Question

How did the position of isotopes of chlorinefixed in periodic table?

Answer 1

Modern periodic table was constructed based on atomic number (in the increasing order).
 Isotopes: The atoms which posses same atomic number but different mass number are called Isotopes.
Since isotopes are considered to be (because of same atomic number) are of same element. So, they kept at only one place as a single element in the Modern periodic table. But not separate place for each of the isotope