Not all elements undergo nuclear decay over timescales that we can observe. Some elements take millions of years to decay. In fact, most living things primarily consist of isotopes of carbon and nitrogen, which have such incredibly long lifetimes that they will essentially never decay within the lifespan of the organism.
This is necessary because the biochemical function of each of these atoms is specifically tied to its atomic number: if a nervous receptor specifically seeks out and binds a carbon-based signalling molecule, then it won’t work if that carbon spontaneously changes into beryllium. Different atoms of the same element can have different masses. For example, an atom of carbon (atomic number 6, so six protons) can have either 6 neutrons or 8 neutrons. The form
er case is more familiar from chemistry class, since a lot of the common light elements used in biology (like oxygen, carbon, and nitrogen) have the same number of protons as neutrons. But it turns out that the case of carbon having 6 protons and 8 neutrons, while not as stable as 6 and 6, is stable enough that it can actually occur in nature in observable amounts. Because the 8 neutron nucleus and the 6 neutron nucleus are technically both carbon, we call them different isotopes of carbon.Since protons and neutrons have roughly the same mass, the more common version of carbon is called carbon-12 (6 protons + 6 neutrons).
The heavier isotope is called carbon-14 (6 protons + 8 neutrons). But when you look up the mass of carbon on the periodic table, it says that the mass is 12.011 atomic mass units (amu). This is because if you went out and weighed a huge batch of carbon atoms, most of the atoms you would find would weigh exactly 12 amu. But within that huge batch you’d occasionally find a carbon-14 nucleus, which would skew the average of your measurements to a value slightly higher than 12.
For reasons that are deeply related to the fundamental forces that act in the nucleus, the tendency of a substance to undergo nuclear decay is related to both the atomic number and the atomic mass of an element. This means that two different isotopes of the same element will have different tendencies to undergo nuclear decay. In the case of carbon, the isotope carbon-14 wants to decay into nitrogen while carbon-12 (which is most of the carbon in your body) would remain stable.As a result, knowing which isotope is present in a sample of element not only tells us the sample’s stability, but also the type of decay it will undergo.