Ok, and why should the periodic tables care about isotope mixtures? They already list the most stable isotope and when I’m dealing with a mixture, I can use this number against experimental data to derive the actual mixture ratio from the ratio of the expected vs actual weights.
By fudging the numbers, I have to look up the common isotope masses every time. Worthless.
Clue: It is 'atomic weight, not isotopic weight. The periodic table lists the atomic weight for the most common ratios of isotopes in nature. You should NOT be using the the weight of the most stable isotope. If you are getting the weight from the periodic table you are most probably already using the weight of the mixed isotopes. If you look at the periodic table for oxygen, you will see that and also see that it has references that the stated uncertainty is inclusive of the variation of the isotopes found in nature. So if you are properly accounting for the error in the atomic weight then your answer would have a range of values. Basically this is nothing really new. Now the uncertainty is just stated as a range.
Because most elements occur in mixtures of isotopes.
They already list the most stable isotope
?The most stable one? Either an isotope is stable, or it isn't. Cl-35 and Cl-37 are both stable. Which one is more stable? Maybe you mean abundant. But a periodic table doesn't tell you the abundance (although you can work it out when there are only two, as in the case of Cl). It just says the atomic weight is 35.453, as a result of it being a mixture of 35 and 37. When there are three or more isotopes, you can't deduce the relative amounts from the average weight.
and when I’m dealing with a mixture, I can use this number against experimental data to derive the actual mixture ratio from the ratio of the expected vs actual weights.
Only when there are two isotopes. And only when you're dealing with a lot of significant figures.
By fudging the numbers, I have to look up the common isotope masses every time. Worthless.
You're confusing a table of isotopes with a periodic table. The isotopic weights as reported on a table of isotopes isn't changing. Only the periodic table atomic masses, averaged over abundance, are changing.