What Do We Know?

An important consequence of the variability of isotopic abundances is that atomic weights are not constants of nature. Because isotopic abundances are used in the atomic weight equation, if the isotopic abundances of a certain element change depending on physical location, the atomic weight of the element must also change.

Why Should We Care?

Since isotopic abundances vary throughout our planet, the atoms of many elements actually have a range of atomic weights across Earth. For this reason, the International Union of Pure and Applied Chemistry (IUPAC) has developed a new periodic table, called the IUPAC Periodic Table of the Isotopes. For some elements, this periodic table does not give a single value for the atomic weight of an element. Instead, the periodic table provides intervals for the atomic weights of these elements. For example, the atomic weight of oxygen is no longer assigned a value of 15.999406. Instead, the atomic weight is given as an interval that varies between 15.99903 and 15.99977. As you can see, the variations in atomic weight are very small, but they are still measurable. In the case of some other elements, such as fluorine, which only have one stable isotope, a single atomic weight suffices. The elements which have an atomic weight interval are colored pink in the Periodic Table of the Isotopes.

However, because the atomic weight intervals are so small, measurements of isotopic abundances must be very precise in order to determine the atomic weight interval for these elements. There are some elements that likely have a range of atomic weights across the Earth, but the measurements of isotopic abundances have not yet been of sufficient precision to establish the exact atomic weight interval. These elements are colored yellow in the Periodic Table of the Isotopes.