Why isn't the atomic mass usually a whole number?

The atomic mass is not given as a whole number because it is a weighted average taken of all of an atom's isotopes found in nature relative to the mass of carbon-12. The measurement unit used for atomic mass is the unified atomic mass that has the symbol "u." This is a derived unit from the carbon-12 isotope, where 12 u is the atomic mass of carbon-12.

On the periodic table, students can find the atomic mass written below the element's name. The number written above the element is the atomic number, which is the number of protons of a given element. For example, lithium has an atomic mass of 6.941 and an atomic number of 3.