Silver is a meta element. Atomic Mass of it is 108.
the relative atomic mass for the element potassium is 39.0983
the Atomic Mass for silver is 107.868200
Atomic mass is so small you can't actually measure it. So, just like carbon dating, you have to use a relative mass.
The relative atomic mass is 83.798. So where did you get that it is not determined.
Sodium is a meta element. Atomic mass of it is 23.