If atomic weight and atomic mass are different, why does the periodic table list the atomic mass as if it was the atomic weight?

Question

I’m so confused with these two. I know they are different terms but the periodic table shows atomic mass when to my understanding it should be the atomic weight because it’s taking into account their isotopes

In: Chemistry

Answer 1

The periodic table is mostly used in chemistry where you normally work with large amounts of chemicals in natural isotopic Distributions. For many calculations you want to do with chemicals the value stated in the periodic table is the one you want. Like if have a bunch of sodium chloride, and want to calculate the mass you need of it to get 1 mol (an specific number of molecules), you need the element weight, as all “normal” chemicals will follow these numbers (and even if not the differences are often pretty small).

The difference between the average weight according to natural Isotope distributions and the true mass of the isotopes only matter, when working with single atoms, or isotopic pure chemicals. And if you do that you normally know where to look up the correct masses.
These values periodic tables are just some quick look up tables for rough calculations, that is not a necessary part of periodic tables. Besides if you know your Isotope it’s not so difficult to calculate the true mass (U-238 is approx. 238 u, if you need a more precise number you can look it up in nuclid maps or other reference tables).