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Valence of atoms in the stationary and excited state. Valence possibilities of atoms of elements in chemical compounds

The structure of the outer energy levels of atoms of chemical elements determines mainly the properties of their atoms. Therefore, these levels are called valence. The electrons of these levels, and sometimes of the pre-external levels, can take part in the formation of chemical bonds. Such electrons are also called valence electrons.
The valence of an atom of a chemical element is determined primarily by the number of unpaired electrons that take part in the formation of a chemical bond.
The valence electrons of the atoms of the elements of the main subgroups are located on the s- and p-orbitals of the outer electronic layer. In the elements of secondary subgroups, except for lanthanides and actinides, valence electrons are located on the s-orbitals of the outer and d-orbitals of the pre-outer layers.
In order to correctly assess the valence capabilities of atoms of chemical elements, it is necessary to consider the distribution of electrons in them by energy levels and sublevels and determine the number of unpaired electrons in accordance with the Pauli principle and Hund's rule for the unexcited (ground, or stationary) state of the atom and for the excited (then there is one that has received additional energy, as a result of which the electrons of the outer layer are depaired and transferred to free orbitals). An atom in an excited state is denoted by the corresponding element symbol with an asterisk. For example, consider the valence possibilities of phosphorus atoms in the stationary and excited states:

In the unexcited state, the phosphorus atom has three unpaired electrons in the p sublevel. During the transition of an atom to an excited state, one of the pair of electrons of the d-sublevel can pass to a free orbital of the d-sublevel. The valency of phosphorus changes from three (in the ground state) to five (in the excited state).
The separation of paired electrons requires energy, since the pairing of electrons is accompanied by a decrease in the potential energy of atoms. At the same time, the energy consumption for the transfer of an atom to an excited state is compensated by the energy released during the formation of chemical bonds by unpaired electrons.
Thus, a carbon atom in a stationary state has two unpaired electrons. Consequently, with their participation, two common electron pairs can be formed, carrying out two covalent bonds. However, you are well aware that tetravalent carbon atoms are present in many inorganic and all organic compounds. Obviously, its atoms formed four covalent bonds in these compounds while in an excited state.

The energy expended on the excitation of carbon atoms is more than offset by the energy released during the formation of two additional covalent bonds. So, for the transfer of carbon atoms from the stationary state 2s 2 2p 2 to the excited state - 2s 1 2p 3, about 400 kJ / mol of energy is required. But during the formation of a C-H bond in saturated hydrocarbons, 360 kJ / mol is released. Consequently, upon the formation of two moles of C–H bonds, 720 kJ will be released, which exceeds the energy of transferring carbon atoms to an excited state by 320 kJ/mol.
In conclusion, it should be noted that the valence possibilities of atoms of chemical elements are far from exhausted by the number of unpaired electrons in the stationary and excited states of atoms. If you remember the donor-acceptor mechanism for the formation of covalent bonds, then you will also understand the other two valence possibilities of atoms of chemical elements, which are determined by the presence of free orbitals and the presence of unshared electron pairs that can give a covalent chemical bond according to the donor-acceptor mechanism. Recall the formation of the ammonium ion NH4+. (We will consider in more detail the realization of these valence possibilities by atoms of chemical elements when studying the chemical bond.) Let us draw a general conclusion.

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Phosphorus valency

Phosphorus P (Is 2s 2/f 3s Zr) is analogous to nitrogen in terms of the number of valence electrons. However, as an element of the 3rd period, it differs significantly from nitrogen, an element of the 2nd period. This difference consists in the fact that phosphorus has a larger atom, less ionization energy, greater electron affinity, and greater atom polarizability than nitrogen. The maximum coordination number of phosphorus is six. As for other elements of the 3rd period, rl - rl binding is not typical for the phosphorus atom, and therefore, unlike nitrogen, the sp- and sp-hybrid states of the phosphorus orbitals are unstable. Phosphorus in compounds exhibits oxidation states from -3 to +5. The most typical oxidation state is +5.

Let's write the formula of the compound that consists of and. phosphorus (V valence) and oxygen (II valence).

In which compounds does phosphorus have the highest valency?

What are the valence capabilities of phosphorus How does it differ in this respect from its counterpart - nitrogen

The electronic structure of the phosphorus atom corresponds to the formula 16F 5 25 2p 33 3p. Phosphorus has valence electrons in the third (outer) energy level, in which, in addition to 5- and three p-orbitals, there are five free -orbitals.

According to another point of view, the difference in the properties of phosphorus and nitrogen is explained by the presence of valence 3-orbitals in the phosphorus atom,

Explain the difference between the first ionization energy of phosphorus, P (1063 kJ mol) and sulfur, 8 (1000 kJ mol), based on a comparison of the valence orbital electronic configurations of the P and 8 atoms.

But in phosphorus, as an element of the 3rd period, the 3-orbitals also play the role of valences. Therefore, along with the commonality of properties in the chemistry of these typical elements of group V, significant differences appear. For phosphorus, sp-, sp-, and 5p types of hybridization of valence orbitals are possible. The maximum coordination number of phosphorus is 6. In contrast to nitrogen, phosphorus is characterized by n - rl binding due to the acceptance of free 3d (-orbitals of electron pairs of the corresponding atoms

The stable coordination number of phosphorus (V) is 4, which corresponds to the sp hybridization of its valence orbitals. The coordination numbers 5 and 6 appear less frequently; in these cases sp4 and sp4 hybrid states are assigned to the phosphorus atom, respectively (p. 415).

A similar behavior is found in the elements of the VA group, but the boundary between metals and nonmetals in this group is lower. Nitrogen and phosphorus are non-metals, the chemistry of their covalent compounds and possible oxidation states are determined by the presence of five valence electrons in the configuration. Nitrogen and phosphorus most often have oxidation states - 3, -b 3 and +5. Arsenic As and antimony Sb are semimetals forming amphoteric oxides, and only bismuth has metallic properties. For As and Sb, the oxidation state + 3 is the most important. For Bi, this is the only possible one, except for the oxidation states that appear under some extremely specific conditions. Bismuth cannot lose all five valence electrons, the energy required for this is too high. However, it loses three br-electrons, forming a Bi ion.

Mendeleev was doing his dissertation work in Germany, in Heidelberg, just in time for the International Chemical Congress in Karlsruhe. He attended the congress and heard Cannizzaro's speech, in which he clearly stated his point of view on the problem of atomic weight. Returning to Russia, Mendeleev began to study the list of elements and drew attention to the periodicity of the change in valence for elements arranged in ascending order of atomic weights: valency of hydrogen 1, lithium I, beryllium 2, boron 3, carbon 4, magnesium 2, nitrogen 3, sulfur 2 , fluorine 1, sodium 1, aluminum 3, silicon 4, phosphorus 3, k1 oxygen 2, chlorine I, etc.

Phosphorus in terms of the number of valence electrons (35 3p) is an analogue of nitrogen

Oxygen atoms bond to at least two different atoms. So do calcium, sulfur, magnesium and barium. These elements have a valence of two, Nitrogen, phosphorus, aluminum and gold have a valence of three. Iron can have a valence of two or three. In principle, the question of valence turned out to be not as simple as it seemed at first, but even such a simple version of this theory made it possible to draw important conclusions.

In the transition from lithium to fluorine G, a regular weakening of the metallic properties and an increase in non-metallic properties occur with a simultaneous increase in valence. The transition from fluorine G to the next element in terms of atomic mass, sodium Na, is accompanied by an abrupt change in properties and valence, and sodium largely repeats the properties of lithium, being a typical monovalent metal, although more active. Magnesium, which follows sodium, is in many respects similar to beryllium Be (both are divalent, exhibit metallic properties, but the chemical activity of both is less pronounced than that of the N-Na pair). Aluminum A1, following magnesium, resembles boron B (valence is 3). Silicon 81 and carbon C, phosphorus P and nitrogen N, sulfur 8 and oxygen O, chlorine C1 and fluorine G are similar to each other as close relatives. valency and chemical properties. Potassium, like lithium and sodium, opens a series of elements (the third in a row), the representatives of which show a deep analogy with the elements of the first two rows.

The effectiveness of the additive depends on the valence state and position of the elements in the additive molecule, the presence of functional groups, their synergism, and other factors. The use of phosphorus-, sulfur-, oxygen- and nitrogen-containing compounds as additives to lubricating oils is closely related to the peculiarity of the electronic structure of these elements. Their interaction with the metal surface of engine parts leads to modification of the latter (change in structure) and, due to the formation of protective films, the anticorrosion, antiwear and extreme pressure properties of these compounds in the oil solution are provided. In addition, additives containing these elements stabilize the oil by terminating the oxidation chain by reaction with peroxide radicals and destroying hydroperoxides.

Halogenation. The most commonly used catalysts for chlorination are metallic iron, copper oxide, bromine, sulfur, iodine, iron halides, antimony, tin, arsenic, phosphorus, aluminum and copper, vegetable and animal charcoal, activated bauxite and other clays. Most of these catalysts are halogen carriers. So, Fe, Sb and P in halogen compounds are able to exist in two valence states in the presence of free chlorine, they alternately add and give chlorine in the active form. Similarly, iodine, bromine and sulfur form unstable compounds with chlorine. Bromination catalysts are similar to chlorination catalysts. Phosphorus is the best accelerator for iodination. No catalyst is required for the fluorination process. In the presence of oxygen, halogenation slows down.

Catalytic chlorination is based on the use of a chlorine carrier, such as iodine, sulfur, phosphorus, antimony, and others, in the form of the corresponding chlorides, which are dissolved in the chlorinated hydrocarbon or, when chlorinating gaseous paraffinic hydrocarbons, in a solvent. Only elements with at least two valency values are used. Radical generating substances such as diazomethap, tetraethyl lead and hexaphenylethane can also be used as homogeneous catalysts. They have the ability to split the chlorine molecule into atoms, which immediately cause a chain reaction.

When an element forms several rows of compounds corresponding to different oxidation states, after the name of the compound in brackets, an indication is given either of the valency of the cation (in Roman numerals) or of the number of halogen, oxygen, sulfur or acid residue atoms in the compound molecule (in words). For example, iron chloride (P1), phosphorus chloride three), manganese oxide (two). In this case, the designation of valence is usually given for less characteristic valence states. For example, for copper in the case of a divalent state, the indication of valence is omitted, while monovalent copper is designated as copper iodide (I).

The conductivity of substances such as silicon and germanium can be increased by introducing small amounts of certain impurities into them. For example, the introduction of boron or phosphorus impurities into silicon crystals effectively narrows the interband gap. Small amounts of boron or phosphorus (several ppm) can be incorporated into the silicon structure during crystal growth. The phosphorus atom has five valence electrons, and therefore, after four of them are used-

Phosphorus, arsenic, antimony and bismuth form stoichiometric compounds that correspond to formal valency, only with s-elements and d-elements of the zinc subgroup.

The fact that the dye and adsorbent constitute a single quantum system is evident from many facts. The most obvious of them is that the absorption of radiation of any, for example, the smallest, frequency within the absorption band of a given phosphorus causes the emission of its entire radiation spectrum, including frequencies much higher than the frequencies of the absorbed light. This means that radiation quanta come into common use, and the energy insufficient to emit frequencies that exceed the low frequency of the absorbed light also comes from the common resources of the solid body. The fact that although the dye is undoubtedly located only on the surface does not allow other interpretations, the absorption of light of its characteristic long waves (for which the crystal adsorbing this dye is practically transparent) is accompanied by the formation of metallic silver in the bulk of the silver bromide crystal. In this case, the sensitivity of silver bromide shifts the further towards long waves, the longer the chain of conjugated bonds in the structure of the dye molecule (Fig. 44). The fact is that the electrons of the dye are in wave motion and that the dye molecule, connecting with the crystal by a valence bond, forms a single whole with it. Crystal and dye form a single quantum system. It is not surprising, therefore, that the mechanism of photolysis of pure

Phosphorus, P, has the valence configuration 3x 3p, and sulfur, 8, has the valence configuration 3x 3p. The atom P, therefore, has a half-filled 3p shell, while atom 8 has an additional electron forced to pair with one of the electrons already present in the 3p orbitals.

SA for the formation of covalent bonds in the crystal structure of silicon, phosphorus has one more electron left. When an electric field is applied to the crystal, this electron can move away from the phosphorus atom; therefore, phosphorus is said to be an electron donor in the silicon crystal. Only 1.05 kJ mol is required to release donated electrons; this energy turns a silicon crystal with a small admixture of phosphorus into a conductor. When a boron impurity is introduced into a silicon crystal, the opposite phenomenon occurs. The boron atom lacks one electron to build the required number of covalent bonds in a silicon crystal. Therefore, for each boron atom in a silicon crystal, there is one vacancy in the bonding orbital. The valence electrons of silicon can be excited into these vacant orbitals associated with boron atoms, which allows the electrons to move freely through the crystal. Such conduction occurs as a result of the fact that an electron of the neighboring silicon atom jumps to the vacant orbital of the boron atom. A newly formed vacancy in the orbital of the silicon atom is immediately filled with an electron from another silicon atom following it. A cascade effect occurs, in which electrons jump from one atom to the next. Physicists prefer to describe this phenomenon as the movement of a positively charged hole in the opposite direction. But regardless of how this phenomenon is described, it is firmly established that less energy is required to activate the conductivity of a substance such as silicon if the crystal contains a small amount of an electron donor such as phosphorus or an electron acceptor such as boron.

White phosphorus consists of P4 tetrahedral molecules, shown schematically in fig. 21.25. As noted in sect. 8.7, part 1, bond angles of 60 ", as in the P4 molecule, are quite rare in other molecules. They indicate the presence of very tense bonds, which is consistent with a high reactivity

Although phosphorus is an electronic analog of nitrogen, the presence of free /-orbitals in the valence mectron layer of the atom makes phosphorus compounds unlike nitrogen compounds.

The electronic structure of organophosphorus compounds and the nature of chemical bonds;

To an even greater extent, aromatic properties are inherent in the phosphorine ring. 2,4,6-Triphenylphosphoric acid does not auto-oxidize and does not quaternize under the action of methyl iodide or triethyloxonium borofluoride. At the same time, its interaction with nucleophilic reagents - alkyl- or aryllithium compounds, easily proceeds in benzene already at room temperature ". In this case, the attack occurs on phosphorus, the valence shell of which expands to a decet, and a resonance-stabilized phosphorine anion appears ( 1) The formation of the anion (I) was confirmed by PMR and UV spectra.Hydrolysis of the reaction mixture, which has a deep blue-violet color, leads to

Preparation of silicate phosphors. The chemical composition of phosphors, the structure of phosphors, the valency of Mn. There are a significant number of different methods for the preparation of silicate-based crystal phosphors. Let's take one of them as an example. A well-purified ammonia solution of zinc oxide, an aqueous solution of manganese nitrate and an alcoholic solution of silicic acid (ethyl silicate) are poured together and a gel is formed. The gel is dried, triturated and calcined to 1200°C in quartz vessels and cooled rapidly after calcination. When the Mn content is low, calcination can always be carried out in air at a large Mn content, in order to avoid its oxidation, calcination is carried out in an atmosphere of carbon dioxide.

Catalytic oxidation of oil residues. There are many attempts to accelerate the process of oxidation of raw materials, improve the quality or give certain properties to the oxidized bitumen using various catalysts and initiators. It is proposed to use salts of hydrochloric acid and metals of variable valence (iron, copper, tin, titanium, etc.) as catalysts for redox reactions. As catalysts for dehydration, alkylation and cracking (transfer of protons), chlorides of aluminum, iron, tin, phosphorus pentoxide as oxidation initiators - peroxides are proposed. Most of these catalysts initiate reactions of densification of feedstock molecules (oils and resins) into asphaltenes without enriching bitumen with oxygen. The possibilities of accelerating the process of oxidation of raw materials and improving the properties of bitumen (mainly in the direction of increasing penetration at a given softening temperature), cited in numerous patent literature, are summarized in, but since the authors of the patents make their proposals without disclosing the chemistry of the process, their conclusions are in this monograph are not considered. Research by A. Heuberg

In most cases, halogenation is accelerated by light irradiation (wavelength 3000-5000 A) or high temperature (with or without a catalyst). As catalysts, halogen compounds of metals are usually used, having two valence states, capable of donating halogen atoms upon transition from one valence state to another, - P I5, P I3, Fe lg. Antimony chloride or manganese chloride are also used, as well as non-metallic catalysts - iodine, bromine or phosphorus.

Lithium and sodium have a moderate electron affinity, the electron affinity of beryllium is negative, while that of magnesium is close to zero. In the Be and M atoms, the valence x-orbital is completely filled, and the attached electron must populate the p-orbital located higher in energy. Nitrogen and phosphorus have little electron affinity because the electron to be added must pair in these atoms with one of the electrons in the half-filled p orbitals.

Atoms of elements of the third and subsequent periods often do not obey the octet rule. Some of them show an amazing ability to bind to more atoms (i.e., be surrounded by more electron pairs) than the octet rule predicts. For example, phosphorus and sulfur form compounds PF5 and SF, respectively. In the Lewis structures of these compounds, all the valence electrons of a heavy element are used by it to form bonds with other atoms.

In these diagrams, the full arrow indicates the position of the coordination bond. The donor elements appearing here (sulfur, arsenic and nitrogen), as well as selenium, phosphorus and others, do not form compounds with the properties of catalytic poisons if they are in the highest valence state, since in this case the molecules do not have pairs of free electrons. The same is true for the ions of these elements. For example, the sulfite ion is a poison, while the sulfate ion is not.

The number of electrons in the outer shell determines the valence states inherent in a given element, and, consequently, the types of its compounds - hydrides, oxides, hydroxides, salts, etc. So, in the outer shells of atoms of phosphorus, arsenic, antimony and bismuth there is the same number (five) electrons. This determines the identity of their main valence states (-3, -f3, -b5), the uniformity of EN3 hydrides, E2O3 and EaO3 oxides, hydroxides, etc. This circumstance is ultimately the reason that these elements are located in one subgroup periodic system.

Thus, the number of unpaired electrons in the excited state of beryllium, boron, and carbon atoms corresponds to the actual valency of these elements. As regards the atoms of nitrogen, oxygen, and fluorine, their excitation cannot lead to an increase in the number of non-ionic electrons in the second level of their electron shells. However, the analogues of these elements - phosphorus, sulfur and chlorine - since at the third level they

The number of unpaired electrons in a phosphorus atom upon excitation reaches five, which corresponds to its actual maximum palency. When a sulfur atom is excited, the number of unpaired electrons increases to four and even up to [yes, and for a chlorine atom, up to three, five, and, at most, up to seven, which also corresponds to the actual values of their valence. Basics of General Chemistry Volume 2 Edition 3 (1973) -

The properties of an atom are largely determined by the structure of its outer electronic layer. Electrons located on the outer, and sometimes on the penultimate, electronic layer of an atom can take part in the formation of chemical bonds. Such electrons are called valence. For example, in a phosphorus atom there are 5 valence electrons: (Fig. 1).

Rice. 1. Electronic formula of the phosphorus atom

The valence electrons of the atoms of the elements of the main subgroups are located on the s- and p-orbitals of the outer electronic layer. For elements of secondary subgroups, except for lanthanides and actinides, valence electrons are located on the s-orbitals of the outer and d-orbitals of the penultimate layers.

Valency is the ability of an atom to form chemical bonds. This definition and the very concept of valency are correct only in relation to substances with a covalent type of bond. For ionic compounds, this concept is not applicable; instead, the formal concept of "oxidation state" is used.

Valence is characterized by the number of electron pairs formed during the interaction of an atom with other atoms. For example, the valence of nitrogen in ammonia NH3 is three (Fig. 2).

Rice. 2. Electronic and graphical formulas of the ammonia molecule

The number of electron pairs that an atom can form with other atoms depends primarily on the number of its unpaired electrons. For example, in a carbon atom, two unpaired electrons are in 2p orbitals (Fig. 3). By the number of unpaired electrons, we can say that such a carbon atom can exhibit a valence equal to II.

Rice. 3. Electronic structure of the carbon atom in the ground state

In all organic substances and some inorganic compounds, carbon is tetravalent. Such a valence is possible only in the excited state of the carbon atom, into which it passes when additional energy is received.

In the excited state, 2s electrons are paired in the carbon atom, one of which passes to a free 2p orbital. Four unpaired electrons can participate in the formation of four covalent bonds. The excited state of an atom is usually denoted by an "asterisk" (Fig. 4).

Rice. 4. Electronic structure of the carbon atom in an excited state

Can nitrogen have a valence equal to five - according to the number of its valence electrons? Consider the valence possibilities of the nitrogen atom.

There are two electron layers in the nitrogen atom, on which only 7 electrons are located (Fig. 5).

Rice. 5. Electronic scheme of the structure of the outer layer of the nitrogen atom

Nitrogen can share three electron pairs with three other electrons. A pair of electrons in the 2s orbital can also participate in the formation of a bond, but according to a different mechanism - a donor-acceptor one, forming a fourth bond.

The depairing of 2s-electrons in the nitrogen atom is impossible, since there is no d-sublevel on the second electron layer. Therefore, the highest valency of nitrogen is IV.

Summing up the lesson

In the lesson, you learned to determine the valence possibilities of atoms of chemical elements. In the course of studying the material, you learned how many atoms of other chemical elements a particular atom can attach to itself, and also why the elements exhibit different valence values.

Sources

http://www.youtube.com/watch?t=3&v=jSTB1X1mD0o

http://www.youtube.com/watch?t=7&v=6zwx_d-MIvQ

http://www.youtube.com/watch?t=1&v=qj1EKzUW16M

http://interneturok.ru/ru/school/chemistry/11-klass - abstract

Rice. 1. Electronic formula of the phosphorus atom

Valence is characterized by the number of electron pairs formed during the interaction of an atom with other atoms. For example, the valency of nitrogen in ammonia NH 3 is three (Fig. 2).

Rice. 2. Electronic and graphical formulas of the ammonia molecule

Rice. 3. Electronic structure of the carbon atom in the ground state

Rice. 4. Electronic structure of the carbon atom in an excited state

Can nitrogen have a valence equal to five - according to the number of its valence electrons? Consider the valence possibilities of the nitrogen atom.

There are two electron layers in the nitrogen atom, on which only 7 electrons are located (Fig. 5).

Rice. 5. Electronic scheme of the structure of the outer layer of the nitrogen atom

The depairing of 2s-electrons in the nitrogen atom is impossible, since there is no d-sublevel on the second electron layer. Therefore, the highest valency of nitrogen is IV.

Summing up the lesson

Bibliography

Novoshinsky I.I., Novoshinskaya N.S. Chemistry. Textbook for grade 10 general. inst. profile level. - M .: LLC "TID "Russian Word - RS", 2008. (§ 9)
Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 11: textbook. for general institution: basic level / G.E. Rudzitis, F.G. Feldman. - M .: Education, JSC "Moscow textbooks", 2010. (§ 5)
Radetsky A.M. Chemistry. didactic material. 10-11 grades. - M.: Education, 2011.
Khomchenko I.D. Collection of problems and exercises in chemistry for high school. - M.: RIA "New Wave": Publisher Umerenkov, 2008. (p. 8)

A single collection of digital educational resources (video experiences on the topic) ().
Electronic version of the journal "Chemistry and Life" ().

Homework

With. 30 No. 2.41, 2.43 from the Collection of tasks and exercises in chemistry for secondary school (Khomchenko I.D.), 2008.
Write down the electronic diagrams of the structure of the chlorine atom in the ground and excited states.
How many valence electrons are in an atom: a) beryllium; b) oxygen; c) sulfur?

concept valence comes from the Latin word "valentia" and was known as early as the middle of the 19th century. The first "extensive" mention of valence was in the works of J. Dalton, who argued that all substances consist of atoms interconnected in certain proportions. Then, Frankland introduced the very concept of valency, which was further developed in the works of Kekule, who spoke about the relationship between valence and chemical bond, A.M. Butlerov, who in his theory of the structure of organic compounds associated valence with the reactivity of a particular chemical compound, and D.I. Mendeleev (in the Periodic system of chemical elements, the highest valency of an element is determined by the group number).

DEFINITION

Valence is the number of covalent bonds that an atom can form in combination with a covalent bond.

The valency of an element is determined by the number of unpaired electrons in an atom, since they take part in the formation of a chemical bond between atoms in compound molecules.

The ground state of an atom (the state with minimum energy) is characterized by the electronic configuration of the atom, which corresponds to the position of the element in the Periodic system. An excited state is a new energy state of an atom, with a new distribution of electrons within the valence level.

The electronic configurations of electrons in an atom can be depicted not only in the form of electronic formulas, but also with the help of electron-graphic formulas (energy, quantum cells). Each cell indicates an orbital, the arrow indicates an electron, the direction of the arrow (up or down) indicates the spin of the electron, a free cell indicates a free orbital that an electron can occupy when excited. If there are 2 electrons in a cell, such electrons are called paired, if electron 1 is unpaired. For example:

6 C 1s 2 2s 2 2p 2

Orbitals are filled in the following way: first, one electron with the same spins, and then the second electron with opposite spins. Since there are three orbitals with the same energy on the 2p sublevel, each of the two electrons occupied one orbital. One orbital remained free.

Determination of the valency of an element by electron-graphic formulas

The valence of an element can be determined by the electron-graphic formulas of the electronic configurations of electrons in an atom. Consider two atoms, nitrogen and phosphorus.

7 N 1s 2 2s 2 2p 3

Because the valency of an element is determined by the number of unpaired electrons, therefore, the valence of nitrogen is III. Since the nitrogen atom has no free orbitals, an excited state is impossible for this element. However, III is not the maximum nitrogen valence, the maximum nitrogen valency is V and is determined by the group number. Therefore, it should be remembered that with the help of electron-graphic formulas it is not always possible to determine the highest valence, as well as all the valences characteristic of this element.

15 P 1s 2 2s 2 2p 6 3s 2 3p 3

In the ground state, the phosphorus atom has 3 unpaired electrons, therefore, the valency of phosphorus is III. However, there are free d-orbitals in the phosphorus atom, therefore, electrons located on the 2s sublevel are able to depair and occupy vacant orbitals of the d-sublevel, i.e. go into an excited state.

Now the phosphorus atom has 5 unpaired electrons, therefore, phosphorus also has a valency equal to V.

Elements having multiple valency values

Elements of IVA - VIIA groups can have several valency values, and, as a rule, the valence changes in steps of 2 units. This phenomenon is due to the fact that electrons participate in the formation of a chemical bond in pairs.

Unlike the elements of the main subgroups, the elements of the B-subgroups, in most compounds, do not show a higher valency equal to the group number, for example, copper and gold. In general, transition elements exhibit a wide variety of chemical properties, which is explained by a large set of valences.

Consider the electronic graphic formulas of the elements and establish, in connection with which the elements have different valences (Fig. 1).

Tasks: determine the valence possibilities of As and Cl atoms in the ground and excited states.

Phosphorus valency

Determination of the valency of an element by electron-graphic formulas

Elements having multiple valency values

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