[SeeSac]

Relative atomic mass is not an intrinsic elemental property. It’s misleading. It assumes experimentally derived information which may or may not be the case with the actual sample. It’s not universally applicable, and there are many applications for which it is not useful at all. This is why periodic tables should state the mass of the most common isotope which is true no matter what sample you use or how you go about doing things.

Atomic mass has been on the periodic tables since Mendeleev made the first one. But then he didn't know about isotopes. The periodic table was reorganize early on with the valence numbers but the atomic mass remains. It's convention.

[BenKenobi]

“Atomic mass has been on the periodic tables since Mendeleev made the first one.”

You’re right. It has been. Periodic tables cited the mass of the most common isotope, and have done so until now.

“But then he didn’t know about isotopes. The periodic table was reorganize early on with the valence numbers but the atomic mass remains. It’s convention.”

The problem is that this change introduces information that has no relevance to the purpose of the Periodic table. Why was Mendeleev able to make predictions? He hit upon the tables and the columns by arranging them according to their physical principles.

Why is it relevant to the chemistry of Chlorine to know that Cl-37 is one quarter as abundant as Cl-35? You see what the fellow above posted, that expressing it as a ratio of C-12 links the whole table together.