegativity reflects the nonmetallicity of elements: the higher the value of electronegativity, the more non-metallic properties are expressed in the element.
Oxidation state
How to calculate the oxidation state of an element in a compound?
1) The oxidation state of chemical elements in simple substances is always zero.
2) There are elements that exhibit a constant oxidation state in complex substances:
3) There are chemical elements that exhibit a constant oxidation state in the vast majority of compounds. These items include:
Element | The degree of oxidation in almost all compounds | Exceptions |
| hydrogen H | +1 | Hydrides of alkali and alkaline earth metals, for example: |
| oxygen O | -2 | Peroxides of hydrogen and metals: Oxygen fluoride - |
4) The algebraic sum of the oxidation states of all atoms in a molecule is always zero. The algebraic sum of the oxidation states of all atoms in an ion is equal to the charge of the ion.
5) The highest (maximum) oxidation state is equal to the group number. Exceptions that do not fall under this rule are elements of a side subgroup of group I, elements of a side subgroup of group VIII, as well as oxygen and fluorine.
Chemical elements whose group number does not coincide with their highest oxidation state (must be remembered)
6) The lowest oxidation state of metals is always zero, and the lowest oxidation state of non-metals is calculated by the formula:
lowest oxidation rate of non-metal \u003d group number - 8
Based on the above rules, it is possible to establish the degree of oxidation of a chemical element in any substance.
Finding oxidation states of elements in various compounds
Example 1
Determine the oxidation state of all elements in sulfuric acid.
Decision:
We write the formula of sulfuric acid:
The degree of hydrogen oxidation in all complex substances is +1 (except metal hydrides).
The oxidation state of oxygen in all complex substances is -2 (except for peroxides and oxygen fluoride OF 2). Let's place the known oxidation states:
We denote the degree of oxidation of sulfur as x:
The sulfuric acid molecule, like the molecule of any substance, is generally electrically neutral, because the sum of the oxidation states of all atoms in the molecule is zero. Schematically, this can be represented as follows:
Those. we got the following equation:
We solve it:
Thus, the degree of oxidation of sulfur in sulfuric acid is +6.
Example 2
Determine the oxidation state of all elements in ammonium dichromate.
Decision:
We write the formula of ammonium dichromate:
As in the previous case, we can arrange the oxidation states of hydrogen and oxygen:
However, we see that the oxidation states of two chemical elements, nitrogen and chromium, are unknown at once. Therefore, we cannot find the oxidation states similarly to the previous example (one equation with two variables does not have a unique solution).
We draw attention to the fact that this substance belongs to the class of salts and, accordingly, has an ionic structure. Then it can rightly be said that the composition of ammonium dichromate includes NH 4 + cations (the charge of this cation can be seen in the solubility table). Therefore, since in the formula unit of ammonium dichromate there are two positive singly charged NH 4 + cations, the charge of the dichromat ion is -2, since the substance as a whole is electrically neutral. Those. the substance is formed by NH 4 + cations and Cr 2 O 7 2- anions.
We know the oxidation states of hydrogen and oxygen. Knowing that the sum of the oxidation states of the atoms of all elements in an ion is equal to the charge, and designating the oxidation states of nitrogen and chromium as x and y accordingly, we can write:
Those. we get two independent equations:
Solving which, we find x and y:
Thus, in the ammonium dichromate, the oxidation state of nitrogen is -3, hydrogen +1, chromium +6, and oxygen -2.
How to determine the degree of oxidation of elements in organic substances can be read.
Valence
The valency of atoms is indicated by Roman numerals: I, II, III, etc.
The valence of an atom depends on the amount of:
1) unpaired electrons
2) lone electron pairs in the orbitals of valence levels
3) empty valence level electronic orbitals
The valence of a hydrogen atom
Let us depict the electron-graphic formula of the hydrogen atom:
It was said that three factors can affect valence potentials - the presence of unpaired electrons, the presence of unshared electron pairs at the external level, and the presence of vacant (empty) orbitals of the external level. We see on the external (and only) energy level one unpaired electron. Based on this, hydrogen can precisely have a valency equal to I. However, at the first energy level there is only one sublevel - sthose. the hydrogen atom at the external level does not have either unshared electron pairs or empty orbitals.
Thus, the only valency that a hydrogen atom can exhibit is I.
The valence of a carbon atom
Consider the electronic structure of a carbon atom. In the ground state, the electronic configuration of its external level is as follows:
Those. in the ground state, at the external energy level of the unexcited carbon atom, there are 2 unpaired electrons. In this state, it can exhibit a valency equal to II. However, a carbon atom very easily goes into an excited state when energy is transmitted to it, and the electronic configuration of the outer layer in this case takes the form:
Despite the fact that a certain amount of energy is spent on the process of excitation of the carbon atom, the waste is more than offset by the formation of four covalent bonds. For this reason, valency IV is much more characteristic of a carbon atom. So, for example, valency IV carbon has in the molecules of carbon dioxide, carbonic acid and absolutely all organic substances.
In addition to unpaired electrons and lone electron pairs, the presence of vacant () orbitals of the valence level also affects valence potentials. The presence of such orbitals at the level being filled leads to the fact that an atom can act as an acceptor of an electron pair, i.e. form additional covalent bonds by the donor-acceptor mechanism. So, for example, contrary to expectations, in the carbon monoxide molecule, the CO bond is not double, but triple, which is clearly shown in the following illustration:
The valence of a nitrogen atom
We write the electron-graphic formula of the external energy level of the nitrogen atom:
As can be seen from the illustration above, the nitrogen atom in its usual state has 3 unpaired electrons, and therefore it is logical to assume its ability to exhibit a valency equal to III. Indeed, a valence of three is observed in molecules of ammonia (NH 3), nitrous acid (HNO 2), nitrogen trichloride (NCl 3), etc.
It was said above that the valency of an atom of a chemical element depends not only on the number of unpaired electrons, but also on the presence of unshared electron pairs. This is due to the fact that a covalent chemical bond can form not only when two atoms provide each other with one electron, but also when one atom having an unshared pair of electrons - a donor () provides it to another atom with a vacant () orbital valence level (acceptor). Those. valence IV is also possible for the nitrogen atom due to an additional covalent bond formed by the donor-acceptor mechanism. So, for example, four covalent bonds, one of which is formed by the donor-acceptor mechanism, is observed during the formation of an ammonium cation:
Despite the fact that one of the covalent bonds is formed by the donor – acceptor mechanism, all N – H bonds in the ammonium cation are absolutely identical and do not differ from each other.
The valency equal to V, the nitrogen atom is not able to show. This is due to the fact that the transition to the excited state is impossible for the nitrogen atom, in which the pairing of two electrons takes place with the transition of one of them to the free orbital, which is closest in energy level. Nitrogen atom does not have dis sublevel, and the transition to the 3s orbital is so energy-intensive that the energy costs are not covered by the formation of new bonds. Many may wonder, what then is the valency of nitrogen, for example, in the molecules of nitric acid HNO 3 or nitric oxide N 2 O 5? Oddly enough, the valency is also IV there, as can be seen from the following structural formulas:
The dashed line in the illustration depicts the so-called delocalized π -connection. For this reason, the terminal bonds of NO can be called "one and a half." Similar one and a half bonds are also present in the molecule of ozone O 3, benzene C 6 H 6, etc.
Valence of phosphorus
Let us depict the electron-graphic formula of the external energy level of the phosphorus atom:
As we see, the structure of the outer layer at the phosphorus atom in the ground state and the nitrogen atom is the same, and therefore it is logical to expect for the phosphorus atom as well as for the nitrogen atom, possible valencies equal to I, II, III, and IV, which observed in practice.
However, unlike nitrogen, the phosphorus atom has at the external energy level also d-sub-level with 5 vacant orbitals.
In this regard, it is able to transition to an excited state by steaming electrons 3 s -orbitals:
Thus, valency V inaccessible to nitrogen for the phosphorus atom is possible. So, for example, a valence of five is a phosphorus atom in the molecules of such compounds as phosphoric acid, phosphorus (V) halides, phosphorus oxide (V), etc.
The valence of an oxygen atom
The electron-graphic formula of the external energy level of the oxygen atom has the form:
We see at the 2nd level two unpaired electrons, and therefore valence II is possible for oxygen. It should be noted that this valency of the oxygen atom is observed in almost all compounds. When discussing the valence potentials of a carbon atom, we discussed the formation of a carbon monoxide molecule. The bond in the CO molecule is triple; therefore, oxygen is trivalent there (oxygen is an electron pair donor).
Due to the fact that the nitrogen atom does not have an external level d-sub-level electron pairing s and p-orbitals is impossible, because of which the valency of the oxygen atom is limited in comparison with other elements of its subgroup, for example, sulfur.
Valence possibilities of the sulfur atom
The external energy level of the sulfur atom in an unexcited state:
The sulfur atom, like the oxygen atom, has two unpaired electrons in the usual state, so we can conclude that a valence of two is possible for sulfur. Indeed, valence of II sulfur is, for example, in the hydrogen sulfide molecule H 2 S.
As we see, the sulfur atom at the external level appears d-sub-level with vacant orbitals. For this reason, the sulfur atom is able to expand its valence potentials, unlike oxygen, due to the transition to excited states. So, when steaming a lone electron pair 3 p-sublayer sulfur atom acquires an electronic configuration of the external level of the following form:
In this state, the sulfur atom has 4 unpaired electrons, which tells us about the possibility of the manifestation by sulfur atoms of a valency equal to IV. Indeed, valence IV sulfur has in the molecules SO 2, SF 4, SOCl 2, etc.
When steaming the second lone electron pair located at 3 s-sub-level, the external energy level takes the configuration:
In this state, the manifestation of valency VII becomes possible. Examples of VI-valent sulfur compounds are SO 3, H 2 SO 4, SO 2 Cl 2, etc.
Similarly, one can consider the valence potential of other chemical elements.
Definition
Under valency it implies the property of an atom of a given element to attach or replace a certain number of atoms of another element.
Therefore, the measure of valency can be the number of chemical bonds formed by a given atom with other atoms. Thus, at present, the valency of a chemical element usually refers to its ability (in a narrower sense, a measure of its ability) to form chemical bonds (Fig. 1). In the representation of the valence bond method, the numerical value of valency corresponds to the number of covalent bonds that an atom forms.
Fig. 1. Schematic formation of water molecules and ammonia.
Valency table of chemical elements
Initially, the valency of hydrogen was taken as the unit of valency. In this case, the valency of another element was expressed by the number of hydrogen atoms that are attached to or replace one atom of this element (the so-called hydrogen valence). For example, in compounds of the composition HCl, H 2 O, NH 3, CH 4, the valence of chlorine hydrogen is equal to one, oxygen — two, nitrogen — three, carbon — four.
Then it was decided that the valency of the desired element can also be determined by oxygen, the valency of which, as a rule, is two. In this case, the valency of a chemical element is calculated as the doubled number of oxygen atoms that one atom of this element can attach (the so-called oxygen valency). For example, in compounds of the composition N 2 O, CO, SiO 2, SO 3, the oxygen valency is equal to one, carbon — two, silicon — four, and sulfur — six.
In fact, it turned out that most chemical elements have different valencies in hydrogen and oxygen compounds: for example, the valence of sulfur in hydrogen is two (H 2 S), and in oxygen - six (SO 3). In addition, most elements exhibit different valencies in their compounds. For example, carbon forms two oxides: CO monoxide and CO 2 dioxide. In the first of which, the valency of carbon is II, and in the second - four. Whence it follows that, as a rule, it is impossible to characterize the valency of an element by any one number.
Higher and lower valencies of chemical elements
The values \u200b\u200bof the higher and lower valencies of a chemical element can be determined using the Periodic table D.I. Mendeleev. The highest valency of the element coincides with the number of the group in which it is located, and the lowest represents the difference between the number 8 and the group number. For example, bromine is located in the VIIA group, so its highest valency is VII, and the lowest is I.
There are elements with the so-called. constant valency (metals of groups IA and IIA, aluminum, hydrogen, fluorine, oxygen), which in their compounds exhibit a single oxidation state, which most often coincides with the group number of the Periodic Table DI Mendeleev, where they are located).
Elements for which several valency values \u200b\u200bare characteristic (and this is not always the highest and lowest valencies) are called alternating. For example, sulfur is characterized by valencies II, IV, and VI.
In order to make it easier to remember how much and what valencies are characteristic of a particular chemical element, valency tables of chemical elements are used, which are as follows:
Examples of solving problems
EXAMPLE 1
| The task | Valence III is characteristic of: a) Ca; b) P; c) O; d) Si? |
| Decision | a) Calcium is a metal. It is characterized by the only possible value of valency, which coincides with the group number in the Periodic table Mendeleev, in which it is located, i.e. valence of calcium is equal to II. The answer is incorrect. b) Phosphorus - non-metal. Refers to the group of chemical elements with variable valency: the highest is determined by the group number in the Periodic table 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. equal to III. This is the correct answer. |
| Answer | Option (b) |
EXAMPLE 2
| The task | Valence III is characteristic of: a) Be; b) F; c) Al; d) C? |
| Decision | In order to give the right answer to this question, we will consider each of the proposed options separately. a) Beryllium is metal. It is characterized by the only possible value of valency, which coincides with the group number in the Periodic table Mendeleev, in which it is located, i.e. valence of beryllium is equal to II. The answer is incorrect. b) Fluorine is non-metal. It is characterized by the only possible value of valency equal to I. The answer is incorrect. c) Aluminum is a metal. It is characterized by the only possible value of valency, which coincides with the group number in the Periodic table Mendeleev, in which it is located, i.e. valence of aluminum is equal to III. This is the correct answer. |
| Answer | Option (c) |
Instruction manual
The table is a structure in which chemical elements are located according to their principles and laws. That is, we can say that the table is a multi-storey “house” in which chemical elements “live”, each of them having its own apartment under a certain number. The “floors” are located horizontally - periods that can be small and large. If a period consists of two rows (as indicated by numbering on the side), then such a period is called large. If it has only one row, then it is called small.
The table is also divided into “porches” - groups of which there are only eight. As in any entrance of the apartment are located left and right, so here the chemical elements are located on the same principle. Only in this embodiment, their distribution is uneven - on the one hand there are more elements and then they talk about the main group, on the other - less and this indicates that the group is secondary.
Valence is the ability of elements to form chemical bonds. There is a valency constant, which does not change, and a variable that has a different value depending on which substance contains the element. When determining the valency according to the periodic table, it is necessary to pay attention to the following characteristics: group number elements and its type (that is, the main or secondary group). The constant valency in this case is determined by the group number of the main subgroup. To find out the value of the variable valency (if any, and usually non-metals), then you need to subtract the number of the group in which the element is located from 8 (a total of 8 groups - hence this figure).
Example No. 1. If you look at the elements of the first group of the main subgroup (alkali metals), we can conclude that they all have a valency equal to I (Li, Na, K, Rb, Cs, Fr).
Example No. 2. Elements of the second group of the main subgroup (alkaline earth metals) respectively have a valency II (Be, Mg, Ca, Sr, Ba, Ra).
Example No. 3. If we talk about non-metals, for example, P (phosphorus) is in the V group of the main subgroup. Hence, its valency will be equal to V. In addition, phosphorus has another valency value, and to determine it, you must perform action 8 - element number. So, 8 - 5 (phosphorus group number) \u003d 3. Therefore, the second phosphorus valency is III.
Example No. 4. Halogens are in group VII of the main subgroup. So their valency will be equal to VII. However, given that these are non-metals, it is necessary to perform an arithmetic operation: 8 - 7 (element group number) \u003d 1. Therefore, the other valence of the halogens is I.
For elements of secondary subgroups (and only metals belong to them), valency must be remembered, especially since in most cases it is equal to I, II, less often III. You will also have to memorize the valencies of chemical elements that have more than two meanings.
From school or even earlier, everyone knows that everything around, including ourselves, consists of their atoms - the smallest and indivisible particles. Due to the ability of atoms to connect with each other, the diversity of our world is huge. Ability of this chemical atoms element form bonds with other atoms called valency element.
Instruction manual
For example, you can use two substance - HCl and H2O. It is well known to everyone hydrochloric acid and water. The first substance contains one hydrogen atom (H) and one chlorine atom (Cl). This suggests that in this compound they form one bond, that is, they hold one atom near them. Consequently, valence and one and the other is 1. It’s also easy to determine valence elements that make up the water molecule. It contains two hydrogen atoms and one oxygen atom. Consequently, the oxygen atom formed two bonds for the addition of two hydrogens, and they, in turn, in one bond. Means valence oxygen is 2, and hydrogen is 1.
But sometimes you have to face substancemore complex in structure and properties of their constituent atoms. There are two types of elements: with constant (oxygen, hydrogen, etc.) and unstable valenceyu. For atoms of the second type, this number depends on the compound in which they are included. An example is sulfur (S). It can have valencies 2, 4, 6, and sometimes even 8. Determining the ability of elements such as sulfur to hold other atoms around it is a little more difficult. To do this, you need to know the properties of other components substance.
Remember the rule: the product of the number of atoms by valence one element in the mix should match the same product for another element. This can be checked by again referring to the water molecule (H2O):
2 (amount of hydrogen) * 1 (its valence) = 2
1 (amount of oxygen) * 2 (its valence) = 2
2 \u003d 2 - then everything is determined correctly.
Now test this algorithm on a more complex substance, for example, N2O5 - nitric oxide. It was previously indicated that oxygen has a constant valence 2, therefore, we can make the equation:
2 (valence oxygen) * 5 (its quantity) \u003d X (unknown valence nitrogen) * 2 (its amount)
Using simple arithmetic calculations, we can determine that valence nitrogen in the composition of this compound is 5.
Valence - this is the ability of chemical elements to hold a certain number of atoms of other elements. At the same time, this is the number of bonds formed by a given atom with other atoms. Determining the valency is quite simple.
Instruction manual
Please note that the valency of the atoms of some elements is constant, while the others are variable, that is, it tends to change. For example, hydrogen in all compounds is monovalent, since it forms only one bond. Oxygen is able to form two bonds, while being divalent. But sulfur can have a valency of II, IV or VI. It all depends on the element with which it connects. Thus, sulfur is an element with variable valency.
Note that in molecules of hydrogen compounds, calculating valency is very simple. Hydrogen is always monovalent, and this indicator for the element associated with it will be equal to the number of hydrogen atoms in a given molecule. For example, in CaH2, calcium will be divalent.
Remember the main rule for determining valency: the product of the valency of an atom of an element and the number of its atoms in a molecule is always equal to the product of the valency of an atom of the second element and the number of its atoms in a given molecule.
Look at the letter formula denoting this equality: V1 x K1 \u003d V2 x K2, where V is the valency of the atoms of the elements, and K is the number of atoms in the molecule. With its help, it is easy to determine the valence index of any element, if other data are known.
Consider the sulfur oxide molecule SO2. Oxygen in all compounds is divalent, therefore, substituting the values \u200b\u200bin the proportion: V oxygen x Oxygen \u003d V sulfur x Xer, we get: 2 x 2 \u003d V sulfur x 2. From here V sulfur \u003d 4/2 \u003d 2. Thus, the valence of sulfur in this molecule is 2.
Related videos
The discovery of the periodic law and the creation of an ordered system of chemical elements Mendeleev became the climax of the development of chemistry in the 19th century. The scientist has summarized and systematized an extensive material of knowledge about the properties of elements.
Instruction manual
In the XIX century there were no ideas about the structure of the atom. Discovery D.I. Mendeleev was only a generalization of experimental facts, but their physical meaning for a long time remained incomprehensible. When the first data appeared on the structure of the nucleus and the distribution of electrons in atoms, this made it possible to take a fresh look at the periodic law and system of elements. Table D.I. Mendeleev makes it possible to visually trace the periodicity of the properties of elements found in nature.
Each element in the table is assigned a specific serial number (H - 1, Li - 2, Be - 3, etc.). This number corresponds to the charge of the nucleus (the number of protons in the nucleus) and the number of electrons rotating around the nucleus. The number of protons is thus equal to the number of electrons, and this suggests that under normal conditions the atom is electrically neutral.
The division into seven periods occurs according to the number of energy levels of the atom. Atoms of the first period have a one-level electron shell, the second - two-level, the third - three-level, etc. When filling a new energy level, a new period begins.
The first elements of any period are characterized by atoms having one electron at an external level - these are alkali metal atoms. Periods end with noble gas atoms having an external energy level completely filled with electrons: in the first period, inert gases have 2 electrons, in the subsequent ones - 8. It is because of the similar structure of the electron shells that the groups of elements have similar physicochemical properties.
In the table D.I. Mendeleev has 8 main subgroups. This number is due to the maximum possible number of electrons at the energy level.
At the bottom of the periodic system, lanthanides and actinides are distinguished as independent series.
Using table D.I. Mendeleev can observe the periodicity of the following properties of elements: the radius of the atom, the volume of the atom; ionization potential; electron affinity forces; electronegativity of the atom; oxidation state; physical properties of potential compounds.
Clearly traceable frequency of the arrangement of elements in the table D.I. Mendeleev is rationally explained by the consistent nature of the filling of energy levels with electrons.
Sources:
- periodic table
The periodic law, which is the basis of modern chemistry and explains the laws of changes in the properties of chemical elements, was discovered by D.I. Mendeleev in 1869. The physical meaning of this law is revealed when studying the complex structure of an atom.
One of the important topics in studying school subjects is the valency course. This will be discussed in the article.
Valency - what is it?
Valence in chemistry means the property of atoms of a chemical element to bind to itself the atoms of another element. Translated from Latin - power. It is expressed in numbers. For example, the valency of hydrogen will always be equal to one. If we take the formula of water - H2O, it can be represented as H - O - N. One oxygen atom could bind two hydrogen atoms to itself. This means that the number of bonds created by oxygen is two. And the valency of this element will be two.
In turn, hydrogen will be divalent. Its atom can be connected to only one atom of a chemical element. In this case, with oxygen. More specifically, atoms, depending on the valency of an element, form pairs of electrons. How many such pairs are formed - such will be valency. A numeric value is called an index. Oxygen has an index of 2.
How to determine the valency of chemical elements according to the table of Dmitry Mendeleev
Looking at the periodic table of elements, you can notice the vertical rows. They are called groups of elements. The valency also depends on the group. Elements of the first group have a first valency. The second is the second. Third - third. And so on.
There are also elements with a constant valency index. For example, hydrogen, a halogen group, silver, and so on. They must be learned necessarily.
How to determine the valency of chemical elements by the formulas?
Sometimes it is difficult to determine the valency from the periodic table. Then you need to look at a specific chemical formula. Take oxide FeO. Here, as in iron, as in oxygen, the valence index will be two. But in Fe2O3 oxide - in a different way. Iron will be trivalent.
It is always necessary to remember different methods for determining valency and not to forget them. Know its constant numerical values. What elements do they have. And, of course, use a table of chemical elements. And also to study individual chemical formulas. It is better to present them in a schematic form: H - O - H, for example. Then the connections are visible. And the number of dashes (dashes) will be the numerical value of valency.