The whole number atomic mass of the elements cobalt (Co) and nickel (Ni) is the same. Therefore there was an ambiguity regarding their sequence in Mendeleev’s periodic table.
Isotopes were discovered 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.
When elements are arranged in an increasing order of atomic masses, the rise in atomic mass does not appear to be uniform. It was not possible, therefore, to predict how many elements could be discovered between two heavy elements.
Position of hydrogen : Hydrogen shows similarity with halogens (group VII).For example, the molecular formula of hydrogen is H2 while the molecular formulae of fluorine and chlorine are F2 and Cl2, respectively. In the same way, there is a similarity in the chemical properties of hydrogen and alkali metals (groupI). There is a similarity in the molecular formulae of the compounds of hydrogen alkali metals (Na, K, etc) formed with chlorine and oxygen. On considering the above propereties it can not be decided whether the correct position of hydrogen is in the group of alkali metals (group I) or in the group of halogens (group VII).
This page was last modified on 26 April 2020 at 13:34