Periodical system of chemical elements

Mendeleev
D. I. Mendeleev

Legend to the color table: 1 - chemical character of the element; 2 - name; 3 - atomic weight atomic weight); 4 - serial number; 5 - distribution of electrons in layers.

Periodical system of chemical elements - classification of chemical elements created by D. I. Mendeleev-based open them in 1869 the periodic law.
According to the present wording of the law, continuous range of elements arranged in order of increasing magnitude of the positive charge of the nuclei of their atoms, periodically repeated elements with similar properties.
Periodical system of chemical elements presented in the form of a table consists of periods, series and groups.
At the beginning of each period (except the first) is the element pronounced metal properties (alkaline metal).
As ascending sequence number of the item that is equal to the positive charge of the nuclei of its atom, gradually weaken the metal, and growing non-metallic properties. The penultimate item in each period is the element with strong non-metallic properties (halogen), and the last inert gas. In the first period are 2 items in the II and III - 8 items, IV and V 18, VI - 32 and in the seventh (not completed the period) - 17 items.
The first three of the period called the small periods, each of them consists of one horizontal row; the rest - large periods, each of which (except VII period) consists of two horizontal lines - even (top) and odd (bottom). In the even rows of long periods are only metals. The properties of the elements in these ranks with increasing sequence numbers change poorly. Properties of elements in an odd series of large periods of change. In the sixth period for lanthanum followed by 14 elements very similar chemical properties. These elements, called lanthanides shown separately below the main table. Similarly presented in the table and actinoid elements following actinium.
periodical system of chemical elements Mendeleyev
In the table there are nine vertical groups. The group number, with rare exception, equal to the highest positive valency of this group. Each group, excluding zero and the eighth, and is subdivided into sub-groups. - main (located to the right and side. In the main subgroups with increasing atomic number intensify metal and weaken the non-metallic elements properties.
Thus, chemical and some physical properties of the elements are defined by the position of this element in the periodic system.
Biogenic elements, i.e. elements included in the composition of organisms and executes certain biological role and occupy the upper part of the periodic table. Blue colored cells, occupied by the elements that make up the bulk (over 99%) living matter, pink - cell occupied by microelements (see).
Periodical system of chemical elements is a major achievement of modern science and a vivid expression of the most common dialectic laws of nature.
Cm. also the Atom, Atomic weight.


Periodical system of chemical elements is a natural classification of chemical elements created by D. I. Mendeleev-based open them in 1869 the periodic law.
In the original formulation of the periodic law of D. I. Mendeleev argued: properties of chemical elements, as well as forms and properties of compounds are periodic, depending on the magnitude of the atomic weights of the elements. In the future, with the development of the doctrine about the structure of the atom was shown that a more precise description of each element is not atomic weight (see)and the value of the positive charge on the nucleus of an atom of the element as a sequence (nuclear) number of this element in the periodic system of D. I. Mendeleev. The number of positive charges in the nucleus of the atom is equal to the number of electrons surrounding the nucleus of the atom, because the atoms in General electrically neutral. In the light of these data, the periodic law is formulated as follows: properties of chemical elements, as well as forms and properties of compounds are periodic, depending on the magnitude of the positive charge of the nuclei of their atoms. This means that in the continuous range of elements arranged in order of increasing the positive charge of the nuclei of their atoms, will be periodically repeated elements with similar properties.
The tabular form of the periodic system of chemical elements presented in its present form. It consists of periods, series and groups. The period is consistent horizontal row of elements arranged in order of increasing the positive charge of the nuclei of their atoms.
At the beginning of each period (except the first) is the element with strong metallic properties (alkaline metal). Then with increasing sequence numbers gradually weaken the metal, and growing non-metallic elements properties. The penultimate item in each period is the element with strong non-metallic properties (halogen), and the last inert gas. I period consists of two elements, the role of alkali metal and halogen free here at the same time performs hydrogen. II and III periods include 8 elements, named by Mendeleev typical. IV and V periods number 18 items, VI-32. VII period yet to be completed and updated with artificially created elements; currently, in this period there are 17 items. I, II and III periods called the small, each of them consists of one horizontal row, IV-VII - large: they (except VII) include two horizontal row - even (top) and odd (bottom). In the even rows of long periods are only metals and change the properties of the elements in the row from left to right expressed weakly.
In odd-numbered rows of large periods of the properties of the elements in the row is changed as well as the properties of typical elements. In the even number VI period after lanthanum 14 elements [called lanthanides (see), lanthanides, rare earth elements], similar in chemical properties of lanthanum and among themselves. The list them separately under the table.
Separately issued and listed under the table the elements that follow actinium - actinides (actinides).
In the periodic system of chemical elements in the verticals are nine groups. The group number is equal to the highest positive valence (see) the members of the group. With the exception of fluorine (happens only negatively monovalent) and bromine (no heptavalent); in addition, copper, silver, gold can be valence more +1 (Cu-1 and 2, Ag and Au-1 and 3), and of the elements of group VIII valence +8 have only osmium and ruthenium. Each group, except for the eighth and zero, is divided into two subgroups: main (located to the right and side. In the main groups are the typical elements and elements of the long periods, in a side - only elements of the long periods and then metals.
The chemical properties of the elements of each subgroup of this group are significantly different from each other and only the highest positive valence the same for all elements of the group. In the main subgroups down intensify metallic properties of elements and weaken non-metallic (France is the element with the most pronounced metallic properties, and fluorine - non-metallic). Thus, the location of the element in paragraph (C. H. E. (serial number) specifies the properties that represent the average of the properties of neighboring elements vertically and horizontally.
Some groups of items are of a special name. So, the main group elements of group I called alkaline metals, II group - alkaline earth metals, VII groups Halogens, items for uranium - transuranium. The elements that are part of the organisms involved in the processes of metabolism and possess a pronounced biological role, called biogenic elements. They all occupy the upper part of the table D. I. Mendeleev. First of all it is About, C, H, N, CA, P, K, S, Na, Cl, Mg and Fe that make up the main mass of living matter (over 99%). The space occupied by these elements in P. C. H. E., painted in a light blue color. Biogenic elements, which in the body very little (from 10-3 10-14%), called micronutrients (see). In the cells of P. S. H. E., painted in yellow, placed microelements vital for which the person is proved.

The average content of biogenic elements in the human body (in weight. %)
N-10,0
-20,0
N-3,0
O-63,0
Na-0,15
Mg-0,04
R-1,0
S-0,2
Cl-0,15
To-0,25
Sa-1,5
Fe-0,004

According to the theory of atomic structure (see Atom) chemical properties of elements depend largely on the number of electrons on the outer electron shell. Periodical changes in the properties of items with increase of the positive charge of atomic nuclei due to the periodic repetition of the structure of the outer electron shell (power level) atoms.
In small periods with increasing positive charge on the nucleus increases the number of electrons in the outer shell of 1 to 2 in the first period, from 1 to 8 in the second and third periods. Hence the change item properties in the period from alkaline metal to the inert gas. The outer electron shell, containing 8 of electrons is completed and sustainable energy (zero elements of a group of chemically inert).
In the long periods, even in the ranks with the growth of the positive charge of the nuclei the number of electrons in the outer shell remains constant (1 or 2) and being filled with electrons second outside containment. Hence the slow change item properties in the even rows. In odd-numbered rows of large periods with increasing energy of atoms being filled with electrons of the outer shell (from 1 to 8) and member properties are changed, as the typical elements.
The number of electron shells of the atom is equal to the number of the period. The atoms of the main group elements have on the external shells of the number of electrons is equal to the number of the group. The atoms of the elements of side-subgroups contain the external shells of one or two electrons. This explains the difference in the properties of the elements of the main and auxiliary groups. Group number specifies the number of electrons that can participate in the formation of the chemical (valence) relations (see Molecule), so such electrons are called valence. The elements of auxiliary groups valence are not only the electrons in the outer shells, but the penultimate. The number and structure of electron shells listed in the accompanying periodic system of chemical elements.
Periodic law of D. I. Mendeleev, and based on it P.F. H. E. are of great importance in science and practice. Periodic law and P. S. H. E. was the basis for the discovery of new chemical elements, the precise definition of their atomic weights, the development of the doctrine about the structure of atoms, the establishment of geochemical laws of distribution of elements in the earth's crust and development of modern views about the living substance whose composition and related laws are in conformity with P. S. H. E. Biological activity of elements and their content in the body is also largely determined by the place they occupy in P. C. H. E. So, with increasing atomic number in some groups increases the toxicity of elements and reduced their content in the body. Periodic law is a striking expression of the most common dialectic laws of nature.