Mendeleev's periodic law is based on

The element or elements whose position is anomalous in the periodic table is

$Li, Na, K$ form a Dobereiner's triad. If the atomic masses of $Li$ and $K$ are $7$ and $39$ respectively,what will be the atomic mass of $Na$?

Fill in the blanks given below:
$(i)$ The first scientist to think about the properties of elements was ......... .
$(ii)$ The relationship of properties of elements suggested by Dobereiner is known as ........... .
$(iii)$ The Law of Octaves was proposed by the chemist ......... .
$(iv)$ The first periodic table was constructed by .......... and .......... .

The telluric helix was given by

On the basis of which important property did Mendeleev classify the elements in his periodic table? Was he able to stick to it firmly? Explain.