Isotopes of a particular element have the same number of protons in their nucleus, but different numbers of neutrons.
This means that although they are very similar chemically, they have different masses.
Some examples of the types of material that radiocarbon can determine the ages of are wood, charcoal, marine and freshwater shell, bone and antler, and peat and organic-bearing sediments.
Could there be other influences that would affect the rate of decay of carbon 14?
If it has generally been established as a constant, at what point does the "constant" break down?
How far can you go back in time, and assume an accurate sample with carbon dating?
It seems limited, how can an observer know the state of the decay of a certain molecular structure even 100,000 calendar years ago?