In the modern periodic table, the elements are arranged according to their atomic number, not atomic masses.
As the isotopes of Cl have the same atomic no.17 and similar chemical properties both will be placed in the same slot.
In Mendeleev's periodic table, The elements are arranged according to their atomic masses. Hence, Both isotopes should be placed in two different slots, but Mendeleev himself kept different isotopes of the same element in the same slot because of their similar chemical property. and it is a limitation of Mendeleev's periodic table.