As I understand it, a uranium atom should exist in a superposition of its three isotopes (U238,U235,U234). Is this correct? If so, the probability of any isotope being observed should be related to its natural abundance. But this would mean that the probabilities are time dependent, which is nonsense. Where I am I going wrong here? What are the states of a uranium atom? According to what I've read about the famous Schrodinger Cat Gedanke the decay of a uranium atom is the critical event. This is taken as the 'realization' of a single quantum state (decayed) although that single state could be U235 or (rarely) U234.