But notice carbon twelve has six neutrons. Carbon 13 has seven neutrons and carbon 14 has eight neutrons thereby giving the atomic mass six plus 612, six plus 713 and six plus eight is 14. So that makes sense because to get the atomic mass to get the a you have to add up the protons and neutrons. Now there is a unit that scientists created to deal with very, very small amounts of atoms. Now the AMU, or atomic mass unit is the unit of mass used for elements and compounds. Now by definition we define that one atom, a single atom of carbon.