How many periods and groups are there in Mendeleev's periodic table?

$Li, Na, K$ 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 $Na$?

Which scientist used atomic volume as the basis for the periodic classification of elements?

Write a note on the genesis of the periodic table.

"The properties of elements are periodic functions of their atomic weights." This periodic law was given by

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 .......... .