Which Atomic Weight Values Should I Use?
Concept Question: When do I use atomic weight intervals, and when do I use conventional atomic weights?
If we know the isotopic composition of a sample containing an element from the pink category on the IUPAC isotopic periodic table, we can figure out where in the interval the sample's atomic weight lies. However, a challenge arises when we don't know the source of a sample, especially when solving problems in the classroom that are based on unspecified samples.
Elements coloured blue have a single isotope that determines atomic weight, and so they will always have a single, constant value for the atomic weight, with uncertainty, which makes calculations simple. Since elements coloured white have only radioactive isotopes and no standard atomic weight values are listed, we will not concern ourselves with calculations involving isotopes of this category. Using the single atomic weight value provided for elements in the yellow category is also acceptable, but you should be aware that these elements might eventually have atomic weight intervals established.
For elements in the pink isotope category, which have an atomic weight interval listed, IUPAC has provided a set of values that can be used in cases where the source and isotopic composition of a particular sample is unknown.