Question

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

Answer 1

because modern periodic table is based on atomic number not on atomic mass.

this is why both the isotopes of CL has same atomic number that is 17 hence it has fixed position in modern periodic table..

I hope it's clear to you

Answer 2

According to the mendeleev's periodic table element are the periodic function of atomic mass

but element are the periodic function of atomic number

these are two isotopes of chlorine