Does the mass number of an element ever change?
Carbon is a mixture of Carbon 12 and Carbonme 14 as time goes on the percentage of Carbon 14 decreases in a sample of Carbon. this will cause a decrease in the average mass of the Carbon Sample.
The mass of an individual atom will not change unless the atom undergoes a nuclear change. The average mass of an element remains essentially staple as an average of the common isotopes. However the mass number of a sample of an element can and will change over time as the radioactive isotopes undergo nuclear changes.
The average mass of an element determined in the 1800s would be slightly different than the average mass of the element determined in the 2000's. In a million years from now the average mass of some elements might be significantly different from the present average mass.