Elements were arranged on the basis of ___________ in Dobereiner’s triads.

When Dobereiner observed the trends of 'Triads'  in the elements, most of the elements were unknown at that time. The atomic number was also not considered a fundamental property and was very difficult to predict.

So, Dobereiner arranged elements on the basis of atomic mass. Elements with similar properties were identified by the German chemist Johann Wolfgang Dobereiner. He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties. Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third elements in a triad would be approximately equal to the atomic mass of the second element in that triad.

The Doberiner's triads are:

1. Calcium, Strontium, and Barium

2. Nitrogen, Phosphorous and Arsenic

3. Chlorine, Bromine and Iodine

Let's understand by an explanation of one example-

Chlorine, Bromine and Iodine

The first and third members of the Dobereiner triads are Chlorine and Iodine and the middle member of the triads is Bromine.

The atomic mass of Chlorine and Iodine is 35.5 u and 126 u.

$\mathrm{Mean} of atomic masses of Cl and Br=\frac{35.5+127}{2}$

$=81.25$

The mean value of the Chlorine and Iodine is equal to 81.25 u.

The mean value of the Chlorine and Iodine is approximately equal to the atomic mass of the Bromine (79.9u).