Question

How did the position of 35,17 Cl and 37,17 Cl get fixed in the modern periodic table?

Answer 1

this is because the modern periodic table is based in atomic no. of element

in detail,

In Mendeleev's periodic table the properties of elements are the periodic function of their atomic masses.

that's why they unable to ensure the correct position of isotopes like cl 35 and cl 37 as the masses are different for the same element ,

but later ,Henry Moseley showed that anomalies in arrangement of elements could be removed when the elements were arranged in order of increasing atomic number

that's why the isotopes of chlorine took correct position in modern periodic table