Answer:
The mass number (represented by the letter A) is defined as the total number of protons and neutrons in an atom. Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus.
The atomic mass of a single atom is simply its total mass and is typically expressed in atomic mass units or amu. By definition, an atom of carbon with six neutrons, carbon-12, has an atomic mass of 12 amu.
