Why cannot the atomic mass in row 6 be calculated the way the total for row 3 is calculated?

Answer 1

The Atomic Mass in row 6 cannot be calculated the same way as in row 3 because row 6 elements, which include heavier and more unstable isotopes, experience significant radioactive decay and have complex isotopic distributions. In contrast, row 3 elements tend to have more stable isotopes that can be averaged to give a clear atomic mass. Additionally, the presence of multiple oxidation states and varying isotopic abundances in heavier elements complicates their average atomic mass calculations. Thus, the methodology for calculating atomic mass differs between these rows due to the inherent instability and variability of the isotopes involved.