Question

How did the cl 35 and cl 37 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 .

I hope this may help you,,:)

Answer 2

This is based on modern periodic law:
THE PROPERTIES OF THE ELEMENTS ARE PERIODIC FUNCTIONS OF THEIR ATOMIC NUMBER.
This law led to solution of the problem caused due to isotopes of chlorine and hence fixing the place of chlorine element in modern periodic table.