Question

Write the merits of Mendeleev’s periodic table.

Answer 1

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 of 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-aluminum and eka-silicon from the known neighbours and their atomic masses were indicated as 44, 68 and 72, respectively. Not only this but their properties were also predicted. Later on, these elements were discovered and named scandium (Sc), gallium (Ga) and germanium (Ge) respectively. The properties of these elements matched well with those predicted by Mendeleev. See the table given below. Due to this success, all were convinced about the importance of Mendeleev’s periodic table and this method of classification of elements was accepted immediately.
   

   Property	eka-aluminum (E) (Mendeleev’s prediction)	Gallium (Ga) (actual)
   1. Atomic mass	68	69.7
   2. Density (g/cm3)	5.9	5.94
   3. Melting point (°C)	Low	30.2
   4. Formula of chloride	ECl3	GaCl3
   5. Formula of oxide	E2O3	Ga2O3
   6. Nature of oxide	Amphoteric oxide	Amphoteric oxide

                      Actual and predicted properties of gallium
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.