The Search For A Classification

In the early era of science, only few elements were known, but new elements were being discovered. Hence, in order to facilitate their study, need arose for a frame work in which the elements may be arranged or classified. Previously the classification was based on atomic mass (atomic weight) of elements because it was thought that the properties of an element depended upon its atomic mass. But recently a complete classification has been made, which is based on atomic number of the elements, instead of their atomic masses.

We trace the history of development of classification of elements, as follows:

4.1.1 Dobereiner’s Classification:

In 1829 Johann Dobereiner, noticed that of the three elements with very similar chemical behavior i.e. Calcium (Ca), strontium (Sr) and Barium (Ba), the atomic mass of the middle element i.e. Sr is almost the arithmetic mean of the other two. This led him to call this group of three elements, a ‘triad’ and proposed the Law or Rule of Triad, which states that:

"Central atom of each set of triad had an atomic mass almost equal to the arithmetical mean of the atomic masses of other two elements".

Table 4.1 Triads



Atomic Mass

Mean Atomic Mass



Sodium Potassium






At. mass of Na =  7+39/2 = 23


Chlorine Bromine






At. mass of Br =35.5+126.5/2 = 81

This law or rule cannot be extended to the classification of all the elements, because it is true only in the cases of very few elements only.


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