Question

how was the positions of different isotopes decided in the modern periodic

Answer 1

The topic ISOTOPE was not included in the modern periodic table..Each element have its own atomic number ..

For example Hydrogen has three isotopes - protium, deuterium and tritium but it has given atomic no 1 .. Not to every isotope but to element only..

Answer 2

All the isotopes of an element have same atomic numbers. In modern periodic table, the elements have been allotted places based on their atomic numbers. Therefore, all the isotopes of an element have been assigned the same position in the modern periodic table