Question

what position of isotopes in the mendeleev's and modern periodic table

Answer 1

There was no place was given to the isotopes in mendeleev's periodic table.

Modern periodic table based on the increasing atomic no. Therefore the isotopes of same element put under the same slot.

For example, isotopes of carbon
$c \times \frac{6}{12} \: and \: c \times \frac{6}{14}$
Are put in the same slot.