Why do isotopes of a element react the same chemically when they have different masses?
It's because they're both part of an isotope family, which has the exact same atomic nucleus and electronic structure. They'll often form stable compounds with other elements from their respective families (i.e. Carbon forms compounds like CO2 and CH4), but they will always prefer to bond to nuclei in that same family rather than anything else due to similar bonding preferences between all atoms of that family.
And there you go! It took me way longer then it should've just by reading the question, lolz.