Why do isotope masses have decimals?

Isotope masses have decimals because they reflect the weighted average of the masses of all the isotopes of an element, taking into account their relative abundances. The mass of an isotope is not a whole number due to the presence of binding energy and the varying number of neutrons, which affects the overall mass. Additionally, Atomic Mass units (amu) are defined based on carbon-12, leading to fractional values when calculating the average for elements with multiple isotopes.