Why are the isotopes of an element chemically similar?
Different isotopes have different numbers of neutrons in their nuclei. Most chemical properties are determined by the arrangement of electrons, especially the outermost electrons. The size of an atom also affects some chemical properties. Having a different number of neutrons does not affect either one of these properties, so isotopes of an element will behave (chemically) the same.
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. Essentially the same process is used for both, which might help you understand why there is concern when a nation wants to use centrifuges to enrich uranium. Are they making fuel-grade (<10%) uranium-235 or weapons-grade (>90%)? Knowing some science can help you understand international politics!