Classification of objects helps in the easy understanding and utility. The first classification of elements as metals and non-metals was, based on utility. Isolation of more and more elements leads to the classification of the elements based on some common physical and chemical properties.
Dobereiner’s law of triads was a classification based on atomic mass. But it is very limited use for only twelve elements of three elements each (Li-Na-K, Ca-Sr-Ba, S-Se-Te & Cl-Br-I). Newland was the first to arrange elements in increasing order of their atomic weight, and proposed a law of octaves indicating the recurrence of with similar properties at regular interval of eight elements. Julius Lothar Meyer was the first to show a relation of physical properties like atomic volume, melting and boiling, boiling point, radii etc., on regular intervals of increasing atomic weight but he related the periodicity of properties to valence of the elements.
Mendeleev, while studying the chemical properties of elements with hydrogen and oxygen, observed that chemical properties of the hydrides and oxides some elements resemble each other and others show a regular variation, related to their atomic weight. Hydrides show an increase in valence from one to four and then decrease to one on increasing atomic weight. Oxides, on the other hand increase from one to eight.