Why is the atomic mass of iron, 55.845 amu, most similar to the mass of iron-56, yet less than 56 amu? The atomic mass is the simple average of the masses of the two most abundant isotopes. Iron‑56 is the most abundant isotope, so the atomic mass of iron is most similar to the mass of iron‑56. The second most abundant isotope is iron‑54, which reduces the atomic mass to less than 56 amu. The atomic mass is the weighted average of the masses of all of the isotopes. Iron‑56 is the most abundant isotope, so the atomic mass of iron is most similar to the mass of iron‑56. The second most abundant isotope is iron‑54, which reduces the atomic mass to less than 56 amu. The atomic mass is the weighted average of the masses of the two most abundant isotopes. Iron‑56 is the most abundant isotope, so the atomic mass of iron is most similar to the mass of iron‑56. The second most abundant isotope is iron‑54, which reduces the atomic mass to less than 56 amu. The atomic mass is the simple average of the masses of all of the isotopes. Iron‑56 is the most abundant isotope, so the atomic mass of iron is most similar to the mass of iron‑56. The second most abundant isotope is iron‑54, which reduces the atomic mass to less than 56 amu.