Modern periodic table

Introduction In the early nineteenth century about 30 elements were known and were classified into three types on the basis of their physical properties as: metals, nonmetals and metalloids. Subsequent noteworthy attempts For classification of the increasing number of elements based on atomic mass were Dobereiner's triads and Newlands' octaves. Development of periodic table Dobereiner's triads : In this arrangement, three elements having similar properties were placed in a group so that the atomic weight of the middle element was average of atomic weights of remaining two elements.  Mendeleev's periodic table : The elements were arranged in the increasing order of their atomic weights, in the horizontal rows or periods and the elements having similar properties were kept in the same group. Hence this periodic table consisted of nine groups (0 to VIII) and eleven periods. Vacant places were left for the undiscovered elements which were filled later on.  After the discov...