Question

why cl35 and cl37 are not different in their chemical properties ?

Answer 1

because an atom of chlorine-35 and an atom of chlorine-37 each contain the same number of protons.
The number of protons an atom has, also known as the atom's atomic number, determines which element it is.
 All atoms which contain 17 protons are called chlorine atoms.

Answer 2

These two chlorines are the isotopes. They differ from each other because they have varying number of neutrons in the nucleus. As the number of neutrons are different the chemical properties happen to be different