Discuss in detail about the classification of elements by Mendeleeff.

Explanation

The periodic classification of elements based on 'atomic weights' was done by Lothar Meyeer (Germany) and Mendeleev (Russia), Independently.
Mandeleev's periodic law: "The physical and chemical properties of elements and their compounds are a periodic function of their atomic weights".
Mendeleev arranged the then known 65 elements in a periodic table. He did not blindly follow the atomic weight but gave more importance to their chemical properties in arranging them in the table.
Explanation of the periodic law: When the elements are arranged in the increasing order of their atomic weights, elements with similar properties appear again and again, at regular intervals, just like the days, weeks, months, seasons, etc. repeat at regular intervals of time. This is called periodicity of properties.
Mendeleev's Table: Mendeleev introduced a periodic table containing the then known 65 elements. In this table, while arranging the elements, he gave importance not only to their atomic weights, but also to their physical and chemical properties. This table was defective in some respects. Then he introduced another table, after rectifying the defects of that table. It is called, 'Short form of periodic table'. He named the horizontal rows as 'periods' and the vertical columns, as 'groups' . It has in all 9 groups, I to VIII and a '0' group. The first 7 groups were divided into A and B sub groups. There are 7 periods in the table.
The VIII group contains three triads, namely, (Fe, Co, Ni), (Ru, Rh, Pd) and (Os, lr,Pt).
Merits of Mendeleev's table:
(1) Actually it formed the basis for the development of other modern periodic tables.
(2) Mendeleev left some vacant spaces in his periodic table, for the then unknown elements. But he predicted the properties of those elements. Later on, when these elements were discovered, they exactly fitted into those vacant places having properties, predicted by Mendeleev.
Ex: Eka-boron (scandium), Eka-silicon (germanium), Eka-aluminium(gallium) etc.
(3) '0' group elements were not known at the time of Mendeleev. Later when they were discovered, they found a proper place in that table under '0' group of elements. Similarly, ther radioactive elements.
(4) In case of these pairs of elements Tellurium-Iodine, Argon-Potassium and Cobalt-Nickel, there is a reversal of the trend. The first element has higher atomic weight than the second one. These are called, anomalous pairs.
However, based on their atomic number, and chemical properties, this arrangement proved quite justified.
Drawbacks of Mendeleev's periodic table:
(1) Dissimilar elements were placed in the same group.
Ex: The coinage metals Cu, Ag and Au are placed along with the alkali metals K,Rb,Cs etc. in the 1 group. The only common property among them is that they are all univalent (valency = 1).
(2) The 14 are rare earths having differents atomic weights are kept in the same place.
(3) Hydrogen could not be given a proper place, as it resembles both alkali metals and halogens in its properties.