Question

Why dont any of the isotopes or iron have the atomic mass of 55.85 amu listed on the periodic table?

Answer 1

The atomic mass is the total number of protons and neutrons in an atom. That's why it is always a whole number (we can't have 0.5 of proton or neutron). But the atomic mass listed on the periodic table is products of the mass of each isotope by its natural abundance, expressed as a decimal, then added to each other. The resulting sum is the weighted average mass of the atoms of the element which we see in the periodic table, and it's the reason why the atomic mass is not a whole number.