Dimitrie Mendeleev,s Periodic Table

DIMITRIE MENDELEEV'S PERIODIC TABLE

In 1871, a Russian chemist, Dimitrie Mendeleev gave a more useful and comprehensive scheme of the classification of elements. First of all he arranged the elements into periods and groups. Vertical column are called group and horizontal rows are called periods. Mendeleev,s periodic table consist of eight(8) groups and twelve(12) periods. He arranged the elements according to their ascending atomic masses and studies their behavior.

❝If elements are arranged according to their ascending atomic masses, he found that elements with similar chemical properties appeared at regular intervals.❞

❝The appearance of properties of elements after regular intervals is called periodic law or periodicity❞

ADVANTAGES OF MENDELEEV'S PERIODIC TABLE:

 Mendeleev left some gaps in his table for elements, which had not yet been discoverer, and by considering their positions in the Periodic Table he predicted properties of these elements. For example, germanium was not known at that time but Mendeleev was confident that this element must exists so he predicted   its properties. A few years later, germanium was discovered its properties were same as predicted by Mendeleev.
Mendeleev corrected the atomic mass of Be. It was though to be 13.5 which was wrong. He proposed the atomic mass of Be to be 9.

DEFECTS OF MENDELEEV'S PERIODIC TABLE:

(i) The basis of Mendeleev,s periodic table was atomic mass. It did not give the idea of structure of atom.
(ii) Position of hydrogen could not be deceived by Mendeleev.
(iii) Lanthanides (85 − 71) and Actinides (90 − 103) were not placed properly.
(iv) Position of isotopes was not indicated.
(v) According to atomic masses, the position of following pair of elements was not regular:

                  Ar           K      Co        Ni         Te           I
                 39.9        39     58.9      58.6      127.6     126.9