concept transition element commonly used to refer to any element with valence d or f electrons. These elements occupy periodic table transitional position between electropositive s-elements and electronegative p-elements.

d-Elements are called the main transition elements. Their atoms are characterized by internal building up of d-subshells. The fact is that the s-orbital of their outer shell is usually filled already before the filling of the d-orbitals in the previous electron shell begins. This means that each new electron added to the electron shell of the next d-element, in accordance with the principle of filling, does not fall on the outer shell, but on the inner subshell that precedes it. The chemical properties of these elements are determined by the participation of electrons in the reactions of both of these shells.

d-Elements form three transition series - in the 4th, 5th and 6th periods, respectively. The first transitional series includes 10 elements, from scandium to zinc. It is characterized by internal building of 3d-orbitals. The 4s orbital fills up earlier than the 3d orbital, because it has less energy (Klechkovsky's rule).

However, two anomalies should be noted. Chromium and copper have only one electron each in their 4s orbitals. This is because half-filled or fully filled subshells are more stable than partially filled subshells.

In the chromium atom, each of the five 3d orbitals that form the 3d subshell has one electron. Such a subshell is half-filled. In the copper atom, each of the five 3d orbitals has a pair of electrons. A similar anomaly is observed in silver.

All d-elements are metals.

Electronic configurations of the elements of the fourth period from scandium to zinc:

Chromium

Chromium is in the 4th period, in the VI group, in the secondary subgroup. It is a medium activity metal. In its compounds, chromium exhibits oxidation states +2, +3 and +6. CrO is a typical basic oxide, Cr 2 O 3 is an amphoteric oxide, CrO 3 is a typical acid oxide with the properties of a strong oxidizing agent, i.e., an increase in the oxidation state is accompanied by an increase in acidic properties.

Iron

Iron is in the 4th period, in the VIII group, in the secondary subgroup. Iron is a metal of medium activity, in its compounds it exhibits the most characteristic oxidation states +2 and +3. Iron compounds are also known, in which it exhibits an oxidation state of +6, which are strong oxidizing agents. FeO exhibits basic, and Fe 2 O 3 - amphoteric with a predominance of basic properties.

Copper

Copper is in the 4th period, in group I, in a secondary subgroup. Its most stable oxidation states are +2 and +1. In a series of voltages of metals, copper is after hydrogen, its chemical activity is not very high. Copper oxides: Cu2O CuO. The latter and copper hydroxide Cu(OH)2 exhibit amphoteric properties with a predominance of basic ones.

Zinc

Zinc is in the 4th period, in the II-group, in the secondary subgroup. Zinc belongs to the metals of medium activity, in its compounds it exhibits a single oxidation state +2. Zinc oxide and hydroxide are amphoteric.

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D. I. Mendeleev, natural classification of chemical elements, which is a tabular (or other graphical) expression periodic law Mendeleev (See Mendeleev's Periodic Law). P. s. e. developed by D. I. Mendeleev in 1869 ... ... Great Soviet Encyclopedia

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The long periods of the Mendeleev system, including the so-called intercalated decades, contain ten elements each, for which the number of electrons in the outer shell is two (two electrons) and which differ only in the number of electrons in second outside shell. Such elements are, for example, scandium to zinc or yttrium to cadmium.

The shell second from the outside plays a lesser role in the manifestation of chemical properties than the outer shell, because the connection of the electrons of the outer shell with the nucleus is weaker than in second outside. Therefore, elements in whose atoms the outer shells are built in the same way and only the second outer shells are different differ much less from each other in chemical properties than elements with different structures of the outer shells. Thus, all elements of intercalary decades, which together form the so-called side subgroups of the main eight groups of the Mendeleev system, are metals; they are all characterized by variable valence. V sixth period Mendeleev's systems, in addition to the intercalated decade, there are 14 more elements following lanthanum, in which the difference in the structure of electron shells manifests itself only in the third electron shell from the outside (filling /-sites in the fourth shell in the presence of filled sites These elements (lanthanides) on -23

As a result of experiments to determine the charges atomic nuclei by the age of 4 total number known elements - from hydrogen (Z = 1) to uranium (Z = 92) - amounted to 86. Six elements with atomic numbers = 43, 61, 72, 75, 85, 87 turned out to be missing in the system. However, despite these gaps, it was already clear that in the first period of the Mendeleev system there should be two elements - hydrogen and helium, in the 2nd and third - eight elements each, in the fourth and fifth - eighteen elements each, in the sixth - thirty-two elements.13

Prior to the elucidation of the structure of the sixth period of the Mendeleev system, element No. 72 was searched among the rare earth elements, and even individual scientists announced the discovery of this element. When it became clear that the sixth period of the Mendeleev system contains 32 elements, of which 14 are rare earth, N. Bohr pointed out that element No. 72 is already behind the rare earths, in the fourth group, and is, as Mendeleev expected, an analogue of zirconium.

Similarly, Bohr pointed out that element 75 was in the seventh group and was Mendeleev's predicted analogue of manganese. Indeed, in the year 3, element No. 72, called hafnium, was discovered in zircon ores, and it turned out that everything previously called zirconium was, in fact, a mixture of zirconium and hafnium.

In the same year, searches for element No. 75 were undertaken in various minerals, where, based on the relationship with manganese, the presence of this element was expected. Chemical operations for the isolation of this element were also based on its supposed similarity in properties to manganese. The search culminated in the year 5 with the discovery of a new element called rhenium.24

But this did not yet exhaust all the possibilities of artificial production of new elements. The boundary of the periodic system in the region of light nuclei is given by hydrogen, because there cannot be an element with a nuclear charge less than one.

But in the region of heavy nuclei this boundary is by no means set by uranium. In truth, the absence of elements heavier than uranium in nature only indicates that the half-lives of such elements are much less than the age of the Earth. Therefore, among the three trees of natural radioactive decay, including isotopes with mass numbers A \u003d 4n, 4n - -2 and 4 4-3, only branches that begin with long-term isotopes Th, and 2 and 2 and 2 and All short-period branches, figuratively speaking, dried up and fell off immemorial times. In addition, the fourth tree of radioactive decay, including isotopes with mass numbers A = 4ga + 1, completely dried up and died, if there ever were isotopes of this series on Earth.
As you know, the fourth and fifth periods of the Mendeleev system contain 18 elements each, while the sixth period contains 32 elements, because between the third group element lanthanum (No. 57) and the fourth group element hafnium (No. 72) there are fourteen more rare earth elements similar to lanthanum .

After clarifying the structure of the seventh period of the D. I. Mendeleev system, it became clear that in the periodic system, the first period of two elements is followed by two periods of eight elements, then two periods of eighteen elements and two periods of thirty-two elements. In the 2nd such period, which should end with the element. volume No., while seventeen more elements are missing, two of them are not enough to complete the family of actinides, and element No. should already be located in the fourth group of the periodic system, being an analogue of hafnium.

At n + / = 5, the levels n = 3, 1 = 2 (M), n = 4, / = 1 (4p) and, finally, n = 5, / = 0 (55) are filled. If up to calcium filling electronic levels went in ascending order of the numbers of electron shells (15, 25, 2p, 33, 3p, 45), then after filling the 5-places of the fourth electron shell, instead of continuing to fill this shell with /7-electrons, the filling of the previous, third, shell with electrons begins. In total, each shell can contain, as is clear from what has been said above, 10 electrons. Accordingly, calcium in the periodic system is followed by 10 elements from scandium (3 452) to zinc (3 452), in the atoms of which the -layer of the third shell is filled, and only then the p-layer of the fourth shell is filled - from gallium (3 (Ncz p) up to krypton 3dShz p). In rubidium and strontium, which begin the fifth period, 55 and 552 electrons appear.19

Investigations of the last fifteen years have led to the artificial production of a series of short-period ones. isotopes of the nuclei of elements from mercury to uranium, to the resurrection of the parents of uranium, protactinium and thorium, long dead in nature - transuranium elements from No. 93 to No - and to the reconstruction of the fourth decay series, including isotopes with mass numbers /4 = 4r- -1. This series can be conditionally called the neptunium decay series, because the longest-lived in the series is the isotope of element No. 93 - the half-life of which is close to 2 million years.

The sixth period begins with the filling of two places for s-electrons in the sixth shell, so that the structure of the outer shells of the atoms of element No. 56 - barium - has the form 4s j0 d 05s2p66s2. It is obvious that with a further increase in the number of electrons in the atoms of the elements following barium, the shells can be filled either with 4f, or with bd, or, finally, with 6 electrons. Already in the fourth and fifth periods Mendeleev's systems, containing 18 elements, filling d-places second outside shells occurred before the filling of the p-sites of the outer shell. So in sixth period the filling of 6/7-places begins only with element No. 81-thallium. - In the atoms of the twenty-four elements located between barium and thallium, the fourth shell is filled with /-electrons and the fifth shell with d-electrons.

Patterns of changes in the activity of d-elements in the period

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d-elements and their compounds have a number of characteristic properties: oxidation state variables; ability to form complex ions; the formation of colored compounds.

Zinc is not among the transition elements. His physical and Chemical properties do not allow it to be classified as a transition metal. In particular, in its compounds it exhibits only one oxidation state and does not exhibit catalytic activity.

The d-elements have some peculiarities in comparison with the elements of the main subgroups.

1. In d-elements, only a small part of the valence electrons is delocalized throughout the crystal (whereas in alkali and alkaline earth metals, the valence electrons are completely given to collective use). The remaining d-electrons participate in the formation of directed covalent bonds between neighboring atoms. Thus, these elements in the crystalline state do not have a purely metallic bond, but a covalent metallic one. Therefore, they are all solid (except for Hg) and refractory (with the exception of Zn, Cd) metals.

The most refractory metals are VB and VIB subgroups. They fill half of the d-sublevel with electrons and realize the maximum possible number unpaired electrons, and consequently, largest number covalent bonds. Further filling leads to a decrease in the number of covalent bonds and a drop in melting temperatures.

2. Due to the unoccupied d-shells and the presence of unoccupied ns- and np-levels close in energy, d-elements are prone to complex formation; their complex compounds are, as a rule, colored and paramagnetic.

3. d-Elements more often than the elements of the main subgroups, form compounds of variable composition (oxides, hydrides, carbides, silicides, nitrides, borides). In addition, they form alloys with each other and with other metals, as well as intermetallic compounds.

4. For d-elements, a large set of valence states is characteristic (Table 8.10) and, as a result of this, a change in acid-base and redox properties over a wide range.

Since some of the valence electrons are in s-orbitals, the lowest oxidation states they exhibit are usually equal to two. The exception is the elements whose ions E +3 and E + have stable configurations d 0 , d 5 and d 10: Sc 3+ , Fe 3+ , Cr + , Cu + , Ag + , Au + .

Compounds in which d-elements are in the lowest oxidation state form ion-type crystals, in chemical reactions exhibit basic properties and are, as a rule, reducing agents.

The stability of compounds in which d-elements are in the highest oxidation state (equal to the group number) increases within each transition row from left to right, reaching a maximum for 3d-elements for Mn, and in the second and third transition rows for Ru and Os, respectively. . Within one subgroup, the stability of compounds of the highest oxidation state decreases in the series 5d > 4d > 3d, as evidenced by the nature of the change in the Gibbs energy (isobaric-isothermal potential) of the same type of compounds, for example:

This phenomenon is due to the fact that with an increase in the principal quantum number within one subgroup, a decrease in the difference between the energies of the (n – 1)d- and ns-sublevels occurs. These compounds are characterized by covalent-polar bonds. They are acidic in nature and are oxidizing agents (CrO 3 and K 2 CrO 4 , Mn 2 O 7 and KMnO 4).

Compounds in which d-electrons are in intermediate oxidation states exhibit amphoteric properties and redox duality.

5. The similarity of d-elements with the elements of the main subgroups E(0) is fully manifested in the elements of the third group ns 2 np 1 and (n – 1)d 1 ns 2 . As the group number increases, it decreases; elements of subgroup VIIIA - gases, VIIIB - metals. In the first group, a distant similarity appears again (all elements are metals), and the elements of the IB subgroup are good conductors; this similarity is enhanced in the second group, since the d-elements Zn, Cd, and Hg do not participate in the formation of a chemical bond.

6. The d-elements of the IIIB–VIIB subgroups in higher oxidation states are similar in properties to the corresponding p-elements. Thus, in higher oxidation states, Mn (VII) and Cl (VII) are electronic analogues. The similarity of electronic configurations (s 2 p 6) leads to the similarity of the properties of compounds of heptavalent manganese and chlorine. Mn 2 O 7 and Cl 2 O 7 in normal conditions unstable liquids, which are anhydrides of strong acids with general formula NET 4 . In the lower oxidation states, manganese and chlorine have a different electronic structure, which causes a sharp difference in the properties of their compounds. For example, lower chlorine oxide Cl 2 O (s 2 p 4) is a gaseous substance that is hypochlorous acid anhydride (HClO), while lower manganese oxide MnO (d 5) is a basic crystalline solid.

7. As you know, the reducing ability of a metal is determined not only by its ionization energy (M - ne - → M n +; + ∆H ionization), but also by the enthalpy of hydration of the formed cation (M n + + mH 2 O → M n + mH 2 O; –∆H hydr). The ionization energies of d-elements are high in comparison with other metals, but they are compensated by the large enthalpies of hydration of their ions. As a result, the electrode potentials of most d-elements are negative.

In the period with the growth of Z restorative properties metals decrease, reaching a minimum for group IB elements. Heavy metals of groups VIIIB and IB are called noble for their inertness.

The redox tendencies of compounds of d-elements are determined by the change in the stability of higher and lower oxidation states, depending on their position in the periodic system. Compounds with the maximum oxidation state of the element exhibit exclusively oxidizing properties, and with the lowest - reducing. Mn (OH) 2 is easily oxidized in air Mn (OH) 2 + 1 / 2O 2 \u003d MnO 2 + H 2 O. Mn (IV) compounds are easily reduced to Mn (II): MnO 2 + 4HCl \u003d MnCl 2 + Cl 2 + 2H 2 O, but strong oxidizing agents oxidize to Mn (VII). Permanganate ion MnO 4 - can only be an oxidizing agent.

Since for d-elements within the subgroup, the stability of higher oxidation states increases from top to bottom, the oxidizing properties of compounds of the highest oxidation state fall sharply. So, chromium (VI) compounds (CrO 3, K 2 CrO 4, K 2 Cr 2 O 7) and manganese (VII) (Mn 2 O 7, KMnO 4) are strong oxidizing agents, and WO 3, Re 2 O 7 and salts of their respective acids (H 2 WO 4 , HReO 4) are difficult to recover.

8. The acid-base properties of d-element hydroxides are affected by the same factors (ionic radius and ion charge) as p-element hydroxides.

Hydroxides of lower oxidation states of d-elements usually exhibit basic properties, and the corresponding higher degrees oxidations are acidic. In intermediate oxidation states, the hydroxides are amphoteric. The change in the acid-base properties of hydroxides with a change in the degree of oxidation is especially pronounced in manganese compounds. In the series Mn(OH) 2 - Mn(OH) 3 - Mn(OH) 4 - H 2 MnO 4 - HMnO 4, the properties of hydroxides vary from weak foundation Mn(OH) 2 through amphoteric Mn(OH) 3 and Mn(OH) 4 to strong acids H 2 MnO 4 and HMnO 4 .

Within one subgroup, hydroxides of d-elements of the same oxidation state are characterized by an increase in basic properties when moving from top to bottom. For example, in the IIIB group, Sc (OH) 3 is a weak base, and La (OH) 3 is a strong base. Group IVB elements Ti, Zn, Hf form amphoteric hydroxides E(OH) 4 , but their acidic properties weaken when moving from Ti to Hf.

9. Distinctive feature transition elements is the formation of phases of variable composition. These are, firstly, interstitial and substitutional solid solutions and, secondly, compounds of variable composition. Solid solutions are formed by elements with similar electronegativity, atomic radii, and identical crystal lattices. The more different elements are in nature, the less they dissolve in each other and the more prone to the formation of chemical compounds. Such compounds can have both constant and variable composition. Unlike solid solutions, in which the lattice of one of the components is preserved, compounds are characterized by the formation of a new lattice and new chemical bonds. In other words, to chemical compounds include only those phases of variable composition that differ sharply in structure and properties from the initial ones.

Compounds of variable composition are characterized by the following features:

a) The composition of these compounds depends on the method of preparation. So, depending on the synthesis conditions, titanium oxides have the composition TiO 1.2–1.5 and TiO 1.9–2.0; titanium and vanadium carbides - TiC 0.6–1.0 and VC 0.58–1.09, titanium nitride TiN 0.45–1.00.

b) Compounds retain their crystal lattice with significant fluctuations in the quantitative composition, that is, they have a wide area of ​​homogeneity. Thus, TiC 0.6–1.0, as follows from the formula, retains the titanium carbide lattice with a lack of up to 40% of carbon atoms in it.

c) The nature of the bond in such compounds is determined by the degree of filling of the d-orbitals of the metal. The electrons of the interstitial non-metal populate vacant d-orbitals, which leads to an increase in the covalence of the bonds. That is why the proportion of the metallic bond in the compounds of the initial elements of the d-series (groups IV–V) is reduced.

The presence of a covalent bond in them is confirmed by large positive enthalpies of formation of compounds, higher hardness and melting point, lower electrical conductivity compared to the metals that form them.

Copper is an element of the eleventh group of the fourth period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 29. It is designated by the symbol Cu (lat. Cuprum). The simple substance copper (CAS number: 7440-50-8) is a golden-pink ductile transition metal (pink in the absence of an oxide film). Since ancient times, it has been widely used by man.