How do isotope masses differ from their mass numbers?

The mass number of an isotope is defined as the total number of protons and neutrons in an atomic nucleus. While this integer provides an approximate value for the isotope's mass on the atomic mass unit scale, the actual measured mass of an isotope differs from its mass number due to the mass defect.12

The mass defect occurs because the total mass of a bound nucleus is less than the sum of the masses of its individual constituent protons and neutrons. This difference in mass is equivalent to the nuclear binding energy—the energy required to separate the nucleus into its components—as described by Albert Einstein’s mass-energy equation, $E=m{c}^2$. Additionally, because an atom's total mass includes the mass of its electrons in addition to the nucleus, the atomic mass unit is not exactly equal to the mass of a single proton or neutron. Consequently, the precise atomic mass of an isotope is rarely an exact integer.3452