Overview
The standard atomic weight can reflect the isotopic composition found in normal terrestrial materials. That is why many displayed atomic weights are averages or intervals rather than simple whole-number mass counts.
How to use this idea
Why the number printed below an element symbol is often not a whole number.
Three similar phrases, three different ideas
“Atomic mass”, “mass number” and “standard atomic weight” are easy to blur together because they often appear near the same element symbol. They answer different questions. Mass number counts nucleons in one nuclide; the mass of an isotope is a measured physical quantity; standard atomic weight describes an element as it occurs in terrestrial materials.
That distinction matters whenever precision matters. A bracketed isotope number for a radioactive element is not a standard atomic weight, and an interval such as the one used for some elements is not indecision: it records genuine variation in nature.
Atomic mass, mass number and standard atomic weight
These terms are related but not interchangeable. The mass number A is the integer count of protons plus neutrons in one nuclide. The relative atomic mass refers to a particular atom or isotope relative to the atomic mass constant. Standard atomic weight is an evaluated quantity for an element in normal terrestrial materials and reflects isotopic composition.