the sum of protons and neutrons in the nucleus of an atom is called the
As mentioned above, note that the atomic mass of an atom is not the same as its mass number. The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus. Mass numbers are always whole numbers with no units.Also, relative isotopic mass is not the same as isotopic mass, and relative atomic mass (also called atomic weight) is not the same as atomic mass.s mentioned above, note that the atomic mass of an atom is not the same as its mass number. The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus. Mass numbers are always whole numbers with no units.