Now in each case because this is a carbon atom it must have the same number of Z. The atomic number must be the same. In other words, all three have six protons. 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.