4 ms·

You can see it done here: https://en.wikipedia.org/wiki/Standard_atomic_weight#List_of_atomic_weights https://en.wikipedia.org/wiki/Standard_atomic_weight#List_

• logfromblammo 7y ago
  The brackets in the table mean that different sources of the element have different proportions of isotopes, so the mean atomic weights for specific deposits may cover a range that differs from the overall mean for every deposit of the element ever measured. I.e. if you measure carbon from the upper atmosphere, it's going to have more C-14 in it, from cosmic rays flipping protons in N-14 to neutrons. And if you measure carbon buried for thousands of years, it's going to have less C-14, from natural decay. If you look at https://en.wikipedia.org/wiki/List_of_physical_constants https://en.wikipedia.org/wiki/List_of_physical_constants you can see that the parentheses are omitted from defined constants, and included for measured constants.