Prout’s Hypothesis
According
to this hypothesis atomic mass of elements is multiple of mass of hydrogen
atom. So elements are related to each other on the basis of their atomic
masses. It fails due to the reason that some elements do not have atomic mass
in whole no. (Like chlorine have atomic mass of 35.5).
Dobereiner’s Triads
According
to Dobereiner’s triads elements arranged in the group of three elements so that
atomic mass of central element is appropriate mean of 1st and 3rd
element. These groups of three elements are known as Dobereiner’s triads.
Example-
Element
Li Na K
Atomic Mass
7 23 39
Mean of Atomic Masses of 1st
and 3rd elements (7+39)/2 =
46/2 = 23
Drawback of Dobereiner’s triads : All
elements are not arranged in triads.