Bromine (Br), at atomic number 35, has a greater variety of isotopes. There are two main isotopes at 79 and 81, which average out to the 79.90amu value. While it won't change the average atomic mass, scientists have made bromine isotopes with masses from 68 to 97. As you move to higher atomic numbers in the periodic table, you will probably find even more isotopes for each element.
If we look at the C-14 atom one more time, we find that C-14 does not last forever.
For carbon, there are a lot of C-12, a couple of C-13, and a few C-14 atoms.
When you average out all of the masses, you get a number that is a little bit higher than 12 (the weight of a C-12 atom).
There are two techniques for dating in archaeological sites: relative and absolute dating.
For example, Christian time counts the birth of Christ as the beginning, AD 1 (Anno Domini); everything that occurred before Christ is counted backwards from AD as BC (Before Christ).
The Greeks consider the first Olympic Games as the beginning or 776 BC.
Natural disasters like floods can sweep away top layers of sites to other locations.
Absolute dating represents the absolute age of the sample before the present.