The periodicity of the chemical elements was formulated by Dmitri Mendeleev in 1869. He stated that: The properties of the elements are periodically dependent on their atomic masses.
However, he did not specify in his law the length of these periods. Mendelee's assumptions were based on the almost triad of J. Doebereiner and the almost octaves of A. R. Newlands. Mendeleev's discovery was a turning point in the development of chemistry, giving the possibility of predicting the properties of undiscovered elements. The periodicity law was the basis for the construction of the Periodic Table.
Today, the right of time is: The properties of chemical elements in order of increasing atomic number are repeated periodically.
The right of periodicity is the basis of the periodic system. In Mendeleev's time there was no logical justification for this law. It was only after the discovery of the structure of the atom that it became physically important. Nowadays it is known that the period number corresponds to the number of the valence shell of the atom. The number of elements in a single period is due to the number of orbits on the valence sheath, each of which can be filled with up to two electrons. These orbitals are filled in as follows: 1st period: 1s - 2 elements 2nd period: 2s 2p - 8 elements 3rd period: 3s 3p - 8 elements 4th period: 4s 3d 4p - 18 elements 5th period: 5s 4d 5p - 18 elements 6th period: 6s 4f 5d 6p - 32 elements 7. Period: 7s 5f 6d 7p - 32 elements 8 period: 8s 5g 6f 7d 8p - 50 elements (hypothetically, none of them has been discovered yet)
The filling of orbitals with increasing numbers of electrons does not always take place exactly in the order given above. Due to the proximity of some energy levels, in the case of transition metals, Electronic promotion.
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