mass number

Meaning: The mass number A counts nucleons, meaning protons and neutrons. Electrons are not counted because their mass is tiny. The atomic number Z is the number of protons and defines the element. Isotopes have the same Z but different A because their neutron counts differ. In isotope notation, A is the upper left superscript and Z the lower left subscript, as in ¹⁴₆C: A = 14, Z = 6, so there are 8 neutrons.

Equation: A = Z + N, where A is the mass number (dimensionless count), Z is the number of protons (dimensionless), and N is the number of neutrons (dimensionless). So N = A − Z.

On the MCAT: Expect nuclear decay problems in which you balance A and Z on both sides of a reaction. Alpha decay lowers A by 4 and Z by 2. Beta-minus decay leaves A unchanged and raises Z by 1. Positron emission and electron capture leave A unchanged and lower Z by 1. Gamma emission changes neither. You may also be asked to count neutrons or to tell which species are isotopes. For ions, the charge changes the electron count, not A.

Common confusion: Mass number is not atomic mass (atomic weight). Mass number is a whole number for one specific nucleus. Atomic mass, in amu or g/mol on the periodic table, is a weighted average over the natural isotopes and is usually not a whole number. Chlorine's atomic mass is about 35.45, but its isotopes have A = 35 and A = 37.