Atomic mass is a measure of the mass of an atom, typically expressed in atomic mass units (amu). It reflects the total number of protons and neutrons in an atom's nucleus, as electrons contribute negligibly to the overall mass. For example, the atomic mass of a carbon atom, which has six protons and six neutrons, is approximately 12 amu. The atomic mass of an element can vary due to the presence of isotopes, which are atoms with the same number of protons but different numbers of neutrons. For instance, the atomic mass of chlorine is about 35.5 amu, accounting for its isotopes, chlorine-35 and chlorine-37.