Question

How were the positions of diff isotopes decided in modern periodic table?

Answer 1

modern periodic table is based on atomic number & we know that isotopes have different masses but same atomic number.

so isotopes are placed in same slot due to same atomic number.

Answer 2

Since the atmoic number of the  isotopes remain the same wheras the mass number alone changes the isotopes are always placed within the corresponding element to which it is related to.

More over the Periodic law states that the Properties of the elements are the periodic function of their atomic numbers.