How can the atomic mass be a decimal?
The atomic mass shown on the periodic table is actually an average of all the masses of the many different isotopes (atoms with different numbers of neutrons) based on the percentages of their occurrences in nature.
For example C-12 has an atomic mass of exactly 12 u and C-13 has a mass of
But C-12 occurs 98.89% in nature and C-13 occurs 1.11% (there is one other natural isotope), so the average is 12.01 u.
BTW, these may not be real numbers but they are close enough and still get to the point that mass number can be a decimal.
I remember all this from class.