Atomic mass is calculated by adding
and

An electrons position cannot be known precisely only it's probability of being in a certain location can be known

Tin has ten stable isotopes. the heaviest, 124sn, makes up 5.80% of naturally occuring tin atoms. how many atoms of 124sn are present in 82.0 g of naturally occurring tin? what is the total mass of the 124sn atoms in this sample?

An observation that requires measurement is called quantitative observable or qualitative