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Limitations of Mendeleev's Periodic Table

Mendeleev arranged 63 elements known at that time in the periodic table. According to Mendeleev “the properties of the elements are a periodic function of their atomic masses.” The table consists of eight vertical column called ‘groups’ and horizontal rows called ‘periods’ Limitations of mendeleev’s classification : (i) He could not assign a correct position of hydrogen in his periodic table, as the properties of hydrogen resembles both with alkali metals as well as with halogens. (ii) The isotopes of the same element will be given different position if atomic number is taken as basis, which will disturb the symmetry of the periodic table. (iii) The atomic masses do not increases in a regular manner in going from one elements to the next. So it was not possible to predict how many elements could be discovered between two elements.
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Drobenier's Traid

Doberiner’s Law of Triads: According to this law, “in certain triads (grout) of three elements) the atomic mass of the central element was the arithmetic mean of the atomic masses of the other two elements.” But in some triads all the threc elements possessed nearly the same atomic masses, therefore the law was rejected. e.g., atomic masses of Li, Na and K are respectively 7, 23 and 39, thus the mean of atomic masses of I St and 3rd element is Limitations of Doberiner’s Triads:  He could identify only a few such triads and so the law could not gain importance. In the triad ofFe, Co, Ni, all the three elements have a nearly equal atomic mass and thus does not follow the above law.
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