atom's mass number

Meaning: The mass number, A, counts the nucleons (protons and neutrons) in one nucleus. It is a whole number for a specific isotope, such as carbon-14, where A = 14. Electrons have negligible mass, so they do not change A. Isotopes of an element have the same number of protons but different numbers of neutrons, so they differ in A.

Equation: A = Z + N, where A is the mass number (a pure count, no unit), Z is the atomic number (number of protons), and N is the number of neutrons. Rearranged, N = A − Z.

On the MCAT: Passages ask you to find neutrons, protons, or electrons from isotope notation. They also ask you to balance nuclear equations by making the sums of A and of Z equal on both sides. 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. Mass spectrometry questions may give peaks for isotopes that differ in A.

Common confusion: Mass number is not the atomic mass (atomic weight) printed on the periodic table. The mass number is a whole number for one isotope. The periodic table value is a weighted average of the isotope masses in amu, so it is usually not a whole number. Chlorine is 35.45 amu on the table, but the individual isotopes have A = 35 and A = 37.