The heavier the atom, the more unstable it gets, right?
That is not true about Uranium and we know it. I wondered why. A brief explanation stated that since more neutrons are there in the nucleus there is more nuclear force to 238U than 235U. Seems legitimate, but then that should have been true about the other radioactive species. Again, we know that's not true.
So, either the explanation about the Uranium exception is not totally true or the principle of the heavier-atom instability (of course, exceptions occur at elements like U) is not a principle. Or maybe there is a relation between these two (nuclear force and atomic weight) which is beyond me.
My hunch is that the latter is true; searched some sites which had an answer to the question, but no acceptable results gained.