Ans. Dobereiner arranged chemically similar elements in the increasing order of their atomic weights in a group of three which is known as “Dobereiner’s triads”.
In a Triad, atomic mass of the middle element is approximately equal to the mean of atomic masses of other two elements in that triad.
Examples: (i) Lithium, Sodium, Potassium.
(ii) Calcium, Strontium, Barium.
(iii) Chlorine, Bromine, Iodine.