An example of an element with a fixed atomic mass is carbon-12. Carbon-12 is often used as a reference isotope for atomic mass calculations and has a defined atomic mass of exactly 12 atomic mass units (u). This means that the atomic mass of carbon-12 is a fixed value and does not vary. Other isotopes of carbon, such as carbon-13 and carbon-14, have different atomic masses due to the presence of additional neutrons, but carbon-12 remains fixed at 12 u.