Atomic mass and mass excess

1. The problem statement, all variables and given/known data
In Clark's tables, the atomic masses of various isotopes are given which are decimal form. Are these experimentally obtained masses? Moreover, while writing nuclear reactions we do not use these, instead they are represented as integers. Is it just for convenience or is there any particular reason for this?In Schaums book on Modern Physics, they have defined the difference between the 2 masses as mass excess.Could somebody please explain the relation between these 2 values?

2. Relevant equations

3. The attempt at a solution

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 Quote by Amith2006 1. The problem statement, all variables and given/known data In Clark's tables, the atomic masses of various isotopes are given which are decimal form. Are these experimentally obtained masses? Moreover, while writing nuclear reactions we do not use these, instead they are represented as integers. Is it just for convenience or is there any particular reason for this?In Schaums book on Modern Physics, they have defined the difference between the 2 masses as mass excess.Could somebody please explain the relation between these 2 values?
My chem teacher in high school said it was about their abundance in the universe (if there's more 16g Oxygen than 15g Oxygen, the decimal will be closer to 16, to put it simply) I don't know if it's abundance on the Earth, or abundance of the Universe. I'd assume they use spectroscopy for measuring the Universe's abundance and have no idea how they'd measure Earth's abundance beyond our little sliver of inhabitation on it's surface.

I think the nuclear reactions round to the nearest integer, or sometimes use the types of isotopes used in your profession (like physics for the health sciences might deal with molybdenum-xx into technetium-99, since radiologists use this to help make detections in living systems).

The excess mass has something to do with the energy that is released (and absorbed for fusion? not sure...) in the fission reactions.

Remember how Einstein equated mass to energy? In Nuclear physics, you get to actually see how that works...