Periodic system of chemical elements. Chapter II. The structure of atoms and the periodic law
The periodic law is the basis of modern chemistry. All scientific directions and research in chemistry are based on the knowledge of the periodic law: the study of the interconversions of substances, the production of new materials, theoretical study structure of substances, types chemical bonds etc.
The nuclear charge determines the number of electrons in an atom, each subsequent element has one more electron than the previous one. The charge of the nucleus determines the structure of the electron shell of the atom in the ground state. Elements are arranged in the periodic table of elements in ascending order of the charge of the nuclei of their atoms. Elements the electronic configurations of atoms are periodically repeated and, as a consequence, are periodically repeated Chemical properties, which are determined electronic configuration atoms. The periodicity of the electronic structure is manifested in the fact that after a certain number of elements, s-, p- and d-elements with the same configurations of electronic sublevels are repeated again. Periodicity is inherent in the entire electron shell of atoms, and not just its outer layers. The periodicity of electronic structures leads to a periodic change in a number of chemical and physical properties elements: atomic radii, ionization energies, electron affinity, electronegativity. Let's discuss this more specifically.
Atomic radii chemical elements change periodically depending on the charge of the nucleus of the atom (or the ordinal number of the element). In periods, the atomic radii decrease from an alkali metal to a halogen. So the atomic radius of the sodium atom is 0.186 nm, magnesium is 0.16 nm, and chlorine is 0.099 nm. The atomic radius of the next alkali metal, which opens the next period, increases sharply, its radius is much larger than the radius of the alkali metal standing above it. For example: the radius of a sodium atom is 0.186 nm, and that of a potassium atom is 0.231 nm.
The decrease in the radii of atoms in periods from left to right, that is, with an increase in the charge of the nucleus of an atom, is explained by the fact that an increase in the charge of the nucleus of an atom contributes to a stronger attraction of electrons of a given electronic level to the nucleus (it acts stronger than the repulsion of electrons from each other).
In groups, with an increase in the charge of the nucleus of an atom (from top to bottom), the radii of atoms increase. This is because each lower element has one more electronic level, so it also has a larger atomic radius. This pattern is more pronounced for the elements of the main subgroups (for s- and p-elements) than for the elements of secondary subgroups (d-elements).
There are exceptions to these considered regularities, but we will not discuss them, since this is not included in the scope of our program.
We also point out that it is necessary to distinguish between the radii of a free atom and the following radii:
a) covalent radius is half the internuclear distance in molecules or crystals of the corresponding simple substances (i.e. substances with a covalent type of bond);
b) the metallic radius is half the distance between the centers of two adjacent atoms in crystal lattice metal;
v) the ionic radii of atoms are considered as half the distance of the sum of the radii of the cation and anion (It should be remembered that the radii of cations are always less than the atomic radii of the corresponding elements, and the radii of anions are always greater than the radii of the atoms of the corresponding elements).
Ionization energy and electron affinity these are parameters that allow us to evaluate the ability of atoms to lose and accept electrons.
Periodic law and periodic system chemical elements of D. I. Mendeleev on the basis of ideas about the structure of atoms. The value of the periodic law for the development of science.
In 1869, D. I. Mendeleev, based on an analysis of the properties of simple substances and compounds, formulated the Periodic Law:
The properties of simple bodies ... and compounds of elements are in a periodic dependence on the value atomic masses elements.
On the basis of the periodic law, the periodic system of elements was compiled. In it, elements with similar properties were combined into vertical columns - groups. In some cases, when placing elements in the Periodic system, it was necessary to violate the sequence of increasing atomic masses in order to observe the periodicity of the repetition of properties. For example, tellurium and iodine, as well as argon and potassium, had to be "swapped".
The reason is that Mendeleev proposed the periodic law at a time when nothing was known about the structure of the atom.
After the planetary model of the atom was proposed in the 20th century, the periodic law is formulated as follows:
The properties of chemical elements and compounds are in a periodic dependence on the charges of atomic nuclei.
The charge of the nucleus is equal to the number of the element in the periodic system and the number of electrons in the electron shell of the atom.
This formulation explained the "violations" of the Periodic Law.
In the Periodic System, the period number is equal to the number electronic levels in an atom, the group number for the elements of the main subgroups is equal to the number of electrons in the outer level.
The reason for the periodic change in the properties of chemical elements is the periodic filling of electron shells. After filling the next shell, a new period begins. The periodic change of elements is clearly seen in the change in the composition and properties and properties of oxides.
The scientific significance of the periodic law. The periodic law made it possible to systematize the properties of chemical elements and their compounds. When compiling the periodic system, Mendeleev predicted the existence of many yet undiscovered elements, leaving free cells for them, and predicted many properties of undiscovered elements, which facilitated their discovery.
Ticket number 2
The structure of atoms of chemical elements on the example of elements of the second period and IV-A group of the periodic system of chemical elements of D. I. Mendeleev. Regularities in the change in the properties of these chemical elements and the simple and complex substances (oxides, hydroxides) formed by them, depending on the structure of their atoms.
As you move from left to right along the period, the metallic properties of the elements become less pronounced. When moving from top to bottom within the same group, the elements, on the contrary, reveal more and more pronounced metallic properties. Elements located in the middle part of short periods (2nd and 3rd periods), as a rule, have a framework covalent structure, and elements from the right side of these periods exist in the form of simple covalent molecules.
Atomic radii change as follows: decrease when moving from left to right along the period; increase as you move from top to bottom along the group. When moving from left to right along the period, electronegativity, ionization energy and electron affinity increase, which reach a maximum for halogens. For noble gases, the electronegativity is 0. The change in the electron affinity of the elements when moving from top to bottom along the group is not so characteristic, but the electronegativity of the elements decreases.
In the elements of the second period, 2s and then 2p orbitals are filled.
The main subgroup of group IV of the periodic system of chemical elements of D. M. Mendeleev contains carbon C, silicon Si, germanium Ge, tin Sn and lead Pb. The outer electron layer of these elements contains 4 electrons (configuration s 2 p 2). Therefore, the elements of the carbon subgroup must have some similarities. In particular, their highest degree oxidation is the same and equal to +4.
And what causes the difference in the properties of the elements of the subgroup? The difference between the ionization energy and the radius of their atoms. As the atomic number increases, the properties of the elements naturally change. So, carbon and silicon are typical non-metals, tin and lead are metals. This is manifested primarily in the fact that carbon forms a simple non-metal substance (diamond), while lead is a typical metal.
Germanium occupies an intermediate position. According to the structure of the electron shell of the atom, p-elements of group IV have even oxidation states: +4, +2, - 4. The simplest formula hydrogen compounds- EN 4, and E-N communications are covalent and equivalent due to the hybridization of s- and p-orbitals with the formation of sp 3 orbitals directed at tetrahedral angles.
The weakening of the signs of a non-metallic element means that in the subgroup (С-Si-Ge-Sn-Pb) the highest positive degree+4 oxidation is becoming less and less typical, and +2 oxidation state is becoming more typical. So, if carbon is the most stable compounds in which it has an oxidation state of +4, then compounds in which it exhibits an oxidation state of +2 are stable for lead.
And what can be said about the stability of compounds of elements in a negative oxidation state -4? Compared with non-metallic elements of groups VII-V, p-elements of group IV show signs of a non-metallic element to a lesser extent. Therefore, for elements of the carbon subgroup, a negative oxidation state is not typical.
The periodic system of chemical elements is a classification of chemical elements created by D. I. Mendeleev on the basis of the periodic law discovered by him in 1869.
D. I. Mendeleev
According to the modern formulation of this law, in a continuous series of elements, arranged in order of increasing magnitude of the positive charge of the nuclei of their atoms, elements with similar properties are periodically repeated.
The periodic system of chemical elements, presented in the form of a table, consists of periods, series and groups.
At the beginning of each period (with the exception of the first) there is an element with pronounced metallic properties (alkali metal).
Symbols for the color table: 1 - chemical sign of the element; 2 - name; 3 - atomic mass (atomic weight); 4 - serial number; 5 - distribution of electrons over the layers.
As the ordinal number of the element increases, equal to the value of the positive charge of the nucleus of its atom, the metallic properties gradually weaken and the non-metallic properties increase. The penultimate element in each period is an element with pronounced non-metallic properties (), and the last is an inert gas. In period I there are 2 elements, in II and III - 8 elements each, in IV and V - 18 elements each, in VI - 32 and in VII (incomplete period) - 17 elements.
The first three periods are called small periods, each of them consists of one horizontal row; the rest - in large periods, each of which (excluding the VII period) consists of two horizontal rows - even (upper) and odd (lower). In even rows of large periods are only metals. The properties of the elements in these rows change slightly with increasing serial number. The properties of elements in odd series of large periods change. In period VI, lanthanum is followed by 14 elements that are very similar in chemical properties. These elements, called lanthanides, are listed separately under the main table. Actinides, the elements following actinium, are similarly presented in the table.
The table has nine vertical groups. The group number, with rare exceptions, is equal to the highest positive valence of the elements of this group. Each group, excluding zero and eighth, is divided into subgroups. - main (located to the right) and side. In the main subgroups, with an increase in the serial number, the metallic properties of the elements are enhanced and the non-metallic properties of the elements are weakened.
Thus, the chemical and a number of physical properties of elements are determined by the place that a given element occupies in the periodic system.
Biogenic elements, i.e., elements that make up organisms and perform a certain biological role in it, occupy the upper part of the periodic table. Cells occupied by elements that make up the bulk (more than 99%) of living matter are stained blue. pink color- cells occupied by trace elements (see).
The Periodic Table of Chemical Elements is the biggest achievement modern natural science and a vivid expression of the most general dialectical laws of nature.
See also , Atomic weight.
The periodic system of chemical elements is a natural classification of chemical elements created by D. I. Mendeleev on the basis of the periodic law discovered by him in 1869.
In the original formulation, the periodic law of D. I. Mendeleev stated: the properties of chemical elements, as well as the forms and properties of their compounds, are in a periodic dependence on the magnitude of the atomic weights of the elements. Later, with the development of the doctrine of the structure of the atom, it was shown that a more accurate characteristic of each element is not the atomic weight (see), but the value of the positive charge of the nucleus of the atom of the element, equal to the ordinal (atomic) number of this element in the periodic system of D. I. Mendeleev . The number of positive charges on the nucleus of an atom is equal to the number of electrons surrounding the nucleus of an atom, since atoms as a whole are electrically neutral. In the light of these data, the periodic law is formulated as follows: the properties of chemical elements, as well as the forms and properties of their compounds, are in a periodic dependence on the positive charge of the nuclei of their atoms. This means that in a continuous series of elements, arranged in ascending order of the positive charges of the nuclei of their atoms, elements with similar properties will be periodically repeated.
The tabular form of the periodic system of chemical elements is presented in its modern form. It consists of periods, series and groups. A period represents a sequential horizontal row of elements arranged in ascending order of the positive charge of the nuclei of their atoms.
At the beginning of each period (with the exception of the first) there is an element with pronounced metallic properties (alkali metal). Then, as the serial number increases, the metallic properties of the elements gradually weaken and the non-metallic properties of the elements increase. The penultimate element in each period is an element with pronounced non-metallic properties (halogen), and the last is an inert gas. Period I consists of two elements, the role of an alkali metal and a halogen is simultaneously performed by hydrogen. II and III periods include 8 elements each, called Mendeleev typical. IV and V periods have 18 elements each, VI-32. VII period is not yet completed and is replenished artificially created elements; there are currently 17 elements in this period. I, II and III periods are called small, each of them consists of one horizontal row, IV-VII - large: they (with the exception of VII) include two horizontal rows - even (upper) and odd (lower). In even rows of large periods, only metals are found, and the change in the properties of the elements in the row from left to right is weakly expressed.
In odd series of large periods, the properties of the elements in the series change in the same way as the properties of typical elements. In an even number of the VI period after lanthanum 14 elements follow [called lanthanides (see), lanthanides, rare earth elements], similar in chemical properties to lanthanum and to each other. Their list is given separately under the table.
Separately, the elements following the actinium-actinides (actinides) are written out and given under the table.
There are nine vertical groups in the periodic table of chemical elements. The group number is equal to the highest positive valency (see) of the elements of this group. The exceptions are fluorine (it happens only negatively monovalent) and bromine (it does not happen heptavalent); in addition, copper, silver, gold can exhibit a valence greater than +1 (Cu-1 and 2, Ag and Au-1 and 3), and of the elements of group VIII, only osmium and ruthenium have a valency of +8. Each group, with the exception of the eighth and zero, is divided into two subgroups: the main (located to the right) and the secondary. The main subgroups include typical elements and elements of large periods, the secondary - only elements of large periods and, moreover, metals.
In terms of chemical properties, the elements of each subgroup of this group differ significantly from each other, and only the highest positive valency is the same for all elements of this group. In the main subgroups, from top to bottom, the metallic properties of elements increase and non-metallic ones weaken (for example, francium is an element with the most pronounced metallic properties, and fluorine is non-metallic). Thus, the place of an element in the periodic system of Mendeleev (serial number) determines its properties, which are the average of the properties of neighboring elements vertically and horizontally.
Some groups of elements have special names. So, the elements of the main subgroups of group I are called alkali metals, group II - alkaline earth metals, group VII - halogens, elements located behind uranium - transuranium. Elements that are part of organisms, take part in metabolic processes and have a pronounced biological role are called biogenic elements. All of them occupy the upper part of the table of D. I. Mendeleev. This is primarily O, C, H, N, Ca, P, K, S, Na, Cl, Mg and Fe, which make up the bulk of living matter (more than 99%). The places occupied by these elements in the periodic table are colored in light blue. Biogenic elements, which are very few in the body (from 10 -3 to 10 -14%), are called microelements (see). In the cells of the periodic system, stained in yellow, trace elements are placed, the vital importance of which for humans has been proven.
According to the theory of the structure of atoms (see Atom), the chemical properties of elements depend mainly on the number of electrons in the outer electron shell. The periodic change in the properties of elements with an increase in the positive charge of atomic nuclei is explained by the periodic repetition of the structure of the outer electron shell (energy level) of atoms.
In small periods, with an increase in the positive charge of the nucleus, the number of electrons in the outer shell increases from 1 to 2 in period I and from 1 to 8 in periods II and III. Hence the change in the properties of the elements in the period from an alkali metal to an inert gas. The outer electron shell, containing 8 electrons, is complete and energetically stable (elements of the zero group are chemically inert).
In large periods in even rows, with an increase in the positive charge of the nuclei, the number of electrons in the outer shell remains constant (1 or 2) and the second outer shell is filled with electrons. Hence the slow change in the properties of elements in even rows. In odd series of long periods, with an increase in the charge of the nuclei, the outer shell is filled with electrons (from 1 to 8) and the properties of the elements change in the same way as for typical elements.
The number of electron shells in an atom is equal to the period number. The atoms of the elements of the main subgroups have a number of electrons on their outer shells equal to the group number. The atoms of the elements of the secondary subgroups contain one or two electrons on the outer shells. This explains the difference in the properties of the elements of the main and secondary subgroups. The group number indicates the possible number of electrons that can participate in the formation of chemical (valence) bonds (see Molecule), therefore such electrons are called valence. For elements of secondary subgroups, not only the electrons of the outer shells, but also the penultimate ones, are valence. The number and structure of electron shells are indicated in the attached periodic table of chemical elements.
The periodic law of D. I. Mendeleev and the system based on it are of exceptionally great importance in science and practice. The periodic law and the system were the basis for the discovery of new chemical elements, exact definition their atomic weights, the development of the doctrine of the structure of atoms, the establishment of geochemical laws for the distribution of elements in the earth's crust and the development contemporary ideas about living matter, the composition of which and the laws associated with it are in accordance with the periodic system. The biological activity of the elements and their content in the body are also largely determined by the place they occupy in the periodic system of Mendeleev. So, with an increase in the serial number in a number of groups, the toxicity of elements increases and their content in the body decreases. The periodic law is a vivid expression of the most general dialectical laws of the development of nature.
Periodic law of D.I Mendeleev.
The properties of chemical elements, and therefore the properties of the simple and complex bodies they form, are in a periodic dependence on the magnitude of the atomic weight.
The physical meaning of the periodic law.
The physical meaning of the periodic law lies in the periodic change in the properties of elements, as a result of periodically repeating e-th shells of atoms, with a successive increase in n.
The modern formulation of D.I. Mendeleev's PZ.
The property of chemical elements, as well as the property of the simple or complex substances formed by them, is in a periodic dependence on the magnitude of the charge of the nuclei of their atoms.
Periodic system of elements.
Periodic system - a system of classifications of chemical elements, created on the basis of the periodic law. Periodic system - establishes relationships between chemical elements reflecting their similarities and differences.
Periodic table (there are two types: short and long) of elements.
The Periodic Table of the Elements is a graphical representation of the Periodic Table of the Elements, consists of 7 periods and 8 groups.
Question 10
Periodic system and structure of electron shells of atoms of elements.
Later it was found that not only the ordinal number of the element has a deep physical meaning, but other concepts previously discussed also gradually acquired physical meaning. For example, the group number, indicating the highest valency of the element, thereby reveals the maximum number of electrons of an atom of a particular element that can participate in the formation of a chemical bond.
The period number, in turn, turned out to be related to the number of energy levels present in the electron shell of an atom of an element of a given period.
Thus, for example, the "coordinates" of tin Sn (serial number 50, period 5, main subgroup of group IV) mean that there are 50 electrons in the tin atom, they are distributed over 5 energy levels, only 4 electrons are valence.
The physical meaning of finding elements in subgroups of various categories is extremely important. It turns out that for elements located in subgroups of category I, the next (last) electron is located on s-sublevel external level. These elements belong to the electronic family. For atoms of elements located in subgroups of category II, the next electron is located on p-sublevel external level. These are the elements of the “p” electronic family. Thus, the next 50th electron of tin atoms is located on the p-sublevel of the outer, i.e., 5th energy level.
For atoms of elements of subgroups of category III, the next electron is located on d-sublevel, but already before the external level, these are elements of the electronic family "d". For lanthanide and actinide atoms, the next electron is located on the f-sublevel, before the external level. These are the elements of the electronic family "f".
It is no coincidence, therefore, that the numbers of subgroups of these 4 categories noted above, that is, 2-6-10-14, coincide with the maximum numbers of electrons in the s-p-d-f sublevels.
But it turns out that it is possible to solve the problem of the order of filling the electron shell and derive an electronic formula for an atom of any element and on the basis of the periodic system, which clearly indicates the level and sublevel of each successive electron. The periodic system also indicates the placement of elements one after another into periods, groups, subgroups and the distribution of their electrons by levels and sublevels, because each element has its own, characterizing its last electron. As an example, let us analyze the compilation of an electronic formula for the atom of the element zirconium (Zr). The periodic system gives the indicators and "coordinates" of this element: serial number 40, period 5, group IV, side subgroup. First conclusions: a) all 40 electrons, b) these 40 electrons are distributed over five energy levels; c) out of 40 electrons only 4 are valence, d) the next 40th electron entered the d-sublevel before the outer, i.e. the fourth energy level.Similar conclusions can be drawn about each of the 39 elements preceding zirconium, only the indicators and coordinates will be different each time.
Periodic law D.I. Mendeleev is a fundamental law that establishes a periodic change in the properties of chemical elements depending on the increase in the charges of the nuclei of their atoms. Discovered by D.I. Mendeleev in March 1869 when comparing the properties of all the elements known at that time and the values of their atomic masses. The term "periodic law" was first used by Mendeleev in November 1870, and in October 1871 he gave the final formulation of the Periodic Law: "the properties of simple bodies, as well as the forms and properties of the compounds of elements, and therefore the properties of the simple and complex bodies formed by them, are in a periodic dependence from their atomic weight.
Hund's rule: atomic orbitals belonging to the same sublevel are each filled first with one electron, and then they are filled with second electrons.
Hund's rule is also called the maximum multiplicity principle, i.e. the maximum possible parallel direction of electron spins of one energy sublevel.
At the highest energy level of a free atom, there can be no more than eight electrons.
Electrons located at the highest energy level of an atom (in the outer electron layer) are called external; The number of outer electrons in an atom of any element is never more than eight. For many elements, it is the number of outer electrons (with filled inner sublevels) that largely determines their chemical properties. For other electrons whose atoms have an unfilled inner sublevel, such as 3 d- the sublevel of atoms of such elements as Sc, Ti, Cr, Mn, etc., the chemical properties depend on the number of both internal and external electrons. All these electrons are called valence; in abbreviated electronic formulas of atoms, they are written after symbol atomic core, i.e. after the expression in square brackets.
2.3. Periodic law and Periodic system of elements
At the beginning of the 20th century, with the discovery of the structure of the atom, it was found that the frequency of changes in the properties of elements is determined by not by atomic weight, but by the charge of the nucleus equal to atomic number and the number of electrons, the distribution of which over the electron shells of an atom of an element determines its chemical properties.
The modern formulation of the Periodic Law reads:
The further development of the periodic system is associated with the filling of empty cells of the table, in which more and more new elements were placed: noble gases, natural and artificially obtained radioactive elements. In 2010, with the synthesis of element 117, the seventh period of the periodic system was completed. However, the problem of the lower boundary of the periodic table remains one of the most important in modern theoretical chemistry.
The graphical (tabular) expression of the periodic law is the periodic system of elements developed by Mendeleev .
More common than others are 3 forms of the periodic table: “short” (short-period), “long” (long-period), “extra-long”.
In the "extra-long" version, each period occupies exactly one line. In the "long" version, the lanthanides and actinides are removed from the general table, making it more compact. In the "short" form of writing, in addition to this, the fourth and subsequent periods occupy 2 lines each.
Elements arranged in ascending Z (H, He, Li, Be...) form seven periods.
In periods the properties of the elements naturally change during the transition from alkali metals to noble gases
