why does the atomic mass decrease when the atomic number increases at nickel and iodine?'?

Question

on the periodic table

Answer 1

Atomic mass includes the protons (which determine the atomic number) and the neutrons. If the atomic mass decreases, it means the most stable isotope of the element has less neutrons in its nucleus than did the previous element. The whys and hows and wherefores of this are still not known.

Answer 2

The atomic mass of an element is calculated from the weighted average of the naturally occurring isotopes. A similar decrease is also noted for potassium. The atomic mass (weighted average) for potassium, nickel, and iodine is less than the previous element on the periodic table resp. Ar, Co, and Te.