the mass number of a carbon-13 (13c) atom is 8.
what is the mass number of a carbon-13 (13c) atom?
8
Na
Na
the mass number gives an estimate of the isotopic mass measured in atomic mass units (u). for c the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of c. for other isotopes, the isotopic mass is usually within 0.1 u of the mass number. for example, cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885. the difference between mass number of an atom and its isotopic mass is known as the mass excess. mass excess should not be confused with mass defect which is the difference between the mass of an atom and its constituent particles (namely protons, neutrons and electrons).
mass number