If you have looked at a periodic table you may have noticed that the atomic mass of an element is rarely an even number. That happens because of the isotopes. If you are an atom with an extra electron, it is no big deal. Electrons don't have much of a mass when compared to a neutron or proton.
Atomic masses are calculated by figuring out how many atoms of each type are out there in the universe. For carbon, there are a lot of C-12, a couple 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). The mass for element is actually 12.011. Since you never really know which C atom you are using in calculations, you should use the mass of an average C atom.
RETURNING TO NORMAL
If we look at the C-14 atom one more time we can see that C-14 does not last forever. There is a point where it loses those extra neutrons and becomes C-12. That loss of the neutrons is called radioactive decay. That decay happens regularly like a clock. For carbon, the decay happens in a couple of thousand years. Some elements take longer and others have a decay that happens over a period of minutes.
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