Elements are identified by their atomic numbers. For example, an element of carbon always has 6 protons in its nucleus, and any element with 6 protons in its nucleus is a carbon atom. If a carbon atom were to lose a proton from its nucleus, it would no longer be identified as carbon. However, the number of neutrons in the nucleus may change without changing the element. Atoms with the same atomic number but with different atomic weights (i.e. different number of neutrons) are called isotopes of each other. Generally, one isotope of an element is more stable than the others, which may tend to break down and experience radioactive decay. A radionuclide is an isotope that tends to break down radioactively.
Isotopes of an element do not all occur with the same frequency. The atomic weight of an element reflects the presence of the different isotopes. Atomic weight is found by averaging the atomic masses of all of the atoms in a given sample of an element.
Example:
In a sample of 1000 carbon atoms, the atomic weight is found to be 12.012 amu. Assuming that the only isotopes present are carbon-12 and carbon-14, how many of the 1000 atoms are carbon 14?
