Physical meaning of the chemical periodicity

Periodic changes of chemical properties of elements are stipulated by the correct repetition of electron configuration of external energy level (valency electrons) their atoms with nuclear charge increasing. Graphic inventing of a Periodic law is a periodic Table (see below). It is kept 7 periods and 8 groups.

Period – horizontal row of elements in the Periodic Table with similar maximum value of the main quantum number of electrons. Period number indicates the number of energy levels in the atom of element.

Period can consist of 2 (first), 8 (second and third), 18 (fourth and fifth) or 32 (sixth) electrons, depending on the amounts of electrons on the external energy level. Seventh period is not completed.

All periods (except the first) are begun with alkaline metal (s-element), but are finished by noble gas (ns²np⁶).

Metallic properties are considered as an ability of atoms of elements to loss electrons easily, but non-metallic – to accept electrons. Filling of external s-sublevel is pointed out to metallic properties of atom, but shaping an external p-sublevel (from 5) intensifies non-metallic properties of atom. Atoms with completely formed, energy firm configuration of the external electronic layer (ns²np⁶) are chemically inert.

At great periods change of the properties from the active metal to the noble gas occurs more smoothly, than at small periods, since a shaping an internal (n-1) d-sublevel occurs at the conservation external ns²-layer. Great periods consist of even and odd rows.

At even row of elements on the external layer ns²- electrons, such metallic properties are dominated and their weakening with the growing of nuclear charge is not great; in odd rows the np-sublevel is formed, that explains a significant weakening of metal properties.

Groups – vertical rows of elements with similar valency number electrons, which is equal to group number. Main and secondary sub-groups are distinguished.

Main sub-groups consist of elements of great periods; valency electrons are placed on the external ns- and np-sublevels.

Secondary groups consist of elements of great periods only. Their valency electrons are on the external ns-sublevel and internal (n-1) d-sublevel (or (n-2) f-sub-level).

Depending on the sub-level, which is filled by valency electrons, the periodic system of elements is subdivided into:

s-block (elements of main sub-group of I and II groups);

p-block (elements of sub-groups of III-IV groups);

d-block (elements of secondary sub-groups);

f-block (lanthanides and actinides).

In the main subgroups from top to bottom the metallic properties are intensified, but non-metallic are weakened.

The number of the group shows the highest valency of element (except N, O, F, elements of Copper sub-group and eighth group).

Formulas of the highest oxides are general for elements of main and secondary sub-groups (and their hydrates). At the highest oxides and hydrates – elements of I-III groups (except boron) the basic properties are dominated, but from IV till VII – acid.

Table 6. General trends in periodicity

For the elements of main sub-groups formulas of Hydrogen compounds are general. Elements of main sub-groups of I-III groups form hard substances – hydrides (Hydrogen has oxidation number –1), but IV-VII – gaseous. Hydrogen compounds of main sub-group of IV group (EH₄) are neutral, V group (EH₃) – base and VII groups (H₂E and HE) – acids.

The properties of atom depend on the position of elements in the Periodic System, which is connected with structure of atoms.

Many characteristics of atoms are changed periodically, for example – electronegativity at the period is increased from left to right, but in sub-group is decreased; ionization energy – at the period increased, but in sub-group is decreased.

According to the position of element in the Periodic system it is possible to forecast its basic properties, as average of its neighbors:

Li	Be	B
Na	Mg	Al
K	Ca	Sc

Date: 2015-01-12; view: 1276