It is impossible for a chemist to study all the elements and their compounds. Hence, classification is a must.

Following are the reasons for the classification of elements:

1. To study elements better
2. To correlate the properties of the elements with some fundamental properties which are characteristic of all the elements
3. To reveal relationships between elements 

The first classification of elements was into 2 groups-metals and non-metals.

At. wt. of A = 7, At. wt. of C = 39

At. wt. of B = 23

i.e. Average of weights of A and C.

1. Döbereiner failed to arrange all the known elements in the form of triads.
2. In the triad of fluorine (19), chlorine (35.5) and bromine (80), it is observed that the mean of the atomic masses of fluorine and bromine is ½(19 + 80) = 49.5, not 35.5.