The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation or all the three attempts.
A, B and C are the elements of a Dobereiner's triad. If the atomic mass of A is 7 and the of C is 39, what should be the atomic mass of B?
State one example of a Dobereiner's triad, showing in it that the atomic mass of middle element is half-way between those of the other two.