In the XIX century there were no ideas about the structure of the atom.Opening DIMendeleev was a generalization of experimental facts, but their physical meaning has long remained unclear.When the first data on the structure of the nucleus and the distribution of electrons in atoms, it is possible to look at the law and the periodic system of elements in a new way.Table D.Mendeleev makes it possible to visually trace the periodicity properties of the elements found in nature.
each element in the table is assigned a specific sequence number (H - 1, Li - 2, Be - 3, etc.).This number corresponds to the nuclear charge (number of protons in the nucleus) and the number of electrons orbiting the nucleu
s.The number of protons is thus equal to the number of electrons, and this suggests that under normal conditions atom is electrically neutral.
division into seven periods occur in the number of energy levels of the atom.The atoms of the first period have single-level electron shell, the second - a two-level, third - a three-level, etc.When filling out a new energy level of a new period.
first elements of every period are characterized by an atom having one electron on the external level - is the alkali metal atoms.Period ends noble gas atoms, which have completely filled outer electron energy level: in the first period rare gases have 2 electrons in the future - 8. It is because of the structure of the electron shells of similar groups of elements have similar physical and chemical properties.
Table D.Mendeleev present 8 major subgroups.This is due to the amount of their maximum possible number of electrons in the energy levels.
the bottom of the periodic system highlighted the lanthanides and actinides as independent series.
Using Table D.The periodic frequency of the following can be observed properties of elements: atomic radius, the volume of the atom;ionization potential;power electron affinity;electronegativity of the atom;oxidation;Physical properties of potential compounds.
For example, the atomic radii, when viewed along the period, decreasing from left to right;grow downwards, when viewed along the band.
clearly observed frequency of the elements in Table D.Mendeleev rationally explained by the consistent filling of the energy levels of electrons.