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.
But that’s not the point of the periodic table. The periodic table is intended to represent how each element relates to each other in a format that is relatively simple to understand.
“The most stable one? Either an isotope is stable, or it isn’t”.
Radioactive elements? Not all isotopes are stable and not always is the most abundant isotope stable.
“It just says the atomic weight is 35.453, as a result of it being a mixture of 35 and 37”
Wrong. You fail chemistry. The atomic mass of CL 35 is 35.453 as a result of CL 35 having 17 protons and 18 neutrons.
See what I mean? You’re already confused as to what atomic mass means because of this bullshit change. I rest my case.
It’s scary that several here claim to work with mass calculations yet say they are using isotopic mass of the ONE most stable isotope and say that they are getting their isotopic weight off the atomic weight chart.
“Only when there are two isotopes. And only when you’re dealing with a lot of significant figures.”
So how is knowing the atomic mass of the mixture at all helpful in solving this problem? It isn’t.
For all of these problems I need to know the atomic mass of the most common occurring Isotope. Not the mixture. The mixture is useless to me, although I can derive from it the expected ratios, which isn’t the point of the periodic table in the first place.