Why do all isotopes of an element have the same chemical properties?
Because each isotope of the element HAS THE SAME NUMBER OF ELECTRONS.
To a first approximation, the chemistry of an element depends on the exchange and sharing of electrons between atoms to make and break strong chemical bonds.
While isotopes has different numbers of neutrons, massive, neutrally charged, nuclear particles, they have the same number of protons, massive, positively charged, nuclear particles. And the neutral atom has therefore the SAME number of electrons, as the isotopes of a given element. And if there is the same number of electrons, the chemistry of that element is essentially the same.
Is this clear?
Just to add that the substitution of