This number tells is how many protons are in the nucleus of every atom in an element. It is different for each element. Therefore, carbon's atomic number is . The nucleus is not only made up of protons. It is also made up of neutrons.
Neutrons are particles within the nucleus that have no charge.
They are neutral and that is how they get their name. The atomic mass is the total number of protons and neutrons in an atom. Although there are an equal number of protons and neutrons, neutrons are slightly heavier in weight than protons. It is like using the weight units of pounds or grams except that amu is for atoms. The atomic mass of carbon is 12.