Atoms with atomic mass greater than iron (atomic mass number 56) have lower binding energies, indicating lower stability and a tendency for radioactive emission. The binding energy per nucleon increases up to iron and then decreases for heavier elements. The mass defect can be calculated using the equation E=Delta{m}c², but specific masses for the elements in question are necessary for precise calculations. In multiple-choice questions, referencing a graph of binding energy per nucleon can provide quick insights without detailed calculations. Iron is noted for its high binding energy and stability, making radioactive forms of iron extremely rare in nature.