Why are the isotopes of an element chemically similar?

$$ However, the greater mass of a heavier isotope does provide some useful differences. For example, isotopes of Uranium (235 and 238) can be separated by using the difference in their masses. This used to be done in very long diffusion chambers, but now it is done with centrifuges. U-235 is thus concentrated so it can be used for nuclear fuel or, unfortunately, weapons.