Merits and Demerits of Mendeleev's periodic table.

Science is progressive. There is a freedom in science to revise the old inference by using more advanced means and methods of doing experiments. These characteristics of science are clearly seen in Mendeleev's periodic table.

While applying the law that the properties of elements are a periodic function of their atomic masses, to all the known elements, Mendeleev arranged the elements with a thought that the information available till then was not final but it could change. As a result of this, Mendeleev's periodic table demonstrates the following merits:

1 Atomic masses of some elements were revised so as to give them a proper place in the periodic table in accordance with their properties. For example, the previously determined atomic mass of beryllium 14.09 was changed to the correct value 9.4 and beryllium was placed before boron.

2 Mendeleev kept vacant places in the periodic table for elements not discovered till then. Three of these unknown elements were given the names Eka boron Eka aluminium and Eka silicon from the known neighbours and their atomic masses were indicated as on these elements were discovered and named as scandium gallium and germanium respectively. The properties of these elements matched well with those predicted by Mendeleev. Due to this success, all were convinced about the importance of the Mendeleev periodic table and this method of classification of the element was accepted immediately. 

3 There was no place reserved for noble gases in Mendeleev's original periodic table. However, when noble gases such as helium, neon, and argon were discovered towards the end of the nineteenth century Mendeleev created the zero group without disturbing the original periodic table in which the noble gases were fitted very well. 

1 The whole number of atomic masses of the elements cobalt and nickel is the same. Therefore there was ambiguity regarding their sequence in Mendeleev's periodic table.

2 Isotopes were discovered a long time after Mendeleev put forth the periodic table. As isotopes have the same chemical properties but different atomic masses a challenge was posed in placing them in Mendeleev's periodic table. 

3 When elements are arranged in increasing order of atomic masses the rise in atomic masses does not appear to be uniform. It was not possible therefore to predict how many elements could be discovered between two heavy elements.

4 Position of Hydrogen 

Hydrogen shows similarity with Halogens. For example, the molecular formula of hydrogen in H₂ while the molecular formula of fluorine and chlorine are F and Cl respectively. In the same way, there is a similarity in the molecular formulae of the compounds of Hydrogens alkali metals formed with chlorine and oxygen. On considering the above properties it cannot be decided whether the correct position of Hydrogen is in the group of alkali metals or in the group of Halogens.