An element with an atomic mass number of 12 is carbon (C). Carbon has an atomic number of 6, which indicates the number of protons in its nucleus. The atomic mass number, on the other hand, represents the total number of protons and neutrons in the nucleus of an atom. Since carbon has 6 protons, an atomic mass number of 12 implies that it has 6 neutrons as well (12 - 6 = 6). Carbon is a fundamental element found in all living organisms and is crucial for the chemistry of life.