I have a quick question here.  Whenever I look at the data about radioisotopes, I always see the energy associated with the decay of said isotope.  (I.E. U-238 gives off 4.10 MeV in gamma rays, or x MeV alpha, etc. etc.).  My question is, on what scale is that energy associated with?  Is is per atom of the isotope?  Per gram?  Per mole?  NO place out there seems to indicate what the scale is.  Due to the number of decays per second, I find it hard to believe that those energies are per atom.  I just wish places would define the units they use.  It's like going to the supermarket and seeing the roast beef priced at $4.00.  Is it four bucks a pound, an ounce, a slab?

I have a quick question here.  Whenever I look at the data about radioisotopes, I always see the energy associated with the decay of said isotope.  (I.E. U-238 gives off 4.10 MeV in gamma rays, or x MeV alpha, etc. etc.).  My question is, on what scale is that energy associated with?  Is is per atom of the isotope?  Per gram?  Per mole?  NO place out there seems to indicate what the scale is.  Due to the number of decays per second, I find it hard to believe that those energies are per atom.  I just wish places would define the units they use.  It's like going to the supermarket and seeing the roast beef priced at $4.00.  Is it four bucks a pound, an ounce, a slab?

Mitch is correct, it is per atom.  Or, more precisely, per event.  An atom can give off numerous gamma rays during a cascade down to ground state.  Each gamma ray would then be x MeV.

A quadrillion seems small, until we start talking about moles of radioactive materials.

It is hopeless, some people just have to die stupid.