In 1869 the Russian chemist Dmitri Mendeleev (1834–1907) and the German chemist J. Lothar Meyer (1830–1895), working independently, made similar discoveries. They found that when they arranged the elements in order of atomic mass, they could place them in horizontal rows, one row under the other, so that the elements in each vertical column have similar properties.

Mendeleev had the idea of arranging ‘families’ of elements that have similar properties in vertical columns or groups.

Shortcomings of Mendeleev’s Periodic Table

(i) Wrong order of the atomic masses of some elements. Certain elements are arranged according to their increasing atomic masses, similarity in the chemical properties of the elements in a group is violated. For example, Ar with atomic mass of 39.9 comes first and K with atomic mass of 39.1 comes after it so that similarity of elements in a group is realized.

(ii) The position of isotopes in the Periodic Table isotopes could not be given separate places in Mendeleev’s Periodic Table. This observation led to the conclusion that atomic mass cannot be the basis of the classification of elements