Mendeleev interpreted this relation between elements on the basis of Periodic Law which he stated as:

“The properties of elements are periodic functions of their atomic weights.”

However, certain discrepancies were noted while arranging the elements on the basis of atomic weights. For example, beryllium was out of place in the table as its atomic mass was 13.5 which should fit in between carbon and nitrogen. Similarly, inert gases had no proper place in the Periodic Table.

These problems were not fully resolved until 1914 when Moseley showed that elements could be arranged in a periodic pattern on the basis of their atomic numbers. The Periodic Law now states as:

“The properties of elements are periodic functions of their atomic numbers.”

By taking atomic numbers as the basis of the periodic classification of’ elements, various anomalies and misfits have been removed. For example, proper positions to cobalt and nickel, potassium and argon, etc., have been given.

The classification of elements was an interesting arrangement and attracted considerable attention. Several gaps in the table suggested discovery of new elements. Chemical and physical properties of unknown elements could be predicted which helped in the search of new elements. Mendeleev suggested that elements similar to aluminium and silicon should exist. Gallium, Ga (similar to aluminium) was discovered in 1875 and Germanium, Ge in 1886.

Mendeleev solved the problem of odd elements (which could not be adjusted properly in a group) by dividing groups into subgroups ‘A’ and ‘W. For example, among metals of the first group, sodium and potassium were placed in Group IA and copper and silver in Group 1B because of the difference in properties. Noble gases were discovered in the last decade of the nineteenth century and were placed in zero group because of their inertness.

The main features of Mendeleev’s Periodic Table were the arruigement of elements in vertical columns or groups and the horizontal rows or periods. He left spaces for the unknown elements and predicted properties of the Germanium which was not discovered until 1886. He called it eka-Silicon as it fell below Silicon.